Utilisation of thermal mass in non-residential buildings: guidance on system design, floor types, surface finish and integration of services

CCIP-020

CI/Sfb

UDC

A cement and concrete industry publication

697.97-033.32

The practical guide will assist designers, architects and engineers wishing to exploit the high level of thermal mass that is available in concrete floor slabs and includes chapters on concrete floor options, integration of services, acoustic considerations, surface finish options, as well as a number of case studies on office buildings utilising thermal mass.

Tom De Saulles is the Building Sustainability Manager at the British Cement Association and The Concrete Centre, specialising in energy use in buildings, particularly the application of passive heating and cooling techniques. Tom is a chartered building services and mechanical engineer and previously spent ten years working for the Building Services Research and Information Association (BSRIA). During his time at BSRIA, he researched and compiled a number of publications for the construction industry including illustrated guides on building services systems, and design guidance on free cooling.

Guidance on system design, floor types, surface finish and integration of services

Tom De Saulles BEng, CEng, MCIBSE, MIMechE

Tom De Saulles BEng, CEng, MCIBSE, MIMechE

Changes to Building Regulations to mitigate the effects of climate change have set tough targets for CO2 emissions, making the application of full air conditioning systems harder to justify, particularly in the face of rising energy costs. This guide provides detailed guidance on the use of thermal mass as a sustainable method of cooling which avoids or reduces the need for air conditioning.

Utilisation of Thermal Mass in Non-Residential Buildings

Utilisation of Thermal Mass in Non-Residential Buildings

Utilisation of Thermal Mass in Non-Residential Buildings

CCIP-020 Published December 2006 ISBN 1-904482-30-9 Price Group P © The Concrete Centre

Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey, GU17 9AB Tel: +44 (0)700 4 500 500 www.concretecentre.com

Utilisation of thermal mass cove1 1

12/12/2006 14:36:46

A cement and concrete industry publication

Acknowledgements The author would like to thank Chris Parsloe of Parsloe Consulting for collating much of the material for this publication including the background material on the case studies. The following companies are thanked for their contribution of photographs and case study details – Termodeck Monodraught Sunpipe Parliamentary Estates Directorate The Concrete Society Troup Bywaters and Anders Faber Maunsell South Cambridgeshire District Council Vodafone Fulcrum Consulting Royal Holloway University of London Gifford and Partners Brighton & Hove City Council Cundall Johnston & Partners National Trust RSPCA Acknowledgement is also given to Derek Chisholm of The Concrete Centre for his assistance with the publication. BSRIA Ltd, the research association serving the building services industry, supports the content of this guide as a welcome addition to the store of knowledge regarding the beneficial role of concrete structures in delivering sustainable low energy buildings.

Published for The Concrete Centre by The Concrete Society CCIP-020 Published December 2006 ISBN 1-904482-30-9 Price Group P © The Concrete Centre The Concrete Centre Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB Tel: +44 (0)1276 606800 Fax: +44 (0)1276 606801 www.concretecentre.com

CCIP publications are produced by The Concrete Society (www.concrete.org.uk) on behalf of the Cement and Concrete Industry Publications Forum – an industry initiative to publish technical guidance in support of concrete design and construction.

CCIP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777 All advice or information from The Concrete Centre and the British Cement Association is intended for those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted by The Concrete Centre, the British Cement Association or its subcontractors, suppliers or advisors. Readers should note that publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version. Cover photo: Jubilee Library, Brighton. Printed by Cromwell Press, Trowbridge, UK.

Utilisation of thermal mass cove2 2

12/12/2006 14:37:41

Utilisation of Thermal Mass in Non-Residential Buildings Contents 1. Introduction

3

2. The environmental case for thermal mass

5

3. Fabric energy storage – operating principles and performance

9

4. Generic FES systems

12

5. Design options for concrete floors

21

6. Integrating the building services

26

7. Ventilation and night cooling

31

8. Acoustic considerations

37

9. Daylighting and shading considerations

43

10. Specifying and achieving good surface finishes

46

11. Construction

50

12. Project planning of high thermal mass buildings

54

13. Conclusions

59

References

60

Glossary

62

Appendix A

65

thermal mass.indd 1

03/01/2007 16:49:39

Foreword The operation of buildings in the UK accounts for around 50% of total UK CO2 emissions, and air-conditioning is responsible for a growing proportion of this. Uptake of airconditioning in the UK is rising by 8% annually, an increase which could lead to six million extra tonnes of carbon emissions per year by 2020.1 The 2006 edition of Part L2A of the Building Regulations sets tough targets for CO2 emissions, making the application of full air-conditioning systems harder to justify, especially in the face of rising energy costs. At the same time, the Building Regulations now include clear overheating limits for non airconditioned buildings to future-proof against the impact of climate change. The net effect is that an increasing number of high thermal mass buildings are being constructed in the UK, and the passive cooling they offer has an important role to play in the twenty-first century and beyond. When used in combination with night cooling, thermal mass provides a sustainable method of cooling, which avoids or reduces the need for air-conditioning. This is often referred to as Fabric Energy Storage (FES), and its use can simplify the design and operation of many types of building. However, it can also introduce new design issues that architects and engineers, more used to conventional design, may not have encountered before. These mostly relate to the exposed concrete soffits, which have implications for acoustics, lighting, routing services and, of course, the design process itself. This publication provides detailed guidance on these issues, along with general information on FES performance and implications of the various slab options. This is supported by numerous case studies, which offer practical examples and feedback on how specific design issues have been tackled and the lessons to be learned. It is intended that the guide will assist designers, architects and engineers wishing to exploit the high level of thermal mass that is available in concrete floor slabs.

2

thermal mass.indd 2

03/01/2007 16:50:21

Introduction

1. Introduction With the on-going tightening of Part L of the Building Regulations, increasing energy prices and a growing demand for more sustainable design, pressure is being put on airconditioned buildings from all directions. Even the speculative office market, which has traditionally paid little attention to energy consumption, is beginning to re-evaluate its largely unquestioned use of air-conditioning. At the heart of low-energy design is the building fabric and the way in which it interacts with the internal and external environment. In this respect, the high level of thermal mass provided by concrete is playing an increasingly important role in ensuring comfortable internal conditions in offices and other types of building. The use of concrete to provide passive cooling can achieve significant savings in terms of capital and operating costs through avoiding or minimising the need for air-conditioning. The basic approach is to expose the soffit of floor slabs, which can then absorb heat gains during warm weather and stabilise the internal temperature. Typically, the cool night air is then used to ventilate the building and remove the accumulated heat from the slab in readiness for the following day. This cycle of heating and cooling using the thermal mass in the building fabric is often referred to as Fabric Energy Storage (FES) – a term used throughout this guide.

Figure 1 National Trust UK headquarters in Swindon.

Over the last decade, a growing number of prestigious owner/occupied office developments have opted for concrete construction and FES cooling, which reflect design briefs that call for a high-quality internal environment and low operating costs. These buildings largely follow the same design format, typified by the UK headquarters of companies such as PowerGen, Canon and Toyota (see Case studies, Appendix A1, A2 and A3). In contrast to this, property developers and investors in the property market such as the large insurance providers have, until recently, not shown interest in using thermal mass, opting instead for a standard air-conditioned format that has traditionally been viewed as a low-risk option with a short payback. However, we are now starting to see evidence of a shift in this sector, which includes projects such as the National Trust HQ in Swindon (Figure 1), Plantation Place in London and Belvedere Court in London.2 Other examples of high-mass

3

thermal mass.indd 3

03/01/2007 16:50:21

speculative office developments include Number One, Leeds City Office Park and the Addison Wesley Longman office in Harlow.3 This reflects a market that is beginning to pay more attention to the running costs of highly serviced buildings and the questionable longer-term popularity of such buildings in a country with an increasingly fickle energy supply. FES can do much to simplify building design and operation, however, it also brings with it specific design issues that are not present in more traditional office design. These issues mostly arise from the use of exposed concrete soffits, which has implications for acoustics, lighting, routing services and the general design process. Information is provided in this guide on a range of design issues including system options, surface finish, integrating services, lighting, acoustics and system control. This is supported by numerous cases studies, which offer practical examples and feedback on how specific design issues have been tackled and the lessons that can be learned. It is intended that the guide will assist designers, architects and engineers considering a high thermal mass approach to cooling.

4

thermal mass.indd 4

03/01/2007 16:50:27

The environmental case for thermal mass

2. The environmental case for thermal mass In making the case for concrete as part of a high thermal mass building design, it is important that whole-life environmental performance is properly considered with regard to embodied and in-use CO2 emissions. Focusing largely on embodied impacts, as some designers do, fails to recognise the significant savings that concrete buildings can achieve over their life through reduced energy use. From a broader sustainability perspective, it is also important to consider the impact of climate change on the internal environment; the use of thermal mass is a key adaptation measure for mitigating the impact of rising temperatures resulting from climate change. It can help to ensure the long-term viability of a building, through its inherent resilience to climate change, which ultimately means cooling costs can be minimised, even when some air-conditioning is required. These issues are discussed in this section.

2.1 Climate change

Current predictions from the UK Climate Impacts Programme (UKCIP) show that, by the 2080s, annual temperatures for the UK may increase by between 2 and 3.5°C (based on the medium to high emissions scenario – UKCIP02 – 2002). During the summer, the increase will be roughly twice that of the winter, giving an increase of approximately 6°C for the average daily maximum temperature during August in the south-east of England. However, we will not have to wait until the end of the century before overheating becomes a problem for a significant number of buildings in the UK; research by Arup4 using the UKCIP02 climate change scenarios indicates that many existing offices, dwellings and other buildings will experience an increasing tendency to overheat towards the middle of this century and beyond. For example, a typical naturally ventilated office built in the 1960s is likely to exceed 28°C for around 15% of its occupied period.

Figure 2 Predicted periods of overheating in office buildings over the 21st century.

100 90 80 70 60 50 40 30 20 10 0

Ground floor 25 - 280c

1980s

100 90 80 70 60 50 40 30 20 10 0

>280c

2020s

2050s

Naturally ventilated 1960’s office⁴

2080s

Ground floor 25 - 280c

1980s

>280c

2020s

2050s

2080s

Modern, naturally ventilated high thermal mass office⁴

5

thermal mass.indd 5

03/01/2007 16:50:27

Current CIBSE guidance suggests that 28°C should not be exceeded for more than around 1% of the occupied period to maintain an adequate level of comfort. The Arup research also looked at the effectiveness of adaptation measures to minimise the occurrence of overheating. These included the uses of: ■ upgraded facades and shading ■ improved ventilation ■ improved air-tightness ■ optimised internal gains from heat-producing equipment ■ application of thermal mass and night cooling The use of thermal mass in conjunction with night ventilation is identified as being particularly effective in helping to maintain comfortable conditions as temperatures rise, especially when used with other adaptation measures. Similarly, CIBSE design guidance identifies the main strategies for reducing cooling loads, in order of preference, as follows: ■ reduce unnecessary heat gains to buildings. ■ adopt passive cooling solutions utilising various features of the building, e.g. thermal mass. ■ utilise cooling energy from naturally occurring renewable sources local to the building. ■ install mechanical cooling plant, but utilise all available opportunities for free cooling during operating periods when full mechanical cooling is not required.

2.2 Overheating limits – Part L2A of the building regulations

The 2006 edition of Part L2 ‘New Buildings Other than Dwellings‘ sets outs very specific overheating limits that must be achieved in spaces that are not served by mechanical cooling. The regulations state that either: a) the combined solar and internal casual gains (people, lighting and equipment) per unit floor area averaged over the period of daily occupancy is not greater than 35W/m2 when the building is subject to the solar irradiances (given as the entry for July in the table of design irradiances given in CIBSE Guide A), or b) the operative temperature does not exceed 28°C for more than a reasonable number of occupied hours per annum when the building is tested against the CIBSE Design Summer Year appropriate to the building location. The number of hours above 28°C considered to be tolerable by occupiers depends on the activities within the space, and clients and designers will agree appropriate limits in order to meet Workplace Regulations. This is around 20 hours or 1% of the occupied period for most office environments. Whilst the use of air-conditioning and mechanical ventilation is in no way prohibited in the 2006 edition of Part L, their use incurs an additional 5% improvement factor in CO2 emissions. The ‘whole-building’ approach adopted in the revised edition of Part L allows this additional improvement to be achieved in many different ways, including the application of thermal mass to reduce the ventilation or air-conditioning load.

6

thermal mass.indd 6

03/01/2007 16:50:28

The environmental case for thermal mass

2.3 Embodied and operational CO2 emissions

The use of concrete in building construction often raises questions regarding the level of embodied CO2 associated with cementitious materials and products when compared to other materials (i.e. the amount of CO2 released during its manufacture). In most cases, the overall difference in embodied CO2 is actually quite small, and becomes insignificant when compared to the CO2 emissions from plant and equipment over the life of a building. In the case of high thermal mass, passively cooled office buildings, the higher embodied CO2 can be offset in the first few years of operation through the avoidance or minimisation of air-conditioning. The ratio between embodied and operational CO2 emissions is obviously dependent on a range of factors such as location, building type and the choice between natural, mechanical or mixed-mode ventilation. However, an indication of the time taken to offset the additional embodied CO2 burden in the structure when compared to an equivalent steelframe format can be obtained by looking at research published by the Steel Construction Institute (SCI), in conjunction with in-use CO2 data published by the British Council for Offices in their Specification for Offices (2000). This is explained below. In the late 1990s, the SCI carried out a comparative lifecycle assessment of modern office buildings.5 They evaluated a small-to-medium-rise building, built to a developer’s standard specification (Building A), and a more prestigious office development representing a large headquarters building (Building B). The research compared the performance of steel, composite, reinforced and precast concrete options for the two building types. One of the conclusions of the study was that, overall, there is no significant difference between the different types of construction with regard to embodied CO2. This can be seen in Figures 3a and 3b which show the make-up of the embodied CO2 for each method of construction. When considering the structure in isolation, the significance of the embodied CO2 increases slightly, but still remains relatively small.

Figure 3a & b Initial embodied C02 for building A and building B.

1000

Carbon dioxide (kg/m²)

Carbon dioxide (kg/m²)

1000 800 600 400 200 0

A1 A2 A3 A4 Slim floor Composite Reinforced Cell beams

Initial embodied C0₂ for Building A

A5 Precast

800 600 400 200 0

B1 B2 B3 B4 Slim floor Composite Reinforced Cell beams

B5 Precast

Initial embodied C0₂ for Building B

7

thermal mass.indd 7

03/01/2007 16:50:29

The largest difference occurred in Building A when comparing the composite option (composite beams and slabs) with the precast option (precast hollow core slabs). The research suggests that the latter would have 92kg more CO2 in the structure per square metre of floor area. To put this figure in context, the operational emissions need to be considered alongside embodied impacts: if the thermal mass provided by the precast option is used as part of a mixed-mode cooling system, the total annual CO2 emissions will be approximately 60kg CO2 /m2 of treated floor area.6 The composite beam and slab option will typically be air-conditioned, and if an energy-efficient solution such as a displacement system is used, will emit approximately 75kg CO2 /m2 of treated floor area.6 The difference between these two figures is 15kg CO2 /m2, suggesting that it will take around six years to completely offset the additional 92kg/m2 of embodied CO2 in the precast option. After this point there will be net savings in the overall CO2 emissions from the concrete frame building. g

Figure 4

kg CO2 ⁄m2

Time taken to offset additional embodied C02 through in-use savings.

1600 1400 1200 1000 800 600 400 200 0

g

Air conditioned Mixed-mode, high thermal mass

0

5

10 6 Years

15

20

25

This is obviously a very simple calculation and is slightly disingenuous as it is comparing an air-conditioned building with one that has mixed-mode mechanical ventilation, However, it does serve to highlight that any additional embodied impacts in a high thermal mass concrete frame building can be offset relatively quickly when compared to an equivalent air-conditioned building. This will also be the case where a combination of thermal mass and air-conditioning (for peak conditions) is used, albeit with a slightly longer offset period.

8

thermal mass.indd 8

03/01/2007 16:50:30

Fabric energy storage – operating principles and performance

3. Fabric energy storage – operating principles and performance 3.1 How it works

The dynamic response of high thermal mass buildings with exposed concrete is characterised by a slow reaction to changes in ambient conditions and the ability to reduce peak temperatures. This is particularly beneficial during the summer, when the concrete absorbs internal heat gains during the day, helping to prevent overheating. In addition to reducing peak internal temperatures, thermal mass can also delay its onset by around 6 hours 7(Figure 5). In an office environment this will typically occur in the late afternoon, or the evening after the occupants have left. At this point the FES cycle is reversed, with solar gains greatly diminished and little heat generated by occupants, equipment and lighting. As the evening progresses, the external air temperature drops, making night ventilation an effective means of removing accumulated heat from the concrete and lowering its temperature in preparation for the next day. The UK variation in diurnal temperature rarely drops below 5°C, making night cooling relatively effective. As an alternative or addition to night ventilation, water cooling may be used, and can offer improved flexibility and control of slab cooling (see Section 4.4). Concrete’s ability to absorb heat and provide a cooling effect comes from the difference between the surface temperature and that of the internal air and other surfaces. Consequently, the greatest cooling capacity is provided when the internal temperature peaks. Therefore, to some extent a variable internal temperature is a prerequisite in FES systems. However, to maintain comfortable conditions and limit overheating, peak temperatures should not exceed 28°C (operative temperature) for more than around than 1% of the occupied period in office environments.8 Operative temperature (also known as ‘dry resultant temperature’) is an important measure of FES. It takes account of radiant and air temperature, providing a more accurate indication of comfort than air temperature alone. The relatively stable radiant temperature provided by the thermal mass in concrete is a significant factor in maintaining comfortable conditions. It enables higher air temperatures to be tolerated than in lighter-weight buildings, which are subject to higher radiant temperatures resulting from warmer internal surfaces.

Figure 5 Stabilising effect of thermal mass on internal temperature.

9

thermal mass.indd 9

03/01/2007 16:50:31

3.2 The difference between structural weight and thermal mass

False ceilings, raised floors and carpets in buildings, particularly offices, effectively isolate the thermal mass of the concrete structure underfoot and overhead. This can severely limit the concrete’s ability to absorb and release heat within the occupied space. Buildings like this can be described as thermally lightweight, even though they may be structurally heavyweight. Consequently, it does not necessarily follow that a structurally heavyweight building will automatically provide a high level of thermal mass; this depends on the extent to which the structural elements can thermally interact with the occupied space, a relationship that is known as ‘thermal linking’. In existing buildings, thermal linking can often be improved during refurbishment by removing wall and floor coverings. Removing false ceilings or introducing a permeable ceiling will help to unlock the thermal mass in the slab. Hard floorings, such as tile, work well from a thermal perspective, but are rarely practical in an office environment. Raised floors prevent radiant heat transfer with the concrete slab below, but still allow good convective heat transfer when used as a plenum for underfloor ventilation (see Section 4.2).

3.3 Admittance: a measure of thermal mass in construction elements

The admittance value of a construction element provides a useful indication of its thermal mass. High values indicate a high thermal mass and vice-versa. ‘Admittance’ describes the ability of a material or construction, such as a wall, to exchange heat with the environment when subjected to a simple cyclic variation in temperature, which for buildings is 24 hours. It is measured in W/m2K, where temperature (K) is the difference between the mean daily value and actual value within the space at a specific point in time. Key variables that determine admittance are thermal capacity, conductivity, density and surface resistance. However, the admittance for structures with a high thermal mass is ultimately limited by the rate of heat transfer between the structure’s surface and the surrounding air. This places an upper admittance limit of 8.3W/m2 K9 on spaces that are naturally ventilated. During the early stages of design, admittance can provide a useful means of assessing the likely performance of different constructions, and values are published by CIBSE.10 A more accurate indication of how a building or construction element will perform requires detailed thermal modelling, taking into account real weather patterns (as opposed to a simple 24hr sinusoidal pattern) and the more varied nature of heat flow to and from the building fabric.

3.4 Modelling the performance of FES systems

Assessing the effectiveness of FES can be complex due to the dynamic nature of the internal and external environment particularly during the summer. Software that uses the admittance method to assess summertime performance is limited by the simple sinusoidal temperature variation upon which it is based. Consequently, to provide anything other than a basic evaluation requires dynamic thermal simulation software using finite difference algorithms e.g. IES Apachesim and TAS Building Designer. These will model the response of a building to real weather data and allow performance to be assessed under a range of conditions including extended periods of hot weather. For a detailed analysis, computational fluid dynamics (CFD) modelling can be used to look at specific areas of the building, and provide a graphical assessment of air movement and

10

thermal mass.indd 10

03/01/2007 16:50:33

Fabric energy storage – operating principles and performance

temperature. CFD is particularly helpful for analysing spaces such as atriums for air flow patterns and localised temperatures under peak conditions. An experienced operator, able to make qualitative judgements regarding the assumptions and simplifications required, is essential to get meaningful answers from thermal modelling tools. For example, profiled ceilings require 3D considerations and accurate surface-heat transfer coefficients. Similarly, radiant heat transfer coefficients will need to take account of radiation between storage elements.11 Many larger building service consultancies employ a dedicated team of thermal modelling engineers with a high level of expertise. The cost of carrying out a detailed assessment of an FES design is generally very worthwhile, given the relatively fine line that can exist between maintaining acceptable conditions and the risk of overheating.

Recommended further reading on modelling FES performance: ■ Modelling the Performance of Thermal Mass, BRE Information Paper IP6/01, BRE, 2001. ■ Dynamic Energy Storage in the Building Fabric, BSRIA Technical Report TR9/94, 1995.

11

thermal mass.indd 11

03/01/2007 16:50:33

4. Generic FES systems 4.1 Natural ventilation with exposed soffits

Description Flat or profiled floor slabs used in conjunction with natural ventilation. This may be wind-driven, or a combination of wind and stack ventilation.

Typical applications: Offices (with low internal gains), schools and universities. Cooling capacity: ≈ 15-20W/m2 (flat slab) ≈ 20-25W/m2 (profiled slab) Key benefits: ■ simple ■ no fan energy ■ minimal maintenance Figure 6

Flat slabs are quick and easy to construct and economical for spans up to 9m (13m with post-tensioning). FES performance can be improved by using a profiled slab with coffers, troughs or a wave-form finish. While this will have little effect on radiant heat transfer, the increase in surface area will improve the convective heat transfer, which can be doubled in some instances.11 The cooling capacity of profiled slabs is in the order of 20–25W/m2. In addition to their architecturally pleasing appearance, profiled slabs assist in maximising daylight penetration and provide improved acoustic control over a flat slab. Formwork costs are generally higher, but precasting is an option, which provides the potential for savings in site time and the quality benefits that a more controlled environment can bring to the manufacturing process. The application of a naturally ventilated solution is limited to buildings with low-to-moderate heat gains and environments where levels of external noise, pollution and/or security issues will not preclude the use of openable windows.

Case studies ■ The Open University design studio, Milton Keynes.12 ■ Park House, Teddington.13 ■ National Trust Headquarters, Swindon (see Case studies, Appendix A5).

12

thermal mass.indd 12

03/01/2007 16:50:34

Generic FES systems

4.2 Underfloor ventilation with exposed soffits

Description The void created by a raised floor is used as a plenum for mechanical ventilation. Air enters the occupied space through floor diffusers. This system is often used in conjunction with an exposed, profiled slab and with openable windows to provide a mixed-mode solution.

Typical applications: Offices, public and commercial buildings. Cooling capacity: ≈ 20-30W/m2 (flat slab) ≈ 25-35W/m2 (profiled slab) Key benefits: ■ provides benefits of mixed-mode ventilation ■ allows convective heat transfer with top of slab, increasing FES performance. Figure 7

Raised floors are generally considered essential in routing small power and communications in commercial buildings. They have also become a popular way to distribute fresh air, by using the void as a supply plenum, which has the advantage that the floor outlets can be readily moved to suit organisational changes. A further benefit of this technique is the direct contact between the air and the slab, which helps to unlock the thermal mass in the upper slab section that would otherwise remain insulated by the raised floor; an underfloor ventilation supply in combination with exposed soffits enables thermal linking of the slab from both sides, increasing the thickness of concrete that can be used to provide thermal mass. A further increase may be realised if air travelling across the floor void is sufficiently turbulent to enhance the convective heat transfer at the surface. This can raise the admittance of the slab to 10–20W/m2K.14 The optimal rate of heat transfer is dependent upon achieving a balance between the mean air velocity and the time it spends in the floor void without incurring excessive fan power. This requires the floor diffusers to be adequately balanced (see Integrating the building services, Chapter 6). When considering mechanical ventilation as part of an FES design, the case for using underfloor ventilation is quite compelling for office environments, not least because exposed soffits leave few options for air distribution and the routing of other services. However, in addition to providing a means of ventilation and good thermal linking of the slab, other advantages2 are offered by an underfloor supply over ceiling-based air conditioning systems, which include: ■ a reduction in the resources required to construct the building, ■ the ability to provide a higher proportion of fresh air to the occupants, ■ lower maintenance, ■ increased flexibility for future change of use, ■ lower energy consumption, ■ reduced carbon emissions.

13

thermal mass.indd 13

03/01/2007 16:50:35

Case studies ■ PowerGen Headquarters, Coventry15 (see also, Case studies, Appendix A1). ■ Portcullis House, Westminster.16,17 ■ Buildings P&T, Best Practice Programme Report 31. 9 ■ Toyota Headquarters, Epsom (see Case studies, Appendix A3).

14

thermal mass.indd 14

03/01/2007 16:50:37

Generic FES systems

4.3 Exposed hollowcore slabs with mechanical ventilation

Description Precast, hollowcore concrete slabs with mechanical ventilation via the cores, providing good convective heat transfer between the air and concrete. Further heat transfer is provided by the exposed underside of the slab. The system is typically referred to by the trade name ‘Termodeck’.

Typical applications: Universities, schools, theatres, offices (owner-occupied) Cooling capacity: ≈ 40W/m2 (basic system) ≈ 50W/m2 (with supplementary cooling) ≈ 60W/m2 (with supplementary cooling and switch flow) Key benefits: ■ well established technology ■ clear spans of up to 16m are possible ■ air can be introduced at high or low level Figure 8

Hollowcore floor slabs are pre-tensioned precast concrete elements with continuous hollowcores to reduce self-weight and achieve structural efficiency. This type of slab can be used very effectively for FES, with mechanical ventilation used to channel air through the cores before entering the occupied space. Air passes through the cores at low velocities, allowing prolonged contact between the air and slabs for good heat transfer. The temperature difference between the slab and the air leaving the cores is not more than 1–2°C. The precast slabs are usually 1,200mm wide, approximately 250–400mm deep (depending on span), incorporating up to five smooth-faced extruded holes along the length. Three of these are used to form a three-pass heat exchanger in each slab, linked to a supply diffuser located on the soffit. Alternatively, displacement ventilation can be used by ducting the air into an underfloor ventilation system. Air supply to the slabs is via a main supply duct, typically located in an adjacent corridor above a false ceiling. Stale air is generally extracted into a central corridor plenum and then drawn back to the plant room. Figure 9 Passage of air through hollowcore flooring. Diagram: Termodeck

15

thermal mass.indd 15

03/01/2007 16:50:37

As with other FES systems, a large proportion of the cooling is radiant, provided by the exposed underside of the slab. Supply diffusers are located about 1–2m from windows to prevent potential down-draughts and/or clashing with partitions. If required, pre-drilled and sealed openings at mid-span make it possible to relocate diffusers in the future. This enables conference rooms or similar spaces to be accommodated in the centre of the building if required. Typical applications for the hollowcore system include universities and colleges. A muchquoted example is the exceptionally low-energy Elizabeth Fry building at the University of East Anglia. This four-storey building has a gross floor area of 3,250m2 and a total energy consumption of approximately 90–100kWh/m2/y,18 well below the good practice values for building types 1, 2 and 3 described in ECON 19,19 all of which share attributes with the Elizabeth Fry building. The system can be configured to suit a variety of applications and cooling duties. In a basic form, it can handle loads of up to 40W/m2, although recent experience at the Meteorological Office in Exeter shows that higher loads of around 47W/m2 are possible with careful design.20 The addition of mechanical cooling can increase the cooling capacity of the basic system to 50W/m2. Performance can also be increased through indirect evaporative cooling, which cools the supply air without increasing its moisture content. The cooling provided by an evaporative system is dependent on ambient conditions, along with the efficiency of the humidifier and heat exchanger, but can lower the air temperature by several degrees under average conditions. The highest cooling performance is provided by using a switch flow system. This adjusts to individual room temperatures and can be used in conjunction with mechanical and evaporative cooling. The system is regulated by a ‘switch unit’ that incorporates a changeover damper to reroute the supply air. When a room has to be cooled, the air-supply route through the slabs is changed directly to the core that contains the ceiling diffuser, rather than the normal route through all three cores. The shorter distance helps to prevent the supply air taking heat from the slab. A novel application for exploiting the thermal mass in hollowcore slabs has been used at the Royal Holloway College, International Building (see Case studies, Appendix A8): hollowcore slabs, 260mm in depth, were used for the first and second floors, and span the full width of the building. The cores can be exposed at each end by means of motorised flaps, allowing fresh air to pass from one side of the building to the other, cooling the slabs.

Case studies ■ Peel Park, Blackpool.21 ■ The Ionica building, Cambridge.12 ■ The Elizabeth Fry building, University of East Anglia.18 ■ Meteorological Office, Exeter.20 ■ Jubilee Library, Brighton (see Case studies, Appendix A9).

16

thermal mass.indd 16

03/01/2007 16:50:40

Generic FES systems

4.4 Water-cooled slabs

Description Precast or cast in-situ slabs with water cooling via embedded polybutylene pipework, which can be used in conjunction with a night ventilation strategy. The precast option is trademarked as ‘Thermocast’.

Typical applications: Offices, museums, hotels, universities, showrooms. Cooling capacity: ≈ 64W/m2 (flat slab) ≈ 80W/m2 (profiled slab) Key benefits: ■ high cooling capacity ■ high chilled water temperature which may allow free cooling from boreholes, lakes, etc. ■ good temperature control Figure 10

The use of water rather than air to cool floor slabs enables higher cooling capacities to be achieved, making this technique suitable for a broad range of applications. Five-layer polybutylene pipe is embedded in the slab about 50mm below the surface, through which water is circulated at approximately 14–20°C during the summer and 25–40°C during the winter for heating. The technology is applicable to cast in-situ and precast slabs. The precast option comprises coffered slabs made in spans up to 16m in length, providing up to 80W/m2 of cooling.22 The overall specification, developed on an individual project basis, is factory-tested before delivery to site. Manufacturer’s details are available from www.tarmacprecast.co.uk/pages/thermocast.asp. The good thermal linking between the concrete and the circulating water significantly increases the response time of the slab. This is because resistance to heat flow between the water and slab is about 100 times less than the resistance when using air to cool the slab, after allowing for the difference in heat transfer surface area.23 The increased response time allows greater flexibility in the night-cooling strategy. In naturally ventilated buildings, maximum use can be made of conventional night cooling of the slabs with fresh air, followed by water cooling if required. The relative speed of the water cooling ensures that a combined night-cooling control strategy can achieve the required start-of-day condition in the time available. It takes around 30 minutes for a change in water temperature to have a discernible effect on the surface temperature.24 Water cooling is not limited to night operation and can be used as required during the occupied periods to maintain a stable internal temperature. This can prove useful under peak-load conditions, when the slab temperature might otherwise increase to a point where overheating is experienced. The relatively short response time of the water cooling makes it possible for the control system to respond to a rise in internal temperature as it occurs.

17

thermal mass.indd 17

03/01/2007 16:50:40

A number of options can be used to supply chilled water, including mechanical refrigeration, natural water sources or a combination of the two. The relatively high chilled-water temperature necessary to avoid condensation problems allows for the use of water from sources such as rivers, lakes and boreholes. Depending on the load profile, these sources have the potential to meet the cooling demand on a year-round basis. In recent years, the use of boreholes has grown in popularity. This has been driven by a number of factors, including the increasing use of chilled beams/ceilings, the ability to avoid the installation of heat rejection plant (e.g. cooling towers, dry coolers, etc.) and a rising water table that has made obtaining an extraction licence relatively straightforward, particularly in the London area. The temperature of the extracted water remains steady all year. For example, at Portcullis House, Westminster, water is extracted at around 13.5°C all year round.16 Lakes and rivers can also be an effective option, but temperatures will be less stable across the year and may be too warm during the summer to meet the full cooling load.25 When using natural water sources, a plate heat exchanger (PHE) separates the chilled water circuit, preventing the occurrence of fouling in the pipework. This will incur an approach temperature (effectively the loss of cooling performance as a result of using a PHE) of approximately 1.25–2°C, which must be accounted for when considering the ability of a natural water source to meet the required water temperature, especially under peak loads. Full-time mechanical refrigeration can be used where natural sources are not an option. However, opportunities still exist to save energy by applying a free cooling technique appropriate to the plant used.26 The elevated chilled water temperature can make this cost-effective, especially where high cooling loads occur for relatively long periods. The ability to run the chiller(s) at night to cool the slab enables cheap-rate electricity to be used, providing further financial savings over conventional air-conditioning systems. Capital savings are possible with the chiller plant, which, as it does not have to meet peak cooling loads due to the stabilising effect of the slabs, can be comparatively small for the size of building.

Case studies ■ Barclays Bank/Basilica, Basildon.24,27 ■ National Maritime Museum, Greenwich.28 ■ British Museum, London.

18

thermal mass.indd 18

03/01/2007 16:50:43

Generic FES systems

4.5 Chilled beams with exposed or partially exposed soffits

Description Concrete soffits (flat or coffered) with chilled beams suspended directly below. A permeable ceiling may be used, or the soffit left exposed. FES is provided in the usual way, using natural and/or mechanical ventilation.

Typical applications: Offices, universities, refurbished 1960s/70s office buildings. Cooling capacity: ≈ 15-30W/m2 from FES. Chilled beams can provide an additional 100-160W/m2 Key benefits: ■ high cooling capacity, while still making effective use of FES ■ high chilled water temperature may allow the use of free cooling techniques ■ good temperature control ■ good for refurbishment projects Figure 11

In recent years, the combination of chilled beams and exposed concrete soffits has become an increasingly popular solution in both new and retrofit projects. In particular, multi-service chilled beams (MSCB) have found favour with many architects and clients. This can be largely attributed to the simplification of ceiling-located services by using what is essentially a packaged system that can, if required, completely avoid the need for a suspended ceiling. Another key feature of chilled beams is their ability to work with the fabric of a building by supplementing the passive cooling provided by thermal mass. A chilled beam is a simple long rectangular unit enclosing a finned tube through which chilled water is pumped. The beams are mounted at a high level where surrounding air is cooled, causing it to lose buoyancy and travel downwards into the occupied space below. Cooling is largely convective, so good air flow around the beams is essential. Air flow can be maximised by suspending beams directly from exposed soffits. Suspended ceilings must have a large open area, typically greater than 50%.29 The maximum cooling output from chilled beams is in the order of 100–160W/m2.30 Further cooling capacity is provided by FES, and potentially from the ventilation system as well, if the fresh air is conditioned. Ventilation is essentially a separate provision, generally via either natural ventilation or a mixed-mode underfloor system. Fresh air can also be ducted directly to the beam, but this approach will limit FES performance. Chilled beams typically operate with chilled or cooled water between 14°C and 18°C, offering the potential to utilise water from sources such as lakes and boreholes. Alternatively, it is possible to use water pumped directly from evaporative coolers, which can satisfy the load for much of the year. This technique has been used to good effect at Leeds Metropolitan University.26 FES can be employed using techniques described in this publication, with the chilled beams operating during the daytime to boost the overall cooling capacity. In some installations, especially those using natural water sources or other forms of free cooling, it may be

19

thermal mass.indd 19

03/01/2007 16:50:43

advantageous to also operate the beams at night during hot weather. This can supplement the night cooling by ventilation, helping to remove heat from the slab. The thermal interaction between the occupied space, chilled beams and slab is highly dynamic, and dependent on variables such as air velocity, air temperature and control strategy. CFD modelling is necessary when an accurate assessment is required (see Chapter 3.4). In existing buildings where the slab-to-slab height is limited, chilled beams are a convenient way of incorporating cooling and other services in a minimal ceiling depth. Typically, the minimum required depth is around 300mm, however shallower depths are possible. This is useful when refurbishing 1960s office buildings which often present a low floor-to-ceiling height. Incorporating a raised floor into these buildings can be difficult, but can often be achieved if chilled beams are used. A good example of a refurbished 1960s property is the Empress State Building, London, which is an ex-Ministry of Defence office block.31 Chilled beams were used as part of the conversion of the floors into modern office space. The beams incorporate cooling, lighting, PIR sensors, primary fresh air and speakers, all in a depth of around 280mm. They were suspended directly from the slab which was left exposed. Basic chilled beams can also be used as part of a permeable ceiling system, useful in existing buildings where the surface finish of the slab is poor. In this type of application, the cooling coil can be left largely exposed, saving the cost of any casing. The open area in the ceiling should be as large as possible to maximise the air flow over the beam and across the slab. MSCBs, and other forms of integrated service module can house a range of services in a simple and effective manner, making them a particularly attractive option for use with exposed soffits. This is discussed in chapter 6, Integrating the building services.

Case studies ■ Barclaycard Headquarters, Northampton.25 ■ Homer Road, Solihull.32 ■ City Campus Learning Centre, Leeds Metropolitan University.26

20

thermal mass.indd 20

03/01/2007 16:50:45

Design options for concrete floors

5. Design options for concrete floors 5.1 Optimising thermal mass and thermal linking

Thermal mass can be provided by all of a building’s structural elements, including walls, frame and floors. Even furniture can, to a limited extent, provide some reduction in heat gains. However, concrete slabs generally provide the bulk of the thermal mass in office buildings. A good-quality surface finish will allow the soffits to be exposed; an important requirement for optimising heat transfer to and from the space. This can be further enhanced through the increase in surface area provided by a profiled finish which increases convective heat transfer (see Section 4.1). Slab thickness is, of course, largely determined by structural requirements; however, in most applications it will be in excess of 200mm, which will provide a high level of thermal mass that can be fully utilised. The notion that beyond 100mm there is no further benefit is misleading, as it is based on the admittance method (see Admittance, section 3.3) which does not take account of real weather patterns, i.e. hot periods when additional capacity will continue absorbing heat long after a 100mm slab is likely to have become saturated. Additionally, it fails to take account of the ability of underfloor ventilation to provide thermal linking on the topside, significantly increasing the effective thickness that can be exploited. These points are explained more fully below, along with the other factors that determine the optimal slab thickness from the perspective of FES performance: 1. In naturally ventilated buildings with exposed concrete soffits and insulated floors (no thermal linking on topside), a concrete slab approximately 100mm thick will provide a sufficient amount of thermal mass for a 24-hour heating and cooling cycle, i.e. based on the admittance method (Figure 12a). 2. Performance based on admittance (24-hour cycle) requires less thermal mass than for a longer cycle, such as an extended period of warm weather. Therefore, slabs with optimal thickness for a 24-hour cycle (approximately 100mm) are unlikely to provide sufficient thermal mass to prevent overheating during prolonged warm summer periods. This will be especially true if night-time temperatures remain relatively high (Figure 12b). 3. A slab with thermal linking on both sides, e.g. exposed soffit and underfloor ventilation, can exploit a significantly greater slab thickness than is the case where only the soffit is exposed. 4. Optimising the turbulence and air flow rate of underfloor ventilation will enhance the convective heat transfer rate at the upper surface. This, in turn, will elevate the cooling capacity and increase the effective slab thickness that can be used for FES. 5. Taking account of points 2, 3 and 4, a building with exposed soffits and underfloor ventilation (providing enhanced convective heat transfer) is capable of exploiting the thermal mass available in concrete floor slabs 250mm thick or more (Figure 12c).

21

thermal mass.indd 21

03/01/2007 16:50:46

(12b)

(12a)

(12c)

Figure 12 Optimising slab thickness.

22

thermal mass.indd 22

03/01/2007 16:50:46

Design options for concrete floors

5.2 Cast in-situ and precast floor options 5.2.1 Cast in-situ reinforced flat slabs

5.2.2 Cast in-situ profiled/ coffered slabs

Cast in-situ flat slabs are a particularly efficient form of construction, and for spans of 5m to 9m thin flat slabs is the preferred option for concrete-frame buildings with a square or near-square grid layout. For spans over 9m, post-tensioning should be considered. Ideally, drops and edge beams should be avoided to ensure simpler formwork and rapid construction. They may also be visually out of place for exposed soffits, and in the case of edge beams, may reduce daylight penetration into the building. With all cast in-situ concrete construction, particular care needs to be taken with specification and the operations onsite, as, unlike precast, there is not the opportunity to reject an element prior to its installation. Refer to Chapter 10.

This is a popular option for high thermal mass office buildings in the UK, and examples include the headquarters for Canon UK, RSPCA and PowerGen. The main benefits of the cast in-situ option include: ■ a good balance between quality and economy ■ can be advantageous in terms of buildability15 (depending on project specifics) ■ less lead-in time (although custom moulds will need adequate procurement time) ■ concrete can be locally sourced. The main disadvantage with in-situ construction is its sensitivity to site operations, but numerous successful projects demonstrate that quality issues can be successfully managed. The specification should make the ready-mix supplier aware of visually important elements to ensure consistency of concrete supply.

5.2.3 Precast profiled/coffered slabs

Precast profiled slabs have been used in a number of offices in the UK, including the Toyota headquarters,33 Portcullis House and the Inland Revenue Building. Precast can offer a number of benefits, including: ■ speed of on-site construction. ■ a reduction in site activity and less skilled on-site labour. ■ a high-quality surface finish is more easily achieved under factory conditions, which

also makes the use of white cement a more practical option, for a light-coloured fairfaced, unpainted finish. Potential disadvantages can include cost, although this is largely influenced by the size of the project and the resulting number of floor units required. The weight of each unit may also have implications for transportation and placing; a single unit can weigh up to 20 tonnes.

23

thermal mass.indd 23

03/01/2007 16:50:50

5.2.4 Precast hollow core slabs

Hollow core slabs are pre-stressed units available in a range of widths, depths and spans up to 16m, all providing a good quality surface finish. The thermal mass can be used as part of a simple passive system with heat transfer to and from the exposed soffit, or as part of an active system which benefits from additional thermal linking provided by ventilation air ducted through the cores before entering the room (see Exposed hollowcore slabs with mechanical ventilation, section 4.3). Hollow core units are cambered and care is needed to install them sympathetically, particularly where a differential camber exists between adjacent units. However, this has not proved to be an issue in buildings such as schools and universities where hollowcore units are a popular option.

5.2.5 Precast biaxial voided flat slabs

A variation to precast hollow core slabs is a precast voided flat slab spanning in two directions up to a 9m x 9m grid. This is more suited to a near square grid layout and eliminates the need for beams on the gridlines. Joints between precast units are at 2.4m centres, giving a visually greater uninterrupted surface area. Rather than using longitudinal voids as in hollow core slabs, the system uses cast-in plastic spheres (bubbles) to reduce the slab dead load. The spheres allow the span capacity of the system to be increased; however, the reduced concrete mass will, to some extent, reduce the thermal capacity of the slab.

5.2.6 Hybrid floors – lattice girder system

This describes precast concrete permanent formwork used in conjunction with a composite in-situ concrete topping. Exposed lattice girder steelwork forms part of the precast unit, and reinforces the in-situ topping. The key benefits of this system are: ■ speed, simplicity ■ high quality soffit finish, only requiring painting ■ avoidance of formwork. A disadvantage of hybrid lattice girder floors is that back propping is usually required. Additionally, the thickness of shorter spans will be around 100–150mm which, for some passive designs, might provide a thermal capacity that is slightly less than optimal, especially if polystyrene void formers are included to reduce self-weight. An example of where this system is used as part of low-energy design is South Cambridgeshire Hall (see Case studies, Appendix A6). Overall, the lattice girder hybrid flooring system provides a very simple means of achieving a high-quality flat soffit which is relatively quick to construct.

24

thermal mass.indd 24

03/01/2007 16:50:51

Design options for concrete floors

5.2.7 Post-tensioned slabs

Concrete has a low tensile strength but is strong in compression, so by pre-compressing a concrete floor slab, its load-carrying ability is increased. For cast in-situ floors, this is achieved by placing concrete around sheaths or ducts containing unstressed tendons. Once the concrete has gained sufficient strength, the tendons are stressed and locked off by special anchor grips. All the tendon forces are transmitted directly into the concrete and the ducts are then grouted up. The benefits of post-tensioned slabs include: ■ rapid construction and good economy ■ good design flexibility and optimum clear spans ■ minimum storey height and number of columns ■ joint-free and crack-free construction. A solid post-tensioned flat slab has an economic span range of approximately 7–13m, which is about 4m more than the maximum economic span of a standard reinforced flat slab. The depth of solid post-tensioned slabs is usually determined by deflection requirements or by the punching shear capacity around the column. Whilst the slab depth for a given span will be less with post-tensioning than standard reinforcing, it will still be in the range of around 200–400mm, which will ensure a high level of thermal mass. An important benefit of post-tensioned slabs with regard to thermal mass is the ability to achieve crack-free construction, which has the potential to give a high-quality surface finish. A good example of this is the PowerGen headquarters, where partial post-tensioning was used to minimise early thermal shrinkage effects and so ensure that there were no visible cracks in the exposed concrete soffit. The maximum design crack width was 0.1–0.2mm so that standard emulsion paint could be applied to the soffit without the cracks showing through.15

25

thermal mass.indd 25

03/01/2007 16:50:51

6. Integrating the building services Exposed concrete soffits bring new challenges for engineers and architects, who must integrate luminaires and conceal services that would normally be housed in the space between slab soffit and the suspended ceiling. This section provides an overview of the key options available.

6.1 Integrated services modules and multi-service chilled beams (MSCB)

Integrated service modules and MSCB’s provide a very convenient and effective means of combining high-level services with exposed soffits. For this reason, they are often used in high thermal mass offices, and can house a range of services including: ■ lighting systems ■ sprinkler systems ■ smoke detectors ■ public address systems ■ voice, data and BMS cabling ■ passive Infra-Red (PIR) sensors ■ acoustic control panels ■ ventilation extract system ■ cooling coils (MSCBs only). They can be stock items or produced as a bespoke design, offering good flexibility for the services engineer and architect; custom-made units can be tailored to meet specific requirements, allowing them to be sympathetic to the overall aesthetics of the interior. Lighting can also be configured to provide a particular effect; for example, up-lighters can be incorporated to avoid dark soffits, and acoustic panels can be included to minimise reflected sound from the soffits. Cover plates allow modules to be linked, providing the appearance of a continuous raft and concealing the connecting services. Alternatively, the modules can be linked directly together. Another option for concealing the connecting cabling and pipe runs is to locate them in the raised floor above and branch off through small holes in the slab located directly above the modules. If the slab penetrations are likely to remain visible, care should be taken to avoid excessive spalling when the drill penetrates the soffit. This option provides a neat appearance, but may not be suited to multi-tenancy buildings as it can introduce right of access complications.

Figure 13 Luminaires fed by a supply in the floor voids above (National Trust, Swindon).

26

thermal mass.indd 26

03/01/2007 16:50:52

Integrating the building services

6.2 Perimeter bulkheads and corridor-located services

The use of perimeter bulkheads or corridor-located services linked to a riser at one or both ends are effective ways of routing services along the length of a room, and can be used to supply service modules and MSCBs, with the rafts running perpendicular to the bulkhead/corridor. They can also be used in their own right as part of an extract system, housing grilles and ductwork, and can provide a convenient surface for locating acoustic panelling. The corridor option provides a particularly discrete solution; services are routed through the separating wall of an adjacent corridor and tee into a high-level service run located above a false ceiling within the corridor. This is a popular option for ‘Termodeck’ systems (see Section 4.3).

Figure 14 Perimeter services bulkhead with extract grilles.

6.3 Permeable/open ceilings

A permeable ceiling provides a compromise between exposing the thermal mass in the slab and the convenience of a suspended-ceiling system. It can also allow soffits with a low-quality surface finish to be concealed from view; a problem exhibited by many refurbished buildings dating from the 1960s and 1970s where the suspended ceiling is removed to expose the mass and allow additional height for a raised floor. Typical options include the ‘egg crate’ type ceiling tile, perforated tiles and horizontal slats which mask the underside of the slab when viewed from an angle.

Figure 15a Permeable ceilings provide some thermal linking while concealing services.

27

thermal mass.indd 27

03/01/2007 16:50:55

Figure 15b Example of permeable ceiling. Photo: Armstrong Building Products

Whilst permeable ceilings are not as effective as a fully exposed soffit, they do provide a compromise solution by allowing a degree of thermal linking between the room air and slab. Thermal performance varies with the type of slab, ceiling tile and percentage of open area. For perforated ceiling tiles, an open area of 20% is about the maximum that can be used if the slab is to remain hidden. An open area of 20% will allow about 40% of the convective heat transfer that would occur with a fully exposed slab.34 Typically, a cooling capacity of around 10W/m2 can be achieved in a naturally ventilated space.34 Performance may be enhanced by leaving a gap in the ceiling tiles around the perimeter of the space, which allows fresh air from the windows to enter the void and travel across the slab before dropping down through the tiles.13 For offices with mechanical ventilation and a conventional suspended ceiling, an alternative to permeable/open ceilings exists, known as CoolDeck. This is a system that allows thermal linking between the slab and mechanical supply, providing a very-highlevel convective heat transfer by means of turbulent air flow at the surface of the slab.35 Whilst this system can be used in conjunction with conventional suspended ceilings, the radiative cooling effect provided by an exposed slab will be absent. To a large extent, this is compensated for by the improved convective heat transfer, but this must be balanced with the increased fan power that is necessary to provide the beneficial turbulence.

6.4 Slab-located services

Where the services design is carried out sufficiently early in the project, the slab can be cast with voids and/or rebates for locating a range of systems such as lighting, smoke detectors and extract grilles. This is true for precast and cast in-situ solutions, although up-front design is likely to be more critical for the precast option. Rebates are created by formers such as extruded rubber sections, which are particularly effective on curved areas of the mould.15 A good example of where an extract system and other services have been integrated in the slab can be found at the Toyota (GB) headquarters in Surrey: this is a hybrid construction with cast in-situ shoulder beams supporting precast slabs containing extract grilles and rebates for light fittings. Two rectangular extract ducts are located in the space formed within the shoulder beam and link to short ducts serving each floor slab. A more detailed description of this arrangement is provided in a short case study of the Toyota building in Appendix A3.

28

thermal mass.indd 28

03/01/2007 16:50:59

Integrating the building services

Figure 16 Extract ducts integrated with cast in-situ shoulder beams and precast floor slabs (Toyota , GB Headquarters, Surrey).

Figure 17 Two views of the cast in-situ slabs at the Vancouver Library, British Columbia. All ceiling mounted services are located in a single, continuous rebate.

6.5 Underfloor ventilation

A raised floor is a prerequisite for the majority of new office developments, providing a convenient way of routing power and structured cabling for data. It also offers the opportunity of using the floor void as a supply plenum for mechanical ventilation. This is discussed in Section 4.2. An important design requirement when using this approach is to ensure that air is distributed evenly over the slab so there is good thermal linking with the surface. One way to achieve this is to divide the floor void into approximately square compartments, each containing several diffusers. Each compartment is supplied via a damper linked to a central plenum duct running across the floor.36 This will help ensure the air velocity and flow patterns within the void can be optimised for effective heat transfer. It also provides a means of zoning for additional control.

29

thermal mass.indd 29

03/01/2007 16:51:03

Figure 18 Distribution ductwork for an underfloor ventilation system being installed at the RSPCA headquarters, West Sussex. Photo: RSPCA

Typically, the depth of raised floors used for ventilation is in the order of 300–450mm,37 which is around 150–300mm more than that required purely for structured cabling and other M&E installations. However, the absence of a suspended ceiling more than compensates for the additional floor depth, even where soffit-mounted chilled beams are used to provide additional cooling. Thus, the overall height of the building remains similar, or less than, solutions with ceilings.

6.6 Slab penetrations for services

Holes for services can significantly affect the design of slabs, and procedures must be established to ensure holes are structurally acceptable. For reinforced flat slabs, holes near columns need special attention as they reduce local resistance to both bending and punching shear. Larger holes, with a dimension up to 1/20 of the span, can be accommodated through the provision of additional localised reinforcement.38 Holes larger than this will require specific consideration in both analysis and design. In all cases, responsibility for checking the significance of penetrations should rest with the Structural Engineer. In general, holes in beams should not exceed 25% of the depth of the beam for horizontal holes. Holes in excess of this limit may start to have a significant effect on the design of beams, particularly for heavily loaded beams. A useful design feature of post-tensioned slabs is that the distribution of tendons on plan within the slab does not significantly affect its ultimate strength. This allows an even prestress in each direction of a flat slab to be achieved with a variety of different tendon layouts, and offers considerable flexibility for holes and subsequent openings. Early decisions in the design process will ensure these can be easily accommodated. Small holes, less than 300mm x 300mm, can generally be positioned anywhere on the slab, between tendons, without any special requirements. Larger holes are accommodated by locally displacing the continuous tendons around the opening.39 Holes are more difficult to accommodate once the slab has been cast, but, where necessary, can still be made with care if the tendon positions have been accurately recorded and identified. The joint Concrete Society and BSRIA publication, Services Integration with Concrete Buildings,40 provides further guidance in this area.

30

thermal mass.indd 30

03/01/2007 16:51:09

Ventilation and night cooling

7. Ventilation and night cooling 7.1 Natural ventilation

Natural ventilation provides the simplest method for night cooling, but is particularly dependent on external night-time temperature and wind speed/direction. The use of stack ventilation can mitigate the effects of a still summer night, but will require an atrium, ventilation tower (Figure 19 and 20) or other structural device. A popular option is to use a mixed-mode approach (natural and mechanical), which will provide more predictable performance whilst retaining the benefits of natural ventilation when the weather permits. However, for buildings with a modest cooling requirement, natural ventilation alone can satisfactorily provide a range of benefits including: ■ simple operation ■ no energy requirement or CO2 emissions ■ occupants empowered to control their environment. This has been shown to result in greater tolerance of higher internal temperatures 41 ■ building space is maximised through avoidance of mechanical plant and distribution systems ■ low capital costs (although a thermally efficient facade with good shading is important) ■ minimal operating and maintenance costs.

Right Figure 19

Ventilation tower at the National Trust HQ, Swindon. Far right Figure 20

Ventilation tower at the Jubilee Library, Brighton.

Effective control of natural ventilation requires a well-designed and user-friendly window system to take maximum advantage of the prevailing conditions. Solar shading, effective in minimising solar gains, is equally important. During the summer months, heat gains offset by passive FES systems are relatively modest, making the performance of windows and shading a significant determinant in the system’s overall success. Consequently, they are likely to represent a significant component in the overall project costs. The effectiveness of building users in controlling their own environment is particularly important with simple, naturally ventilated buildings, especially the manual switching off of equipment and lighting when not in use, along with the appropriate use of windows and shading. Users need to understand the basic design intent and the extent to which they are responsible for their own comfort – something that will be new to individuals

31

thermal mass.indd 31

03/01/2007 16:51:13

more used to a fully air-conditioned environment. Where heat gains exceed 30W/m2, and the plan depth is greater than 7m for single-sided ventilation or 12m for cross ventilation, natural ventilation is unlikely to be suitable on its own and will require an active system in the form of a mixed-mode or full mechanical system.29

7.1.1 Use of atriums for stack ventilation

Atriums can provide a pleasant working environment, and increase social interaction between floors. They can also greatly assist air flow, especially on hot, still nights when there is not enough wind pressure to move air through the building. Fresh air enters through perimeter windows and moves across the occupied space into the atrium. Increased buoyancy from the heat gains provides a stack effect, allowing the air to be exhausted at a high level through windows with powered actuators. This process may also be assisted by wind pressure, with the balance of driving forces being largely dependent on ambient conditions. Central atriums are a key feature in many owner-occupied high thermal mass office buildings. The atrium works in unison with narrow floor plates and an open balcony arrangement, to provide an unobstructed path into the atrium. Natural ventilation in these buildings is often supplemented by a mechanical underfloor supply to provide a mixed-mode solution.

7.1.2 Combined wind and stack ventilators

Where atriums and openable windows are not an option, combined stack and wind ventilators may provide an effective alternative (Figure 21). These contain a volume control damper that can be programmed to fully open at night and close again at a predetermined time, or when a lower temperature limit is reached to avoid overcooling.

Figure 21 Combined stack/wind ventilators. Photo: Monodraught Sunpipe Ltd

32

thermal mass.indd 32

03/01/2007 16:51:19

Ventilation and night cooling

7.1.3 Ventilator panels

Ventilator panels can overcome the security problems that may prevent the use of openable windows, especially at low level. They can incorporate bars to stop intruders and wire mesh to keep out vermin and cats, etc. A short case study describing the use of ventilator panels is provided in BRE Information Paper 4/98.42

Figure 22a and 22b Side and bottom hung ventilator panels (National Trust HQ, Swindon).

7.2 Mixed-mode systems

Most active FES systems adopt a mixed-mode approach, which offers the benefits of natural ventilation during favourable conditions, and mechanical ventilation at other times. This can be particularly beneficial during hot, still weather when adequate ventilation needs to be maintained for effective night cooling. Winter performance is also optimised through more predictable ventilation rates, which help to minimise heat loss. The majority of high thermal mass offices with mixed-mode ventilation use an underfloor supply system, which provides some beneficial convective heat transfer between the air and the top of the slab, enabling the thermal mass to be more fully utilised. Whilst the fan energy associated with mechanical ventilation can be relatively high, it must be balanced against the improved thermal performance it can provide, particularly through the enhanced thermal linking afforded by underfloor systems, and the ability to minimise ventilation heat loss in winter. Centralised systems can also reduce energy consumption through the use of heat-recovery devices, such as a cross-flow heat exchanger, which preheats incoming fresh air. This type of device can also be used on very hot summer days to pre-cool the incoming fresh air at times when the exhaust air is at a lower temperature. During night cooling, a damper-controlled bypass prevents the heat recovery device from warming the incoming fresh air. The effectiveness of pre-cooling can be enhanced through the addition of evaporative cooling of the exhaust air before it passes through the heat-recovery device. This cools the fresh air, without increasing its moisture content and depending on the ambient and internal conditions, can lower the supply temperature by several degrees.26

33

thermal mass.indd 33

03/01/2007 16:51:23

7.2.1 Mixed-mode control

The main design objective in a mixed-mode system is to sustain passive operation as long as possible whilst maintaining comfortable conditions. A simple operating strategy might use temperature as the controlling parameter, with a rise triggering a change over to mechanical ventilation. With a more complex strategy, a rise in temperature may signal a change to one of two or three operating modes. This requires intelligent controllers to resolve the optimum control action. However, experience has shown that many systems have too many options and are too complex to commission and manage.43 This can undermine system performance, and a poor control strategy often results in operators abandoning the change-over concept in favour of a concurrent approach, with the mechanical system running continuously. This is wasteful of energy, and can significantly undermine the energy-saving potential of passive cooling. A particular problem that can occur in high thermal mass buildings is the use of control strategies that respond too quickly to changes in conditions. The room response rate can be particularly slow, leading to controllers running fans at 100% to get the required response in internal conditions. To avoid this problem, large dead-bands should be allowed to account for the normal swing of daily temperature. Beyond this, proportional control alone may be sufficient.43 An effective control strategy for mixed-mode ventilation in a high thermal mass office is to combine manual and automated window control; high-level fanlight windows are operated by powered actuators linked to a building management system (BMS) which controls their opening in response to temperature, wind speed and direction etc. At the end of the day the manually operated low-level windows are closed, and the BMS continues to control the upper fanlight windows and mechanical ventilation (if required) to achieve optimal night cooling of the building fabric. For much of the year, occupants are free to open or close the low-level windows as they please. At times when the external temperature is too high (or low) for natural ventilation, the mechanical system takes over and the occupants are advised to close the windows. An effective way to do this is to send a group email to occupants in perimeter locations, advising them of the most appropriate window setting. Examples of where this system has been used include the RSPCA headquarters in West Sussex and Plantation Place, London,44 where staff are alerted that ‘today is a natural ventilation day, feel free to open your window’. A novel alternative to email is a BMS-controlled traffic-light system, which provides a visual guide to the optimal window position for the conditions. This approach has been used to good effect at the BT Brentwood Building.2 In addition to helping limit simultaneous natural and mechanical ventilation, a semi-automatic approach to window control can help to minimise the design risk associated with occupant control. The use of a fully automated system would avoid this, but psychological benefits are provided by empowering individuals with some control over their environment, which is an important aspect of passive building design.

34

thermal mass.indd 34

03/01/2007 16:51:30

Ventilation and night cooling

7.2.2 BMS control strategy for night cooling

Night cooling should take maximum advantage of ambient conditions whilst avoiding overcooling, which would result in uncomfortable conditions at the start of the day, and may result in the subsequent need to reheat the space. This can be a problem with simple set-point control that operates night cooling to achieve an internal space temperature regardless of the previous day’s internal conditions. Mixed-mode systems should default to natural ventilation whenever possible so the energy consumed by running fans is minimised. To achieve these objectives, a number of different control strategies that vary in their approach and complexity can be used. The relative attributes of these strategies have been investigated by BSRIA, who undertook site monitoring of four high thermal mass office buildings constructed in the 1990s. Each employed a different night-cooling control strategy as detailed in the study.45 The buildings featured in the study were: ■ Inland Revenue Building, Durrington ■ Inland Revenue Buildings B and F, Nottingham ■ Ionica Building, Cambridge ■ PowerGen Headquarters, Coventry. The BSRIA’s key conclusion was that a complex control strategy is not necessary to maintain comfortable conditions and achieve energy savings in systems with mechanical ventilation. The careful selection of the control set-point to initiate night cooling was, however, identified as being of great importance. As a result of the monitoring, and further research using computer simulations, BSRIA recommended the following night-cooling strategy: 1. select one, or a combination, of the following criteria, to initiate night cooling: • Peak zone temperature (any zone) >23°C • Average daytime zone temperature (any zone) >22°C • Average afternoon outside air temperature >20°C • Slab temperature >23°C. 2. Night cooling should continue, providing the following conditions are satisfied: • Zone temperature (any zone) > outside air temperature (plus an allowance for fan pick-up if mechanical ventilation is used) • Zone temperature (any zone) > heating set point • Minimum outside air temperature > 12°C. 3. Night cooling should be enabled (potentially available): • Days: seven days per week • Time: entire non-occupied period • Lag: if night cooling is operated for five nights or more, it should be continued for a further two nights after the external air temperature falls below the control set-point.

35

thermal mass.indd 35

03/01/2007 16:51:30

Recommended further reading on natural ventilation: ■ Control of Natural Ventilation, BSRIA Technical Report TN11/95, 1995. ■ Making Natural Ventilation Work, BSRIA Guidance Note GN 7/2000, 2000. ■ Natural Ventilation in Non-Domestic Buildings, CIBSE Guide AM10, 2005. ■ Air Distribution in Naturally Ventilated Offices, BSRIA Guide TN 4/99, 1999.

Recommended further reading on mixed-mode systems: ■ Mixed-Mode Ventilation, CIBSE Application Manual AM13, 2000. ■ Energy Efficiency in Buildings, CIBSE Guide F, 2004.

Recommended further reading on the control of night ventilation: ■ Night Cooling Control Strategies, Technical Appraisal TA14/96, BSRIA, 1996. ■ Night Ventilation for Cooling Office Buildings, BRE Information Paper IP4/98, BRE, 1998.

36

thermal mass.indd 36

03/01/2007 16:51:31

Acoustic considerations

8. Acoustic considerations 8.1 Background noise

Passively cooled buildings provide a quiet environment due to the absence or infrequent use of mechanical ventilation. In a conventionally serviced office, the air-distribution system provides a degree of background noise which is generally beneficial as it provides a masking sound that is particularly useful in open-plan offices. Removing this from the acoustic environment can have the effect of increasing the impact of other sounds, leading to noise sources of the same loudness being audible over long distances and unwanted speech perceived to be up to 15dBA louder.46 The use of partitions and other high-absorption materials will help to address this problem and clearly have a role to play. However, further benefit can also be provided through the use of electronic sound conditioning. This describes a system that generates white noise in the occupied space via a network of speakers normally concealed above the luminaires/service modules. The noise is broadband and characterless, similar to that of a fan-coil installation. The system operates on the principle that audibility of a sound depends on the level of that sound relative to the background noise. If the former is greater than the later then it will be audible. Conversely, if the background noise is greater, the sound is effectively masked; the greater the excess, the greater the masking effect. Examples of high thermal mass buildings where electronic sound conditioning have been used include the headquarters of PowerGen and Vodafone (see Case studies, Appendix A1 and A7).

8.2 Reflected sound

The absence of a suspended ceiling with its acoustic absorbency can give rise to increased reverberation time and increased reflected sound across an open space. Floor coverings and furniture can provide significant sound absorption and may be sufficient to meet acoustic requirements. However, in environments where this is likely to be inadequate, a number of options exits which are outlined in this section. The key challenge is to provide sufficient acoustic absorption whilst minimising or avoiding the location of sound-absorbing acoustic materials on the soffit. This would have detrimental impact on thermal performance by reducing thermal linking, particularly when secured directly to the soffit as this will impede both radiant and convective heat transfer.

8.3 Acoustic coatings

Exposed concrete soffits can be treated with absorptive spray coating to improve acoustics, but careful consideration needs to be given to the thermal implications. These coatings comprise a blend of Portland cement and exfoliated vermiculite which provides a lightweight material with good sound-absorption characteristics. However, the low density also results in a low thermal conductivity, significantly reducing the heat-transfer properties of the slab.47 To address this problem, coatings are available with an enhanced thermal conductivity, providing a compromise between acoustic and thermal performance. Standard coatings have thermal conductivity of around 0.14W/mK . This increases to around

37

thermal mass.indd 37

03/01/2007 16:51:31

0.225W/mK for the enhanced coating,48 but is still only about 16% of the conductivity provided by a dense concrete (approximately 1.4W/mK).10 Thinner coatings could potentially be applied to improve heat transfer, but this is likely to impact on absorption properties, potentially leading to less than ideal thermal and acoustic performance. To ensure thermal mass is maximised, the use of acoustic coatings should ideally be limited to construction elements where it will provide a useful level of sound absorption without significantly affecting the overall thermal linking of the building fabric, e.g. on structural columns, lightweight internal walls/partitions or problem areas where other forms of acoustic treatment would be unacceptable. Aesthetically, acoustic coatings can provide a good visual appearance. The monolithic spray texture can be applied in a variety of colours. Ideally, a fair-faced finish is required prior to application, since any surface irregularities will show through. However, for soffits it is possible to lightly rule the initial coats to correct any local high spots caused by shutter joints, providing they are no more than approximately 3mm. Figure 23

absorption co-efficient

Sound absorption for acoustic coatings.

1.2 1 0.8 0.6 0.4 0.2 0

125

250

500 1000 Frequency Hz

2000

4000

High thermal conductivity acoustic coating (Firespray - Audex W) Standard mineral fibre ceiling tile (Armstrong - Prima Adria)

8.4 Permeable/open cell ceilings

Permeable ceilings without any form of acoustic backing generally offer very little sound attenuation, and fulfil more of a visual role by hiding services whilst maintaining a degree of thermal linking with the slab. Acoustic performance may be marginally improved with relatively small free areas, but this will be at the expense of thermal performance due to the reduced air flow between slab and occupied space. Radiative heat exchange between the slab and occupants will also be limited to the line of sight between the two.

38

thermal mass.indd 38

03/01/2007 16:51:32

Acoustic considerations

8.5 Baffle ceilings

Vertical-baffle ceiling systems provide a high level of sound absorption and can be an effective option in areas with high noise levels, e.g. entrance halls and restaurants. Depending on the baffle height and spacing, a high degree of sound absorption can be achieved by using suitable perforations in conjunction with sound-absorbing infill materials. As with permeable ceiling tiles, the ceiling-located services can be largely hidden, but thermal linking of the slab is likely to be reduced as a result of the restriction to flow and reduce radiative exchange.

8.6 Suspended luminaires with acoustic panels

A widely used and effective method for reducing reflected sound is to mount acoustic panels, or ‘wings’, on either side of suspended luminaires or service modules. This approach is particularly effective in combination with a coffered slab designed to focus the sound onto the panels. The shape of the downstand elements of the coffer can be designed to direct the sound energy onto the upper and lower surface of the panel. This ensures an optimised solution that provides a high level of absorption whilst also minimising the surface area of the panel, thus minimising the impact on the thermal linking of the slab. The use of this technique will require early collaboration and coordination between members of the design team responsible for lighting, acoustic and slab design. Luminaires with acoustic wings can be custom-made for the project, but a more cost-effective solution is to adapt an existing product. Whatever the approach, adequate time must be allowed for performance testing and the modification/ manufacture of the fittings. This form of acoustic control is also applicable to flat slabs, although performance may be reduced without the focusing effect of a coffered profile. An example of this type of application is the Jubilee Library in Brighton, which incorporates the Termodeck system (see Case studies, Appendix A9).

Figure 24 Coffered slabs can be designed to focus reflected sound onto acoustic wings (RSPCA headquarters, West Sussex).

39

thermal mass.indd 39

03/01/2007 16:51:33

Figure 25 Accoustic wings used in conjunction with flat slabs (Jubilee library, Brighton).

8.7 Acoustic panels and partitions

Acoustic panels are available in a wide range of shapes, sizes and materials, providing a bespoke approach to sound absorption that can be tailored to meet the specific requirements of a project. They can be located in a variety of ways; however, care must be taken to ensure that, as far as practicable, they do not restrict ventilation paths across the occupied space and restrict heat transfer to and from the soffit. Standard acoustic ceiling tiles can often be used in other locations to provide aesthetically pleasing solutions; one option is to mount panels at high level around the perimeter, leaving the soffit exposed. Another option, where a services bulkhead is used, is to mount panels on the underside. A novel approach, applied at the National Trust headquarters in Swindon (see Case studies, Appendix A5), is to use suspended acoustic panels in the atrium seating, which can be raised and lowered by the occupants to form private meeting areas.

Figure 26 High level perimieter mounted acoustic panel (DIN HQ, Berlin).

40

thermal mass.indd 40

03/01/2007 16:51:37

Acoustic considerations

Movable partitions are ubiquitous in the open-plan format of the low-energy office. They can provide very effective attenuation and may, in combination with the carpet and office furniture, provide sufficient sound control without the need to apply additional measures. The size and location of partitions must be selected in sympathy with the ventilation strategy so as not to restrict air flow across the occupied space. Tall partitions should be avoided. Figure 27 Suspended acoustic panels that can be raised and lowered to form private meeting areas (National Trust HQ, Swindon).

8.8 Acoustic blocks

Although not commonly used in the UK, acoustic blocks are included in this guide since they combine sound control with thermal mass in the form of a structural block containing a Helmholtz resonator cavity. Specific absorption characteristics can be adjusted through block type and the use of a rockwool filler. They can be combined with standard masonry blocks and are available in a range of finishes. This system is particularly effective in absorbing low-frequency sound and typical applications include sports halls and lecture theatres. It may, however, be suitable for other, less-obvious commercial applications, and could be used with other control measures to provide the required absorbency across the frequency spectrum.

absorption co-efficient

Figure 28 Sound absorption properties of acoustic block with Helmoltz resonator cavity.

1.4 1.2 1 0.8 0.6 0.4 0.2 0

125

250

500 1000 Frequency Hz

2000

4000

Structural blocks with Helmoltz cavities (Oscar Acoustics -Soundbox) Standard mineral fibre ceiling tile (Armstrong - Prima Adria)

41

thermal mass.indd 41

03/01/2007 16:51:44

Recommended further reading on acoustics: ■ Acoustics in Buildings, Thomas Telford, 1996. ■ Architectural Acoustics, McGraw-Hill Publishing, 1988.

42

thermal mass.indd 42

03/01/2007 16:51:49

Daylighting and shading considerations

9. Daylighting and shading considerations 9.1 Windows and shading

Exposed concrete soffits can help to provide excellent daylight penetration when designed in unison with the façade. The design objective is to maximise the daylight within the space without causing excessive glare and solar heat gains. A high window head will allow light to be reflected off the soffit and travel well beyond the perimeter zone. This can be enhanced by the use of light shelves to bounce light off the soffit. An optimum window size for minimising solar gain whilst providing adequate daylight is frequently in the range of 20–40% of the internal area of sun-exposed walls, which will result in a heat gain of between 50 and 75W/m2.43 This is still too high for most FES systems since they typically offset around 20–40W/m2, and it is therefore essential that high performance solar shading is used to reduce heat gains. Shading options range from the very good performance of opaque external louvres, through mid-pane blinds, to the less-efficient internal venetian blind. CIBSE Guide LG1049 and CIBSE Guide A50 give extensive information and guidance on the selection of shading systems. Internal blinds intercept the solar radiation after it has entered the room, and are therefore generally insufficient to attenuate solar gain on their own. This is due to the warming of the air by convection and re-radiated heat from the blind. Therefore, to minimise internal heat gains, shading should ideally be outside the room space to reduce the radiant temperature and peak gain to 5–25W/m2.43 Recent advances in glass technology have provided coatings that can distinguish between longer-wavelength solar heat and shorter wavelength visible light. These can be beneficial, however, given the large overlap in wavelength between the two, there is a limit to how far this technology can be used to limit heat gains, thus retaining the need for additional solar shading. Horizontal overhangs on south-facing facades provide a very efficient option for June and July, although their efficiency reduces with lower solar altitudes. This can lead to problems in September when the unshaded solar gains from the lower-angle sun may not be sufficiently offset by lower outdoor air temperatures, resulting in the need for supplementary shading. In the UK, the combination of fixed external shading in combination with some form of adjustable blind has been widely used to good effect in high thermal mass office design. The blind is often electrically operated, and is controlled by a simple switch adjacent to the window.

Figure 29a and 29b Optimised glazing area with fixed external overhangs and internal blinds (RSPCA headquarters, West Sussex).

43

thermal mass.indd 43

03/01/2007 16:51:49

Main shading options for offices External blind/awning

Solar transmittance ≈ 0.15 (through fabric)

Mid-pane blind Advantages ■ combines best features of a miniawning with a blind. ■ can be fully retracted when not required.

Advantages ■ blind does not restrict window opening. ■ unaffected by wind.· ■ greater solar control than normal internal blinds. ■ good glare control. ■ avoids the need for regular cleaning.

Disadvantages ■ requires occupant control. ■ high quality fabric required to ensure low maintenance/deterioration.

Disadvantages ■ requires occupant control. ■ less solar control than an external blind or louvre. ■ view restricted when in use.

External overhang

Solar transmittance ≈ 0.50

Solar transmittance ≈ 0.34 (closed)

Internal venetian/roller blind and external louvre blind Advantages ■ good at blocking high-angle sun on facades orientated towards the south. ■ unobstructed view. ■ largely maintenance free. ■ no occupant control required.· ■ c an be combined with a light shelf to increase daylighting.·

Advantages ■ internal blinds are relatively inexpensive and easy to fit. ■ good glare control. ■ some external louvre blinds can be retracted during the winter.

Disadvantages ■ not effective for lower angle sun in spring and autumn, although risk of over heating is diminished at this time. ■ no glare control ■ effectiveness limited to south facing facades

Disadvantages ■ some of the heat absorbed by internal blinds is transferred to the room. ■ internal blinds can become unwieldy in a breeze. ■ requires occupant control. ■ view restricted when in use.

Solar transmittance for : ■ venetian blind ≈ 0.66 ■ roller blind ≈ 0.59 ■ reflective roller blind ≈ 0.34 ■ external louvre 0.17 (closed)

Note The solar transmittance through a plain, unshaded double glazed unit is 1

9.2 Minimising the need for artificial lighting

Around one-third of the energy used by a building goes into lighting,51 making efficient lighting design particularly important in the drive to minimise internal heat gains. The use of daylight and movement sensors can help to optimise the control of artificial lighting, but it is important that the overall control strategy takes proper account of occupant requirements and the way in which they are likely to interact with their environment. This can often have unexpected consequences; occupants located in perimeter areas adjust the blinds on a sunny day to prevent glare, resulting in occupants seated away from the windows switching the lights on. This problem has as much to do with facade design as lighting control, but highlights the importance of a holistic approach to design. Modern systems where individual lights dim in response to local light levels can help to resolve this problem, but appropriate system settings are not always easy to determine.

44

thermal mass.indd 44

03/01/2007 16:51:54

Daylighting and shading considerations

9.3 Profiled slabs

9.4 Soffit reflectance

The use of profiling running parallel to the path of daylight can be used to enhance daylight penetration, and coffered slabs are particularly effective. Slabs can also be shimed upwards towards atria or windows to assist daylighting. In addition to aiding daylighting, profiled slabs provide a positive visual aspect to the lighting design by creating areas of contrast that help to define room geometries.

Ideally, a high surface reflectance of at least 70–80% should be achieved, and a gloss factor of no more than 10% to prevent lamps from becoming visible. A simple painted finish using white emulsion is a particularly effective way to achieve this, and provides a cost-effective solution that has been widely used. Another option is to use white cement in the mix to provide a light surface finish that is largely maintenance free. The use of an unpainted soffit made with white cement requires a high standard of casting to achieve a consistent, fair-faced finish and will add to project costs. This approach has been used to good effect at Portcullis House where the high reflectance of white, precast concrete floor units has been fully exploited through the use of light shelves. A degree of uplighting is now a requirement in the latest version of the CIBSE Lighting Guide (LG3).52 To avoid the ceiling appearing dark, the guide states the following:

Figure 30 Light shelf and wave form slab made with white cement (Portcullis House, Westminster). Photo: Parliamentary Estates Directorate

The ceiling average illuminance from both the direct and reflected component should be at least 30% of the average horizontal illuminance. This could be from the sides of surface mounted downlights; from uplighting elements of suspended luminaires; from dropped elements of recessed downlights or from supplementary uplights. In large spaces with unusually low ceilings this may be difficult to achieve and in such circumstances the proportion of light on the ceiling should be as high as is practicable.

Recommended further reading on shading and daylighting: ■ Design for Improved Solar Shading Control, CIBSE Guide TM 37, 2006. ■ Daylighting: Natural Light in Architecture, Elsevier, 2004 (available through the CIBSE). ■ Solar Shading of Buildings, BRE, 1999. ■ Daylighting for Sustainable Design, MacGraw-Hill Publishing, 1998.

45

thermal mass.indd 45

03/01/2007 16:51:56

10. Specifying and achieving good surface finishes 10.1 General considerations

The exposure of a concrete surface to enable the thermal mass of a concrete element to be utilised will usually mean that more care needs to be taken in forming or finishing the concrete surface. Thus the concrete specification will need to cover any requirements for higher standards of finish on concrete surfaces exposed to view. There will be cost implications for the higher finish standard although this will be balanced by the fact that there are no applied finishes. Previous experience in achieving a high-quality finish may be a significant factor in the choice of contractor (or sub-contractor). In general, architectural concrete made in a precast factory can achieve a more consistent standard of finish than in-situ concrete, thus the use of precast concrete for any important visual concrete should be considered. The choice of pre-stressed flooring is important as there are different standards of finish achieved between different types. For in-situ concrete, high standards of in-situ finish can be achieved provided the appropriate quality assurance is in place.

10.2 Specification considerations

Before the specification is written, decisions need to be made on the visual importance of a surface. Often, with a design utilising thermal mass, the exposed ceiling will be used to reflect light into the working spaces below. Light ceiling colours will provide good light reflectance but will also show up any defects in the surface texture of the ceiling. In general, painting will overcome unacceptable colour variation but it will not mask any physical imperfections in a concrete surface. Thus, such surfaces may need remedial treatment to remove such imperfections prior to painting. Flat soffits formed as the underside of a mould will generally produce a surface with a minimum of defects. However, mould joints and formwork joints will be visually emphasised, particularly if light shines across the surface at an oblique angle. For surfaces always in shadow, for instance the soffit of a suspended floor with hanging lights, the finish will not be as critical. Also, dark painted surfaces will assist to mask any defects. Good surface finishes on formed vertical or an inclined-upward surface are more difficult to achieve than on horizontal-formed surfaces. Any blowholes on vertical surfaces will require careful filling before painting.

10.3 Specification of concrete finish

The specification of off the form surface finishes is not assisted significantly by current British or European standards. The National Structural Concrete Specification (NSCS) produced by the National Concrete Structures Group (CONSTRUCT) is currently being

46

thermal mass.indd 46

03/01/2007 16:51:58

Specifying and achieving good surface finishes

updated for introduction when BS 8110 is withdrawn in 2008. NSCS edition 4 will become the UK National Annex to EN13670 ‘Execution of concrete structures’ and is likely to have more guidance on surface finishes.

10.4 Formed concrete finishes

Where a particular type of finish is required to the concrete surface, it should be indicated on the architectural drawings. The finish needs to be clearly defined via the specification, as ambiguity in describing the finish can lead to disputes later in the contract. The following sections give an example of the sort of clauses which could be included in a specification: Finish shall be identified from sample panels XYZ with the following additional requirements concerning blemishes, blowholes, colour, lips/steps and other visual criteria.

10.4.1 Blemishes

10.4.2 Blowholes

Blemishes, such as segregation, honeycombing, water scour, discoloration from leakage or cracking from plastic settlement shall be prevented. Any concrete so affected may not be accepted in the final works.

Blowholes shall not exceed the size and frequency stated below:No more than five blowholes of diameter greater than 5mm diameter in any 1m² of surface (Note: these numbers can be varied to suit. Alternative descriptors can be reference projects or graphical representation of blow-holes – including aggregate bridging voids – can be used.)

10.4.3 Colour variation

Colour variation is an important consideration from a light reflectance as well as a visual point of view: Colour variation, within construction panels or between adjacent panels, shall not be greater than that shown in the quality benchmark panels for this project or as agreed with the architect. The quality benchmark panels could be: ■ Sample panels made specially to set a benchmark for the particular contract. ■ Panels on a nominated concrete building in the vicinity of the construction site. ■ CONSTRUCT reference panels if these are conveniently located. (See section 10.6.)

47

thermal mass.indd 47

03/01/2007 16:51:58

10.4.4 Lips, steps abrupt changes in surface

10.5 Treatment of panels

Notwithstanding the general requirements for tolerances given elsewhere in the specification, all surfaces shall be dimensionally accurate within the limits below (note: these numbers can be varied to suit): ■ straightness or bow of a wall, or column or beam: deviation from a 3m straight edge held in contact with the surface – 3mm. ■ steps or misalignments at board, panel or construction joints – 1mm. ■ roundness of circular columns: permissible deviation from a true semi-circular template held in contact with the surface – 2mm.· ■ any variation in the width of joints between precast concrete panels shall be within a tolerance of 5mm for any one joint.

Surface treatment of concrete to improve the appearance is a specialist task, and care and attention with the formwork to prevent steps across joints or grout loss is preferable to trying to make good the concrete surface after casting. Surface defects, blowholes, etc. can be filled with a cement and fine aggregate paste to match the colour of the concrete. Surface colour differences can be improved by surface rubbing which removes the outer ‘skin’ of cement, just exposing the sand and providing a more uniform appearance. Highpressure water blasting or grit blasting for in-situ work, or acid etching for precast, can be used to affect a greater degree of surface removal than surface rubbing.

10.6 Sample panels

Where high-quality fair-face finishes are specified, it is recommended that contractspecific sample panels be made to act as reference for assessing acceptability of finishes and colour during construction. As an alternative, CONSTRUCT has placed seven sample panels 3m x 1m x 0.25m exhibiting various examples of the form finishes in locations around England and Scotland to be used as reference panels against which suitability of finish can be assessed. These sample panels are intended to be used in conjunction with the NSCS specification. Sample panels have two objectives. They serve to establish an acceptable standard of finish before construction commences and they act as a reference against which the finish actually achieved in the structure is measured. The sample panels may form part of the works in non-visually sensitive areas such as plant rooms and areas with false ceilings; alternatively, bespoke sample panels may be cast for use during the construction period, and/or CONSTRUCT sample panels may be used if these are convenient to the works site. All panels should be subject to approval before similar construction begins in the works.

48

thermal mass.indd 48

03/01/2007 16:51:58

Specifying and achieving good surface finishes

The samples should employ the materials, plant and concrete mix proposed for the works. They should be representative of the actual construction and of a size agreed with the client. They should be of similar thickness and similarly reinforced to the elements they represent and should incorporate all features that may contribute to the final appearance of the work, i.e.: ■ horizontal and vertical construction joints, ■ horizontal and vertical panel joints, ■ arrises and chamfers, ■ tie bolts or other fixing devices, ■ kickers, ■ means of maintaining concrete cover to reinforcement, ■ release agent, ■ any other feature.

10.7 Assessment of surface finish

The appearance of the concrete in the works should be monitored by comparison with the standards and reference set out above. Concrete will generally lighten as it dries out and needs to be left to dry for an extended period before evaluating any colour differences. The assessment of surface finish of either construction samples and/or the works should be made under comparable lighting conditions and from a distance at which the structure will normally be seen. The angle of the sun on a surface may have a marked bearing on the surface imperfections observed. In the absence of other specific requirements, a viewing distance of 3m should be adopted.

49

thermal mass.indd 49

03/01/2007 16:51:59

11. Construction 11.1 Formwork

The preparation of formwork is critical for visual concrete; any defects in the formwork surface will be reflected in the concrete finish. All formwork for visually important surfaces should be of new construction at the beginning of the project. The design of the formwork should enable complete sections between the designated construction joints to be concreted in a single operation. Sections of formwork between supporting members should be sufficiently robust so as to not permit significant differential movement or fluttering, which could cause unacceptable variation in the colour of the concrete finish. The formwork joints should be adequately supported to prevent differential movement across the sheet joints. Mismatching across formwork joints will be reflected as an abrupt change in the concrete surface. All formwork joints should incorporate compressible sealing gaskets and the formwork shall be sufficiently rigid and tightly fitting to prevent loss of water or grout from the concrete during placing, compaction and finishing.

Figure 31 The consequence of mismatching across formwork joints.

An alternative to a highly smooth fair-face finish is to form a textured finish in the concrete surface. Joints between formwork sheets can be masked by forming a rebate or a v-joint at regular centres to coincide with the joints in the formwork. Such a textured finish can be achieved by a variety of board finishes, e.g. a board marked finish or a tongue and groove finish. The formwork pattern for such finishes needs to be clearly detailed giving groove spacing, and nail-head spacing if the nailheads are a visual feature. The formwork needs to be maintained at all times in good condition as regards accuracy of shape, strength, rigidity, water-tightness and surface finish. Concrete cast against impermeable forms such as steel is liable to have more blowholes compared to slightly permeable forms such as plywood. Controlled permeability formwork can produce finely

50

thermal mass.indd 50

03/01/2007 16:51:59

Construction

textured finishes free from blowholes even on surfaces that are inclined or upward facing. Consideration should also be given to using permanent formwork such as GRC or GRP which can give very smooth finishes. The best release agent to use for visual concrete is a chemical release agent. Oil-based or cream-based release agents may leave staining in the concrete surface and promote blowholes. Release agents should be applied sparingly and evenly to the clean formwork immediately prior to placing the reinforcement and any excess wiped off. Release agents adhere better to matt surfaces and thus should enable a more uniform colour. Treated form surfaces should be protected from heavy rain, dust or other contamination prior to concreting.

11.2 Reinforcement

Care should be taken to avoid the reinforcement cage resting on the formwork after the release agent has been applied as this may result in reflective reinforcement lines on the concrete surface. All types of reinforcement spacers will be visible to some extent on the surface. Spacers either with recessed contact surfaces, colour-matched plastic wheel spacers with nibs, or concrete spacers colour matching the in-situ concrete are preferable to spacers with flat contact surfaces. Care should be taken to bend back or remove tie wire from the cover concrete.

11.3 Concrete supply and placement

For the supply of the concrete, BS 8110-1 53 calls up British Standard BS 8500-Pt 1 54, ‘Method of specifying and guidance for the specifier’, Pt 255 ‘Specification for constituent materials and concrete’. BS 8500 is the conforming standard to the UK National Annex to Eurocode 2. In general, the type of concrete used for a good off-the-form finish will have more fine material (cementitious or fine sand) than a concrete specified for structural purposes alone. Under BS 8500-1 clause 4.3, a designed concrete needs to be specified with specific requirements to achieve a particular finish or placing capability (4.3.3m). The concrete specifier shall ensure that all the relevant requirements for the concrete properties are included in the specification given to the supplier/producer. These are covered as ‘Basic’ and ‘Additional’ requirements. The specifier is also required to inform the supplier of the concrete properties needed for transportation, delivery, placing, compaction, curing and further treatment. This is covered under Exchange of information in BS 8500-1, Section 5.1. In the majority of cases the contractor will be the specifier. Concrete shall be supplied from a plant that holds current certification meeting the requirements of the NACCB, category 2, for product uniformity. The supplier shall be responsible for the concrete mix that needs to be proven, either from previous experience or by a preconstruction trial, to achieve a good visual uniformity. Once the mix proportions and materials have been agreed, they shall not be changed without notification.

51

thermal mass.indd 51

03/01/2007 16:52:04

In general, visual concrete should have a minimum cement content of 350kg, which may be higher than the minimum cement or combination content specified for compressive strength or durability. Preferably the fine aggregate should be a light-coloured natural sand. Sand content would typically be around 35–45% of the total aggregate mass depending on the concrete sand fineness. The 10mm aggregate fraction would typically be higher than for structural concrete, even to the extent that the 20mm fraction may be deleted altogether. The colour of ‘as struck’ concrete will generally be determined from the colour of the cement, and the water content of the concrete mix. Different types and sources of cement or cementitious blends will produce various shades of grey. Slag cement blends will generally produce a lighter colour, while a pulverised fuel ash (PFA) blend will generally make the concrete darker. Availability of a particular blend should be checked with the concrete supplier. Sufficient supplies of both cementitious materials and aggregate should be available from single sources for the duration of the contract, to minimise concrete batch colour variation. To optimise colour control in visual concrete, supply should be to one mix design sourced from the same batching plant with good workability control to keep the water content within tight limits. The supply rate of concrete should reflect the capacity of the placer to place and compact the concrete. Any significant delay before discharging the concrete from a truck can result in a differential colour in concrete between trucks. Concrete should not be placed directly against a vertical form face, but will tend to flow towards this surface during the compaction process. Care needs to be taken to avoid the form face being splashed with mortar during the placing operation. Vibration of visual concrete is important – concrete should be placed in layers no thicker than can be effectively compacted with the equipment available; whilst immersion vibrators must not be allowed to touch and potentially damage the formwork surface, even near-surface vibration can result in ‘swirl lines’ in the concrete surface. Self-compacting concrete (SCC) is a recent advancement in concrete technology which is gaining more and more acceptance for special applications. SCC has a very high powder content combined with special admixtures to achieve good self-levelling properties and allow the concrete to flow into place, requiring no vibration. In this context, very good blemish-free concrete surfaces can be achieved with SCC, but it has limitations.56 Trials would need to be carried out for a particular concrete, with a particular release agent– formwork combination to optimise the finish achieved. The age at which the formwork is removed from the concrete may affect the colour. Formwork stripped at a later age will result in darker shading, reflecting a well-cured, more durable surface. Thus, stripping times should be consistent between pours as colour differences in two concretes adjacent to each other will be more of a concern than the colour itself.

52

thermal mass.indd 52

03/01/2007 16:52:05

Construction

11.4 Curing

Generally, interior concrete utilised for thermal mass may not require any active curing, thus eliminating a potential source of discolouration. Some spray-on curing agents can cause staining in the concrete surface. A 90% water-retentive efficiency liquid curing membrane is an effective curing agent that should not stain; however, it needs to be demonstrated that the membrane can be fully removed by post-treatment. Unfortunately water curing is not an option for an unpainted smooth finish as the water curing typically leaves streak marks that remain in the surface indefinitely. Considering the effect of age at stripping on concrete colour, both curing and colour control may be affected by striking formwork at a constant concrete age, say 3 to 7 days after pouring. After striking formwork exposed concrete surfaces should be protected against the weather and other construction activities so as to retain the specified surface finish.

Recommended further reading on specifying and achieving a good surface finish: ■ Plain Formed Concrete Finishes, Concrete Society Technical Report 52. ■ Assessing as Struck In Situ Concrete Surface Finishes, Concrete Advice No. 16 – Concrete

Society. ■ Achieving Good Quality as Struck In-Situ Concrete Surface Finishes, Concrete Advice No.

17 – Concrete Society. ■ Visual Concrete – Design and Production, Appearance Matters 1 – Concrete Society. ■ The Control of Blemishes in Concrete, Appearance Matters 3 – Concrete Society. ■ National Structural Concrete Specification, Third Edition, May 2004, CS 152.

53

thermal mass.indd 53

03/01/2007 16:52:05

12. Project planning of high thermal mass buildings In order to achieve the best possible result, the normal design process applicable to conventional air-conditioned buildings must be modified to suit the special requirements of a high thermal mass concrete building utilising FES for cooling. At the outset of any project it is essential that the client amasses a project team with the required skills and with good cross-discipline communication and cooperation within the design team. Traditional responsibilities and boundaries will, to some extent, be challenged, as the floor slabs shift from being a purely structural element to something that has implications for a range of design issues, i.e. aesthetics, lighting, acoustics, thermal performance and, of course, structure. The following sections describe the main stages and considerations for procuring a high thermal mass building.

12.1 Developing the brief

The earliest stage of any project will involve an assessment of future client requirements that would lead to the conclusion that a building project is the only way of meeting those requirements. This assessment will largely be undertaken by the client in conjunction with a facilities management or project-management company. The assessment should also involve input from the likely end-users of the building. The resulting ‘client brief’ is essentially a statement explaining the background to the project, why it is needed, and what are its objectives. It does not go into any detail as to how an objective will be achieved. The content should be seen as the basis for undertaking feasibility studies and engineering reports to assess the different design options listed. The criteria for assessing the suitability of a particular option, linked to the project objectives, should also be established. The likelihood of some variability in the building’s thermal performance needs to be properly conveyed to the client. The significance of company culture and good occupant control should also be emphasised.

12.2 Design team appointment

The choice of architect and building-services engineer in particular should take into account the type of building the client has in mind. Currently, high thermal mass buildings tend to be designed by a relatively small number of innovative architects and building-services engineers who have specialist skills in building science (or building physics). The intention of this publication is to move this capability into a wider arena of building design. An early appointment of the design team is critical if the outcome of a thermal mass project is to be successful. The approach to selection should focus primarily on the technical suitability of the design team.

54

thermal mass.indd 54

03/01/2007 16:52:06

Project planning of high thermal mass buildings

Standard terms of engagement can be applied to projects utilising the thermal mass for cooling, but the agreed terms may need to be modified to ensure that: ■ Additional duties relating to FES cooling are defined and recognised. For example, with high thermal mass projects there is more likely to be a requirement for Computational Fluid Dynamics (CFD) analysis and for post-handover monitoring and fine-tuning by the design team.· ■ There is a clear demarcation between the responsibilities of the architect relative to the building-services design engineer. For high thermal mass buildings there is far more overlap between the roles of these parties and, hence, potential for confusion with regard to responsibilities. ■ The structural and building-services engineers’ views are given equal weighting alongside the architect’s on matters that will affect the eventual performance of the thermal cooling solution. Opportunities for increasing the cooling output from the thermal mass may be lost where aesthetic considerations alone are given precedence. It is essential where an option to use the thermal mass for cooling is being contemplated, that this forms an integral part of the brief, and key decisions regarding this certainly need to be taken before any significant architectural design work on the building is undertaken. If the building form and layout is commenced without regard to FES cooling, it is probable that the design options available will be compromised if a subsequent decision is made to use it.

12.3 Feasibility

The feasibility stage involves developing the design and associated budget costs to a point where proper comparison can be made between different project options, leading to a recommendation based on criteria established at the briefing stage. Typical output from the feasibility stage would be a ‘project brief’ which should state all technical, managerial and design objectives and show how these objectives are to be met. A feasibility study to determine the suitability of FES cooling for a particular project would involve consideration of a range of issues important to the client, such as performance, risk, installed cost, running cost, etc. The study would include one or more of the generic FES systems outlined in Chapter 4. Based on the conclusions of the feasibility study, a final decision on the use of thermal mass cooling should be made, which can then be incorporated in subsequent design stages. It should embody decisions based on the outcome of feasibility studies, surveys cost appraisals, etc. A review of design team responsibilities should also be undertaken at this stage to ensure that the overlap between architect and building-services engineer’s responsibilities are clearly defined and are suited to the subsequent design-stage activities associated with designing a high thermal mass building.

55

thermal mass.indd 55

03/01/2007 16:52:06

12.4 Conceptual design

Having agreed an approach to the design of the building that incorporates FES cooling, work can begin to develop a scheme design. At this stage the design parameters for the building can be finalised and documented. An integrated approach to design is required involving direct collaboration between the architect, structural engineer and building-services engineer. For example, the architect must take into consideration the design issues addressed in this guide, such as building shape and orientation, surface finish, external shading, the need for atria or ventilation towers, etc. Similarly, liaison between the structural engineer and other members of the design team will be required to establish the best structural slab solution. The issues raised in Chapter 5 relating to the alternative slab options and finishes will also need to be addressed. The full conceptual design involves producing finalised scheme design drawings incorporating all of the main features of the building and building services systems. The scheme design drawings will form the basis for obtaining planning permission for the building. If appropriate, the potential energy-saving features of the building may be used in support of the planning application.

12.5 Detailed design

The detailed design stage involves preparing general structural and architectural drawings (including services drawings) suitable for tendering purposes. All design issues that have a significant impact on project cost should be resolved at this stage. It is particularly important to ensure that all features that impact on the FES cooling solution are properly detailed and specified. Aspects that will require particular attention include aesthetics (Chapter 10), lighting (Chapter 9), acoustics (Chapter 8), treatment of services (Chapter 6) and thermal performance (Chapters 4 and 7). The drawings and specifications prepared at this stage will form the basis for obtaining tenders from potential trade contractors for the various elements of work. For high thermal mass concrete buildings, the appointment of the specialist concrete formwork sub-contractor is particularly important. If slabs are to be cast in situ, the formwork, pouring and finishing of concrete will be critical activities for a successful outcome. Every effort should be made to appoint a contractor with a good track record in producing high-quality concrete finishes in compliance with the highest standards.

12.6 Production information

The main building contractor (or management contractor), and their sub-contractors should have been appointed by this stage and be available to contribute to final construction details. This may include producing construction or installation drawings and method statements themselves, explaining the detail of various construction activities, or commenting on existing design and construction proposals.

56

thermal mass.indd 56

03/01/2007 16:52:07

Project planning of high thermal mass buildings

The design team members should also be available to provide comment and feedback on the contractors’ drawings and method statements to ensure that the FES design principles are not compromised.

12.7 Construction

Aspects of FES construction that require special attention are as follows: ■ Precast floor slabs require a longer lead time than in-situ slabs. If slabs are to include holes and rebates for services, this will mean that the M&E design will need to be finalised earlier to allow time for moulding. Slab weight/size may have implications for transport, cranage and site access (a single unit can weigh around 20 tonnes). ■ Cast in-situ slabs have less lead-in time than precast, although custom moulds will need adequate procurement time, which in turn may require the M&E design to be finalised early in the project (see Concrete floors, chapter 5). ■ Consideration should be given to using the contractor for the detailing, and the level of reinforcement rationalisation, i.e. the extent to which variations in reinforcement configuration is permitted; this will have time and cost implications. ■ For post-tensioned slabs, early identification in the design process of penetration requirements for services will ensure the tendon layout can be appropriately designed and easily accommodated. ■ To help ensure good results, the ready-mix supplier needs enough lead time to ensure that a constant supply of materials can be arranged for the duration of the contract (Chapter 11). ■ The shape of the soffit of profiled/coffered slabs will influence acoustic, daylighting and thermal performance. Therefore, any change in slab design will require the M&E design appraisal to be revisited.

12.8 Commissioning

The specification should make adequate provision for commissioning and long-term fine-tuning of the controls (This may only be possible when a period of hot weather is experienced.) The commissioning specification must detail the sequence of commissioning activities and must make clear the indoor and outdoor data to be recorded and the monitoring frequencies appropriate to these. It should be noted that the purpose of these checks is to confirm whether the system, at the particular time of the test, is operating within the designers’ anticipated performance limits. Progressive monitoring of the system over a prolonged period after handover is the most effective method of demonstrating that the intended building performance is being achieved.

12.9 Handover and maintenance

Adequate provision must be made for briefing the building users. This will require time to properly convey the design intent and to help ensure that there is general ‘buy-in’ to the overall approach.

57

thermal mass.indd 57

03/01/2007 16:52:07

At handover, it is essential that the building occupants and maintenance staff (or maintenance contractor) are made fully aware of how the system is intended to operate and which maintenance tasks need to be carried out to ensure that system performance is maintained. It is therefore essential that pre-handover meetings are held so that the operation of the building can be fully explained. Training should combine one or more presentations covering: ■ an overview of the project; ■ importance of good occupant control; ■ methods of control during each season. Involvement in this activity of the architect or other key members of the design team can be beneficial. The training should also comprise a walking tour of the building, during which the use of shading systems, windows and lighting, etc., are fully explained. If possible, particular emphasis should be put on training occupants who will be located in perimeter positions. It is also important that the building services design engineer is permitted to carry out regular measurements to determine the effectiveness of the system under different conditions, and to fine-tune operation as required. Feedback from the building occupants should be considered during this period. Subjective comments from occupants on the building environment and its comfort levels will usually provide a better overall indication of success than individual measurements. The handover information must include comprehensive O&M manuals that include a full explanation of the design and a detailed explanation of how the building should be controlled. Occupiers and designers should meet regularly for at least a year after initial occupation to review the performance of the building. The findings of post-handover monitoring and any fine-tuning carried out should be reported at these meetings.

58

thermal mass.indd 58

03/01/2007 16:52:08

Conclusions

13. Conclusions The advantages offered by modern, high thermal mass buildings are now well established and include simplicity of design, reduced operating costs and a comfortable working environment. More recently, the drive for greater sustainability has highlighted other advantages, such as low in-use CO2 emissions and good adaptability in the face of climate change. Collectively, all of these benefits add up to excellent whole-life performance; an increasingly important requirement in many project briefs. First cost has been slightly higher on some projects when compared to alternative options, but this is not always the case as savings can be made through the avoidance of suspended ceilings and reduced mechanical services. Evidence that costs are comparable to other options can be found in the commercial office market, which has traditionally required a highly serviced, fully airconditioned environment. However, in recent years there have been a number of speculative developments that have adopted a high thermal mass FES approach. This marks a turning point in the market and reflects a growing requirement for lower operating costs and recognition that this type of building offers long-term usability, leading to a good return on investment. FES design offers a good deal of flexibility, ranging from simple, naturally ventilated systems, to more sophisticated mechanically ventilated or water-cooled systems suitable for projects with higher cooling loads. There are also a range of in-situ and precast floor systems, each providing specific attributes to match project requirements. Overall, system design does not present any significant challenges, but does require a closely integrated project team, able to resolve aspects of the project that straddle structural, architectural and building services design issues. It is also important that the initial briefing process is managed well to ensure client expectations are understood. Similarly, at the end of the project there must be a comprehensive handover process to ensure occupants are familiar with the design intent and the way in which the building is controlled. Whatever the motivation to construct a high thermal mass building, it will provide a high-quality, well-ventilated space, in which occupants are empowered to take control of their environment. The building operator will benefit from lower operating costs, and everyone will benefit from reduced CO2 emissions; a saving that can outweigh the additional manufacturing impacts of cement many times over, resulting in low whole-life (embodied and operational) CO2 emissions. For many developments, FES design will be combined with mixed-mode ventilation, and/ or air-conditioning. There are a number of reasons for this, including the need for enhanced cooling performance in more-demanding office environments, and the need to overcome issues relating to security, noise and the potential design risk associated with occupant control.

59

thermal mass.indd 59

03/01/2007 16:52:09

References 1. Constructing the Future, Building Research Establishment, Issue 24, Spring 2005. 2. Sustainable Buildings are Better Business: Can We Deliver Them Together?, Arup

Associates, British Council for Offices, 2002. 3. ‘Speculative build warms to passive design’, Reinforcing Links, Reinforced Concrete 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Council, Issue 10, 1996. Climate Change and the Indoor Environment: Impacts and Adaptation, TM36, Chartered Institute of Building Services Engineers (CIBSE), 2005. Eaton, K.J., Amato, A., Comparative Environmental Life Cycle Assessment of Modern Office Buildings, SCI Publication 182, Steel Construction Institute, 1998. BCO Guide 2000, Best Practice in the Specification of Offices, British Council for Offices. Braham, D., Barnard, N., Jaunzens, D., Thermal Mass in Office Buildings: an Introduction, BRE Digest 454, Part 1, 2001. Approved Document L2 (2006) Conservation of Fuel and Power, The Building Regulations 2000. BRECSU, Avoiding or Minimising the Use of Air-Conditioning, Report 31, HMSO, 1995. CIBSE Guide A, Environmental Design, Chartered Institute of Building Services Engineers, 2006. Barnard, N., Concannon, P., Jaunzens, D., Modelling the Performance of Thermal Mass, Information Paper IP6/01, BRE, 2001. Case Studies of Low Energy Cooling Technologies (International Energy Agency, Annex 28), Faber Maunsell, 1998. Martin, A., Kendrick, C., Booth, W., Refurbishment of Air-Conditioned Buildings for Natural Ventilation, Technical Note TN8/98, BSRIA, 1998. Braham, D., Barnard, N., Jaunzens, D., Thermal Mass in Office Buildings: Design Criteria, BRE Digest 454, Part 2, BRE, 2001. O’Neill, B.T., Shaw, G., Flynn, M., PowerGen Headquarters (Project Profile), British Cement Association, 1996. Bunn, R., ‘Lord of the files’, Building Services Journal, September 2000. Thornton, J.A., Deavy, C.P., Mitchell, D.M., The New Parliamentary Building: Portcullis House, A paper presented at the Institution of Structural Engineers 2000. ‘Probe 14 – Elizabeth Fry Building’, Building Services Journal, April 1998. Energy Consumption Guide 19, Energy Use in Offices, Energy Efficiency Best Practice Programme, 2003. Kennett, S., ‘Location location location?’, Building Services Journal, June 2004. Bunn, R., ‘Mass control (building analysis: Peel Park)’, Building Services Journal, November 1997. Happold, B., Thermal Performance of the Thermocast System, Research Report 008939 for Tarmac plc, 2004. Arnold. D., Building Mass Cooling: Case Study of Alternative Cooling Strategies, CIBSE National Conference 1999. Arnold, D., Othen, P., What a Mass, HAC, 2002. ‘Probe 20 – Barclaycard Headquarters’, Building Services Journal, March 2000. De Saulles, T., Free Cooling Systems, Guide BG 8/2004, BSRIA, 2004.

60

thermal mass.indd 60

03/01/2007 16:52:09

References

27. Bunn, R., ‘Cold silence’, Building Services Journal, March 1999. 28. Under Floor Heating and Cooling, ENERGIE Technical Note, BSRIA, 2001. 29. Gold, C.A., Martin, A.J., Refurbishment of Concrete Buildings: Structural and Services

Options, Guidance Note GN 8/99, BSRIA, 1999. 30. De Saulles, T., The Illustrated Guide to Mechanical Building Services, Guide AG/15, BSRIA, 2002. 31. Trox Technik, Multi-Service Chilled Beams, September 2002. 32. Dawson, S., ‘Solihull hybrid sheds light on office design’, Concrete Quarterly, Issue 209, 2004. 33. Dawson, S., Cast in Concrete, The Architectural Cladding Association, 2003. 34. Kendrick, C., ‘Permeable ceilings for energy storage’, Building Services Journal, August 1999. 35. Barnard, N., Thermal Mass and Night Ventilation: Utilising ‘Hidden’ Mass, CIBSE/ ASHRAE conference, 2003. 36. Gody, R., Willis, S., Bordass, B., ‘Cutting out the cooling’, Building Services Journal, April 1994. 37. Best Practice in the Specification of Offices, British Council for Offices (BCO) Guide 2000, BCO, 2000. 38. Flat Slabs for Efficient Concrete Construction, British Cement Association, 2001. 39. Stevenson, A.M., Post-Tensioned Concrete Floors in Multi-Storey Buildings, British Cement Association, 1994. 40. Bunn, R., Simpson, D., White, S.. Services Integration with Concrete Buildings, The Concrete Society, 2004. 41. Oseland, N., A Review of Thermal Comfort and its Relevance to Future Design Models and Guidance, Proceedings from BEPAC Conference (pp. 205–216), York, 1994. 42. Maria, K., Night Ventilation for Cooling Office Buildings, Information Paper IP4/98, BRE, 1998. 43. Mixed-Mode Ventilation, CIBSE Guide, Application Manual AM13:2000. 44. Kennett, S., ‘Bucking the trend’, Building Services Journal, November 2004. 45. Martin, A., Fletcher, J., Night Cooling Control Strategies, Technical Appraisal TA14/96, BSRIA, 1996. 46. Bunn, R., ‘Too quiet for words’, Building Services Journal, December 1996. 47. Brister, A., ‘For your ears only’, Building Services Journal, December 1996. 48. Audex W acoustic coating – Firespray International Ltd. 49. Daylighting and Window Design, CIBSE LG10, CIBSE. 50. CIBSE Guide A: Design Data, CIBSE. 51. Doing More with Less, EU Green Paper on Energy Efficiency, 2006. 52. CIBSE Lighting Guide 3 ‘The Visual Environment for Display Screen Use’. Chartered Institute of Building Services Engineers, 1996. 53. BS 8110-1:1997, ‘Structural Use of Concrete- Part 1: Code of practice for design and construction’. 54. BS 8500-1 – ‘Complementary British Standard to BS EN 206-1- Part 1: Method of specifying and guidance to the specifier’. London, BSI, 2006. 55. BS 8500-2 – ‘Complementary British Standard to BS EN 206-1- Part 2: Specification for constituent materials and concrete’. London, BSI, 2006. 56. UK Concrete Society, Self Compacting Concrete – a Review, Technical Report No. 62, TCS 2005. 57. Interface, issue 6, Trent Concrete Limited, Autumn 2001. 58. Goddchild, C.H. Best Practice Guidance for Hybrid Concrete Construction. Reinforced Concrete Council (RCC) 1995.

61

thermal mass.indd 61

03/01/2007 16:52:10

Glossary Admittance

BSRIA

Admittance describes the ability of a material or construction element to exchange heat with the environment when subjected to a cyclic variation in temperature (typically 24 hours for buildings). It is measured in W/m2 K, and provides a useful indication of the thermal mass in construction elements and materials.

Building Services Research and Information Association.

CFD modelling

Computational fluid dynamics (CFD) modelling is a computer-based tool that uses numerical methods and algorithms to analyse fluid flow such as air movement within a building.

CIBSE

The Chartered Institution of Building Services Engineers (CIBSE): An international body which represents and provides services to the building services profession.

Coffered slab

Convective heat transfer

Cross ventilation

Displacement ventilation

A floor slab with a curved, sunken soffit repeated approximately every 2 to 4m. Coffers are often used to reduce the slab weight of large spans whilst also providing visual interest for exposed soffits and an increased surface area for better thermal linking (see thermal linking).

Heat transfer that occurs between a surface and the surrounding air when they are at a different temperatures and there is air movement.

Ventilation from openings/windows on two or more sides of a ventilated space.

Ventilation from air entering a space at low level across a large area (e.g. from floor outlets) and displacing warmer air with minimal mixing. This is then extracted at a higher level.

62

thermal mass.indd 62

03/01/2007 16:52:10

Glossary

Diurnal temperature variation

Dynamic thermal response

Effective slab thickness/depth

The difference in temperature between the warmest and coolest parts of the day.

In the context of this guide, the term describes the variation in floor, wall and ceiling temperature and the occupied space temperature throughout the day, in response to continually varying internal and external conditions.

In the context of this guide, the term refers to the maximum depth that heat will penetrate a slab used for fabric energy storage (see fabric energy storage). This is determined by: 1. The conductivity of the concrete. 2. The rate of heat transfer at the surface. 3. Whether both surfaces (top and bottom) are exposed. 4. The period for the heating and cooling cycle (typically 24 hours, but can be several days during a heatwave).

Embodied CO2

Fabric energy storage (FES)

Life cycle assessment

Mixed-mode ventilation

In the context of this guide, the embodied CO2 of a material, construction element or building is the CO2 emitted from the processes associated with their production, including the mining of natural resources, manufacturing of materials and transportation.

A process that uses the thermal mass in a building to absorb excess internal heat during the day and remove it at night by means of night ventilation. A variation on this process is the use of polybutylene pipes embedded in concrete floor slabs, through which low temperature water is circulated to remove heat.

In the context of this guide, the term describes the cradle to grave analysis of the economic and/or environmental impact of a material, construction element or building throughout its lifespan.

A term used to describe servicing strategies that combine natural ventilation with mechanical ventilation.

63

thermal mass.indd 63

03/01/2007 16:52:11

Operative temperature

Operative temperature is a measure of comfort that takes account of the air and radiant temperature in a space, giving a better indication of comfort than air temperature alone.

Passive cooling

Cooling by natural means i.e. without the need for mechanical systems. The combined use of thermal mass and natural ventilation is an example of a passive cooling technique.

Radiant heat transfer

Single sided ventilation

Stack ventilation

Thermal linking

Thermal mass

Thermally lightweight

UKCIP

Heat transfer by radiation between two surfaces at different temperatures.

Ventilation that relies on one or more openings on one side only of a ventilated space.

A method of ventilation in which buoyant, warm air rises upwards within a building and exits at high level. The displaced air causes cool external air to be drawn into the building at low level, where it is warmed and the cycle repeated.

Describes the thermal interaction between structural elements and the occupied space by means of radiant and/or convective heat transfer.

A term used to describe the ability of a material to absorb and retain heat.

A term used to describe buildings or construction elements that provide minimal thermal mass. In addition to lightweight materials and construction elements, the term also applies to structurally heavyweight buildings in which much of the thermal mass is effectively isolated by false ceilings and floor coverings etc.

United Kingdom Climate Impacts Programme: UKCIP is funded by the Department for Environment, Food & Rural Affairs (DEFRA) and co-ordinates research on how climate change will have an impact at regional and national levels.

64

thermal mass.indd 64

03/01/2007 16:52:11

Appendix A1 – PowerGen Headquarters

Appendix A PowerGen Headquarters and two buildings of a similar design format: Toyota Headquarters (UK) and Canon Headquarters Building a new headquarters can be a risky business for any organisation. The brief generally requires a sustainable approach to design while also being sensitive to the needs of staff, but without creating something too extravagant; a particularly important consideration for charities and companies with shareholders. In meeting this requirement, the last decade has seen a number of high-profile organisations opting for a low-energy design, where the thermal mass provided by exposed concrete soffits is at the heart of a passive approach to cooling. These buildings follow a similar format characterised by three floors comprising long narrow floor plates with an open balcony arrangement onto a central atrium, which enhances both daylight penetration and natural ventilation during the summer. Mechanical ventilation is also used when necessary, providing a mixed-mode solution that ensures effective night cooling of the building fabric in summer and controllable background ventilation in the winter. This type of office design has proved successful in meeting the needs of the owner/occupier organisations that procure them, and will no doubt continue to be built in the UK, hopefully with uptake in the more mainstream speculative office market. The next three case studies outline three buildings that have adopted this design format, starting with the ground-breaking PowerGen headquarters which largely pioneered the high thermal mass office in the UK.

PowerGen Headquarters, Coventry (1994) Architect: Bennetts Associates Structural engineer: Curtins Consulting Engineers M&E engineer: Ernest Griffiths & Son

In many respects the PowerGen headquarters, completed in 1994, represents a landmark in high thermal mass, passive office design, and provided a successful format that was to be repeated in a number of buildings which followed. It offers a good balance between daylighting, natural ventilation, thermal mass and office layout which has proved effective in providing a comfortable, low-energy environment.

Figure 32 Photos: The Concrete Society

The PowerGen headquarters is a purpose-built administrative centre located at Westwood Business Park in Coventry. The three-storey building provides 140,000ft2 of office space and includes an undercroft carpark. The long narrow floor plan consists of two parallel floor plates separated by a central atrium and lies on an east–west axis. The office layout is generally open-plan. The building’s orientation and layout provides good daylighting and air-flow patterns which are both important considerations in a naturally ventilated building. Even more significant in reducing energy costs is the choice of reinforced concrete for the structural fabric. Cooling of the exposed concrete soffit is by night-time

65

thermal mass.indd 65

03/01/2007 16:52:12

Appendix A1 – PowerGen Headquarters

ventilation, providing free, passive cooling during hot summer days. The coffered ceiling is perhaps the most obvious example of the fulfilment of functional, structural and aesthetic requirements in one feature, and it was achieved by the careful exploitation of the properties of in-situ concrete. Every aspect of the design contributed to the production of a spacious, integrated work environment. This was achieved through the team approach adopted by the client, designers and contractors for all aspects of the design and construction. The building performed well during the summer of 1995, one of the hottest on record. The actual performance closely matched predictions from thermal modelling undertaken at the design stage. Detailed analysis of PowerGen’s overall design intent established that the office space requirements would be best met by a series of narrow floor plates. This would allow connection across the office space and encourage the personal communication between occupants. The size of the floor plates also needed to accommodate a variety of departmental offices and allow for future flexibility. Within the 10.8m x 7.2m structural grid are three coffers, each 2.4m wide, that span from atrium to external window. The coffers’ elliptical cross-section is designed to improve the acoustic performance of the office space by focusing unwanted noise onto the acoustically absorbent wings of the interior lighting rafts suspended beneath each coffer. The lighting rafts partially uplight the coffers to enhance their sculptural form. They also incorporate smoke detectors and fire detectors, and the PA system, which would normally be hidden behind a suspended ceiling. In long section, the coffers taper towards their ends to increase the penetration of natural light into the office space from the external windows and the atrium. As well as providing the circulation routes that link the floors, the stair towers contain much of the shared office equipment. This functional planning fulfils the dual purpose of bolstering the stair towers’ role in promoting social contact and of removing heatproducing equipment from the office areas to ‘hot spots’ where heat gains can be locally treated. The 113 m-long central atrium was designed to further enhance natural ventilation and lighting. The open-balconied configuration of offices on either side of the atrium contributes to the open, communicative environment and aids ventilation. The good thermal mass properties of concrete, and its low cost in comparison with other structural alternatives, focused the design effort on choosing between an in-situ, a precast and a hybrid concrete frame solution. The exposed structure required high-quality finishes that initially led the team to consider a precast or hybrid concrete frame. However, both the precast and the hybrid options presented disadvantages of cost, buildability and procurement. Each precast unit would have spanned 10.8m and weighed up to 20 tonnes. This would have created transporting and placing problems. The units would have cost some 10% more than in-situ concrete and would have had to be ordered in advance of letting the main building contract. The balance between quality and economy was achieved by careful design. To enhance the quality of finish, specially designed GRP moulds were used to form the coffered ceiling profile (Figure 33).

Figure 33

The decision to use prefabricated reinforcement considerably improved the speed and accuracy of its fixing. It was important that the speed of construction of the coffered solution should not differ significantly from traditional in-situ concrete floors, otherwise the economies of the in-situ solution would be lost. It was decided to use partial post-

66

thermal mass.indd 66

03/01/2007 16:52:15

Appendix A1 – PowerGen Headquarters

tensioning to minimise early thermal shrinkage effects and so ensure that there were no visible cracks in the exposed concrete coffers. The maximum designed crack width was 0.1–0.2mm so that standard emulsion paint could be applied to the soffit without the cracks showing through. The floorplates are supported by 400 mm-diameter circular columns. Whilst the design was being developed, key aspects were tested by modelling and mock-ups. A 1:40 scale model of a typical cross-section through the building was made to develop the atrium glazing, office glazing and concrete profiles for maximum lighting performance. A full-size mock-up of a 7.2m x 10.8m structural bay was also built using glass-reinforced plaster to form the coffers. This was invaluable for confirming and tuning the design of the coffer profile and light fittings, and for testing the acoustic performance and artificial lighting levels. When Laing Midlands were appointed as design and build contractors, they adopted the approved scheme and worked closely with the design team. The on-site mock-ups played an important role in incorporating key refinements such as the prefabrication of slab reinforcement into the final design solution. The choice of natural ventilation required the service engineers Ernest Griffiths & Son to consider all aspects of the building design. The arrangement of relatively narrow openplan office areas on either side of the three-storey atrium provided the ideal layout for good natural ventilation. By minimising solar gain effects and providing clear air paths, the exposed concrete soffit helps to control the internal environment. The building management system controls the top row of windows, which are opened at night to allow cool air to flow over the coffered concrete soffit. Computer simulations by environmental-modelling specialist EDSL were used to accurately model the office environment and predict peak internal temperatures, taking into consideration external effects, internal heat loads and the passive cooling effects of the exposed concrete. The modelling helped to develop the design strategy and establish the right mix of thermal mass and natural ventilation. It also showed that night-time ventilation was able to exploit the long-term thermal dynamics of the floor. The latter were provided by the careful use of exposed concrete with sufficient thickness to absorb heat gains over many days. Internal heat gains are minimised by placing areas that require air-conditioning, such as the computer suite and kitchens, at the east and west ends of the building. The larger, heat-generating office equipment, such as photocopiers, is grouped into segregated rooms, out of the open-plan space. Staff have considerable control of their environment as the lower windows may be opened manually during the day. Underfloor ventilation ductwork has been installed to accommodate any potential future change of use or the addition of cellular offices. The electrically powered building uses a warm water heat-bus, which gathers heat from various sources including IT equipment, the atmosphere and a diesel generator. This heat is then distributed through underfloor pipes in the atrium, in anti-downdraught radiators at atrium roof level and in air-heater batteries that serve the underfloor ventilation system. A seven-month appraisal of the building using the Building Research Establishment Environmental Assessment Method (BREEAM) earned the building a ‘Very Good’ classification, including a maximum score for its ventilation system.

67

thermal mass.indd 67

03/01/2007 16:52:16

Appendix A2 – Canon Headquarters

Canon Headquarters, Reigate (2000) Architect: David Richmond & Partners Structural engineer: Curtins Consulting Engineers M&E engineer: Ernest Griffiths & Son

Completed in 2000, Canon’s headquarters are situated on a 25-acre site of mature, sheltered landscape at Woodhatch near Reigate. The 11,500m2 of office space has been designed in sympathy with its surroundings, taking advantage of natural ventilation which is ideally suited to the quiet, unpolluted environment. The ventilation strategy works in unison with a coffered soffit to provide passive cooling. The risk of overheating is further reduced by night ventilation during hot weather and cross-ventilation during the day. A BMS controls the night ventilation and uses a very simple algorithm that correspond to BSRIA recommendations (see Section 7.2.2).

Figure 34 Photos: The Concrete Society

Stack ventilation is also provided by a central atrium, but this is not relied upon as part of the cooling strategy in extreme conditions; like PowerGen, a cross-ventilated condition was assumed in favour of a hot still day for the summer design condition. The buildings have been orientated to minimise solar gains and a 3m roof overhang structure with cloisters keep direct sunlight out while minimising the need for artificial light. West and east-facing windows have external shading that is automatically controlled to prevent excessive solar gains in the morning and evening. Working areas are open-plan and comprise a narrow floor plan of 11m on three levels with a central atrium and an open balcony arrangement. This ensures that all workstations receive a high level of natural light, and that there is minimal restriction to air flow. This is particularly important with stack ventilation which, although effective with atria, only offers a very small driving force that is easily disrupted by obstructions. To improve air flow across the soffit, downstand edge beams were substituted with upstand edge beams within the raised floor. Occupants can open perimeter windows, whilst clerestory and atrium windows are controlled by mechanical actuators linked to the BMS. Heat-producing office equipment such as photocopiers have been grouped together in a controlled area with cooling provided by fan coils served by a single chiller, which also provides cooling to meeting rooms and a lecture theatre. Background ventilation is via a two-speed fan system that delivers air via ductwork linked to swirl diffusers in the floor. These produce a column of air and avoid pooling, making supply temperatures as low as 15°C possible without discomfort. Return air is via extract grilles located at the top of the atrium. These can be accessed from a high-level balcony situated above the second floor. This is at the same level as five roof-mounted airhandling units. The post-tensioned coffered slabs were cast on-site in re-usable moulds which were a cost-effective option given the number of re-uses. The coffers were designed to work in unison with suspended luminaires incorporating acoustic wings to reduce reflected sound from the soffit. The surface finish is of a high quality and has a white finish to aid daylight penetration. A BREEAM assessment has confirmed the good environmental credentials of the development by archiving the highest rating of ‘Excellent’.

68

thermal mass.indd 68

03/01/2007 16:52:17

Appendix A3 – Toyota Headquarters

Toyota Headquarters, Surrey (2001) Architect: Sheppard Robson Structural engineer: Whitbybird M&E engineer: Arup

Figure 35 Photos: The Concrete Society

Figure 36

Toyota’s UK headquarters in Epsom, Surrey, were completed in 2001 and provide 14,200m2 of high-quality office space that met the client brief for flexibility and low energy. The building started as an architectural competition and led to a design by Sheppard Robson being adopted. It comprises four two-storey office wings that radiate outwards from a glazed ‘street’ that houses communal facilities including an entrance rotunda. In many respects the general design and operating strategy of the building is similar to that of the PowerGen, Canon and RSPCA headquarters. However, the concrete construction technique does differ, with a hybrid of hidden in-situ reinforced concrete in conjunction with exposed precast coffered floor units and structural columns that are mostly precast (with an internal steel column). After considering the available construction options, the design team decided upon the use of hybrid concrete construction (HCC), largely driven by the desire for snag-free lean construction57 and the high-quality finish provided by the use of precast elements. HCC allowed a high proportion of the frame to be manufactured in a factory-controlled environment and led to high-speed construction on-site. Trent Concrete worked closely with the design team to create a floor unit that incorporated ventilation ducts, electric access points and lighting units. This involved a full-scale polystyrene pattern, which was modified until it could be accurately recreated in precast manufacture. Detailed models were also made of the office floor showing the relationship of the precast units to other key components and interfaces, such as the glass façade, the suspended lighting units and the central services distribution zone;58 an in-situ ‘shoulder’ beam comprising two downstands are hung from an upstand column head. Two rectangular extract ducts are located in the space formed within the shoulder beam and with regular branches to short ducts within the floor units, which draw through openings formed in the underside of the soffit (see Figure 16, Chapter 6). The underside of the shoulder beam is concealed by ceiling panels that also house down-lighters and additional extract diffusers. Air is supplied to the office space in the usual way, i.e. via floor outlets, with the floor void used as a supply plenum.

Figure 37

69

thermal mass.indd 69

03/01/2007 16:52:20

Appendix A4 – The Basilica

The Basilica, Basildon, Essex (1999) Client: Barclaycard Architect: Fitzroy Robinson Services Engineer: Troup, Bywaters and Anders Case study material: Troup, Bywaters and Anders

Figure 38 Photos: Troup, Bywaters and Anders

The Basilica office built for Barclaycard uses natural ventilation and water to cool the slab. A man-made lake adjacent to the building provides cooling water that serves the slabs, or alternatively a mechanical chiller can be used, depending on external conditions and cooling loads. The building client was committed to achieving the most energy-efficient design possible, having previously achieved a successful high thermal mass building at the Barclaycard headquarters in Northampton. The aim was to achieve an even better solution this time. As a result, the design team that had performed so well on the Northampton project were re-employed to carry out the design for the Basilica. In recognition of their importance, all members of the design team (architects, structural and building-services engineers) were employed from the early concept stage of the project. The client accepted a maximum internal temperature of 25°C to help to optimise the potential of an FES solution. The exposed undersides of concrete floor slabs are cooled by natural ventilation using cool night air, and by water from an adjacent man-made pond circulated indirectly through the slabs via embedded pipes. When insufficient cooling is available from natural sources, a mechanical chiller supplements the system using the same pond water to reject heat from the refrigeration plant condensers. Two rectangular offices are located either side of a central atrium. One of the office blocks faces south while the other faces north. The façade on the south side is deeply recessed with a brises-soleil. Night-time cooling of the slab is achieved by natural ventilation using the stack effect in the atrium to draw night air in through high-level window openings on each floor. The windows are opened by motorised actuators. Occupants can control these during the day, with the BMS taking over in the evening to control the night cooling, and also when the mechanical cooling is operating. Manually openable trickle vents at high and low levels allow background ventilation during the summer. In winter, ventilation is provided via floor-mounted swirl diffusers supplied by a floor void distribution system. The chilled slab is a conventionally reinforced, with a continuous serpentine pipework attached to the reinforcing mesh and buried around 70mm above the underside of the soffit. Each of the precast units is 3m x 7.5m in size and contains a single 110m long x 20mm diameter polybutylene tube. The slabs were constructed in a precast factory with the pipes pressurised (to avoid compression) during the concrete pouring. The pipes were then pressure-tested at the factory after the concrete had cured. The soffits are fully exposed and profiled, which improves their appearance and enhances their thermal performance. The ends of the cooling pipework extended outside the slab and are connected to a flow and return system installed in the false floor above.

70

thermal mass.indd 70

03/01/2007 16:52:24

Appendix A4 – The Basilica

A man-made lake was dug at the bottom of the sloping site so that water would naturally drain into it. Water is drawn from the lake through an inverted pipe inlet and is then filtered before passing through a plate heat exchanger, thereby providing cooling indirectly and maintaining the integrity of the building’s chilled water circuit. The water is then returned to the lake. The closed-circuit cooling water system distributes water to separate zoned plant rooms from where it is pumped through the serpentine coils in each slab. When the lake water is too warm to be of use for cooling, mechanical chillers operate to reduce the circulating water temperature to the required level. During this time the same closed-circuit cooling system is used to remove heat from the chiller condensers. The chillers are de-centralised to avoid the need to cool the entire building when only one area needs it. There are six zones, each with their own plant rooms, housing water-cooled chillers, boilers, hot water calorifiers, pumps and fans.

Figure 39

Laboratory testing of the precast concrete slabs showed that the cooling output achieved using water cooled overnight to 19°C was 27W/m2. The results also showed that the floor could achieve a cooling capacity of 64W/m2 with the space maintained at 25°C and chilled water flow and return temperatures of 13–16°C. This was sufficient to meet the client’s predicted maximum cooling requirement. Although some cooling was anticipated from the top of the slabs, it was estimated that 90–95% of cooling would be from the lower surfaces. From the test work, it had been estimated that the length of time to cool a slab was around 3.5–4 hours and that the temperature at the slab surface began to fall around 30 minutes after commencing water circulation. The BMS controls the system in stages so that, depending on the cooling load, the lowest-cost energy option is used first, i.e.: ■ Stage 1: night-time cooling by natural ventilation. ■ Stage 2: night-time cooling by natural ventilation, plus slab cooling using lake water. ■ Stage 3: supplementary cooling using the chillers.

71

thermal mass.indd 71

03/01/2007 16:52:26

Appendix A5 – National Trust Headquarters

National Trust Headquarters, Swindon (2005) Client: National Trust Architect: Feilden Clegg Bradley Services engineer: Max Fordham

Figure 40

The new National Trust headquarters, which opened in July 2005, is built on a brown field site in Swindon, once occupied by a foundry that in years gone by supported the town’s thriving railway industry. Heelis, the name of the building, was the married name of Beatrix Potter, a major benefactor who died in the 1940s, leaving 4,000 acres of Lake District land to the Trust. The architects, Feilden Clegg Bradley, worked closely with the services consultant, Max Fordham, to meet the design brief, which centred on the need for excellent environmental performance and a high level of sustainability. Extensive use has been made of natural ventilation, thermal mass and renewable energy in the form an 80kW photovoltaic array, making the development almost carbon neutral. The environmental impact of staff travel has also been minimised through the requirement that anyone wishing to use the carpark must car share. Enforcement of this policy is no doubt helped by a carpark that will take a maximum of 150 cars, while staff numbers exceed 400. The building occupies 7,300m2 spread over two floors, with a public area at the front comprising a café, shop and seating. Natural light is abundant throughout the building, aided by generous perimeter glazing and a double height atrium incorporating roof lights. Internal finishes are to a high standard and extensive use has been made of oak selected from the Trust’s own woodland, along with carpets made from wool produced on the Trust’s farms. Whilst Heelis is clearly a prestigious HQ building, surprisingly it was built by a commercial developer within the budget for a typical speculative office of a similar size. While closely involved in the brief and overall development, the Trust will in fact be leasing the site from the developer for a period of 25 years. The basic structure is steel frame with precast concrete floor slabs. The office space is open-plan, allowing good air flow from the perimeter windows across the occupied space to the atrium. High-level vents allow air to leave the building, aided by the stack effect. Night cooling during hot weather is an integral part of the passive cooling strategy. All windows and ventilation openings have powered actuators controlled by a BMS system. However, occupant control is also provided by simple rocker switches located next to perimeter windows. Manual control will override the BMS setting for one hour, after which the window will revert to the BMS setting. This approach offers a good compromise between optimal BMS control and the psychological benefit provided by allowing occupants some control over their environment. Ventilator panels are used on the ground floor which incorporate security bars and wire mesh allowing secure night-time ventilation and preventing vermin from entering.

72

thermal mass.indd 72

03/01/2007 16:52:27

Appendix A5 – National Trust Headquarters

Figure 41

A high level of thermal mass is provided by the 150mm-thick precast floor slabs, which have a fair-faced finish on the underside allowing the soffit to be fully exposed. White emulsion paint has been used on the concrete to provide a bright finish and to aid daylight penetration. Additional thermal mass is provided by 75mm precast panels incorporated into the roof structure (Figure 41). These have been specifically included to improve cooling performance at high level; a problem identified with offices such as Environmental Building at BRE is that lightweight roof structures can contribute to overheating problems for occupants located on the top floor. A raised floor is used for routing services, and perimeter floor-level ventilation outlets provide heating from a mechanical system during the heating season. Extraction is via grilles located at the top of a central riser, which, in turn, is linked to a heat-recovery system.

Figure 42

Lighting is either surface mounted or suspended from cables secured to the underside of the soffit. Power for the lighting is provided from cabling located in the floor void above, which passes through holes drilled through the slab adjacent to each light fitting (Figure 42). This is a simple and effective approach, but in other developments could lead to right-ofaccess problems where two more tenants occupy different floors within a building. No specific acoustics treatment has been provided in the office areas; however, a novel system of acoustic panels has been installed in the atrium which is used as an area for staff to have lunch or hold informal meetings. The panels double as partitions that can be raised or lowered by a manual winding mechanism to provide small private areas or a larger communal area for staff to gather. Woven tapestry coverings produced by the Trust have been used. The panels are a clever design touch which shows that the specific design challenges posed by buildings of this type can also provide opportunities to try new ideas and combine functional requirements.

73

thermal mass.indd 73

03/01/2007 16:52:30

Appendix A6 – South Cambridgeshire Hall

South Cambridgeshire Hall (2005) Client: South Cambridgeshire District Council Architect: Aukett Structural engineer: Whitby Bird Services engineer: Faber Maunsell Contractor: Alfred McAlpine Case study material: Faber Maunsell and South Cambridgeshire District Council Figure 43 Photos: Faber Maunsell and South Cambridgeshire District Council

South Cambridgeshire Hall uses mixed-mode ventilation to cool both sides of its concrete floor slabs, with supplementary mechanical cooling to air delivered via a displacement ventilation system. The client, South Cambridgeshire District Council, had previously occupied a 1960s naturally ventilated office building. Parts of the building suffered from over-heating during the summer. A property developer was approached to help to identify a new site and to develop a building suitable to re-house the Council. The developer already had a close working relationship with the building-services design consultants and so was able to obtain direct input from them at an early stage. As a result, the building-services consultant was able to consider and propose alternative forms of cooling before the architectural design had been developed. Three alternative approaches were proposed: ■ natural ventilation with night cooling of the thermal mass. ■ mixed-mode ventilation with night cooling of the thermal mass. ■ conventional air-conditioning, e.g. fan coil units. The advantages and disadvantages of each option were explained to the client by the building-services engineers. The final choice was made by council members. The requirement was for a low-energy building that achieved a maximum internal temperature of 25°C. A mixed-mode solution was selected to satisfy these requirements. This was based on natural ventilation with night cooling of the thermal mass during most of the year. A displacement ventilation system provides top-up cooling during peak summer periods to maintain comfort. The displacement system also provides ventilation during winter with heat recovery to further reduce energy requirements. In recognition of the importance to the success of the building, the building-services design consultant was involved in all stages of the subsequent briefing process. The building layout conforms to that commonly used for naturally ventilated, high thermal mass office buildings, i.e. three floors, a central atrium with rectangular office areas either side. The office areas open onto the central atrium. A steel frame supports 150mm concrete floor slabs, constructed using the lattice girder, permanent formwork system, i.e. precast concrete used for the soffit/formwork, with an in-situ concrete topping (see Section 5.2).

74

thermal mass.indd 74

03/01/2007 16:52:35

Appendix A6 – South Cambridgeshire Hall

Figure 44

The building operates in natural ventilation mode during mid-season periods. Air enters through automated high-level windows in the office spaces. This can be supplemented by manually openable windows. Air discharges through automated high-level vents in the atrium. The external façades of the building are extensively glazed with automatic motorised louvres on the south-east-facing walls to control solar gains. Photocells detect when the solar gain is high, causing the louvres to adjust towards a closed position. To satisfy the occupants’ desire for a view out of the window at all times, louvres are limited so that they will not close beyond a 30% open position. An underfloor displacement ventilation system provides summertime cooling to supplement and enhance the cooling available from the thermal mass. An air-handling unit with a chilled water-cooling coil delivers 4l/s/m2 of air at 17–18°C to the raised floor, and then into the space via floor diffusers. The air is discharged naturally via high-level openings in the atrium. At night, cool air is introduced via automatically controlled fanlight windows on each floor. Windows at the top of the atrium also open to encourage a stack effect. Rain and wind sensors ensure that windows around the building will open to take best advantage of the prevailing wind direction, but close if driving rain is detected. All windows with powered actuators are fitted with a mesh screen to keep insects out. During daytime, occupants of the building are free to open low-level windows on each floor if they wish, although these need to be manually shut at night. Low-level windows only open by 100mm for security reasons. If summertime night-air temperatures are too warm to be of use for cooling the slab, the chiller plant and displacement system operate overnight to provide some pre-cooling to the top side of each slab. During the winter, the displacement system is used to supply minimum fresh air volumes to the occupied spaces. Since all of the atrium vents are kept closed during winter, a ducted extract system draws air from the top of the atrium and back to the air-handling unit. Heat is recovered from the extracted air using a thermal wheel and is transferred back to the supply side.

75

thermal mass.indd 75

03/01/2007 16:52:38

Appendix A6 – South Cambridgeshire Hall

The control system therefore operates the system in three modes depending on the outside air temperature: < 10°C 10°C–20°C > 20°C

Heating and heat recovery Natural ventilation Natural ventilation with mechanical cooling from the displacement ventilation.

Thermal modelling was used to determine the combined effects of the displacement ventilation system and night cooling of the thermal mass. The aim was to ensure that internal resultant temperatures would remain within the maximum value (25°C) agreed with the client. Furthermore, computational fluid dynamics (CFD) software was used to analyse the stack and wind effects that drive night-time natural ventilation. To maximise the degree of cooling from the slab, all concrete soffits are painted with white emulsion and fully exposed, with no false ceilings or other features which might disrupt air-flow patterns. To retain some flexibility over the future use of the building, floor-to-ceiling heights were sufficient to permit the installation of false ceilings at a future date. A predominantly glass exterior maximises natural daylight. Automatically operated solar louvres prevent excessive solar gains, reduce glare and also bounce sunlight back into the building. Lights are switched on automatically by sensors to save energy and minimise heat gains. The services designer specified reflectance levels for painted surfaces that ensure adequate lighting levels at work surfaces, which is in accordance with CIBSE Lighting Guide LG3, which requires 30% of the light at work desks to be reflected light from walls and ceilings. This was easily achievable using light fittings suspended from the slab that directed part of their light directly onto the soffit. Electrical cabling for the lights was encased within the continuous linear light fittings. It was not considered desirable to use the floor void above the slab to distribute electrical cables because the developer wished to retain the ability to let the building to multiple tenants at a future date. Therefore the services on each floor had to be self-contained. Acoustic absorption was required to minimise sound reverberation from the hard, flat concrete ceilings. To achieve this, light fittings were fitted with acoustic wings. The building services design consultant’s appointment included post-handover monitoring and fine-tuning duties to enable the system performance to be checked, and improvements made to control and operation where necessary. It was found that the cooling performance of the system was satisfactory. The building has been assessed using the Building Research Establishment Environmental Assessment Method (BREEAM) for new and existing office designs and has been awarded an ‘Excellent’ rating. BREEAM assessment seeks to minimise the adverse effects of new buildings on the environment at global and local scales, whilst promoting healthy indoor

76

thermal mass.indd 76

03/01/2007 16:52:46

Appendix A6 – South Cambridgeshire Hall

conditions for the occupants. The environmental implications of a new building are assessed at the design stage, and compared with good practice by independent assessors. The Environmental Performance Index (EPI) scale provides a comparative measure of a building’s performance between buildings assessed at different stages. It is based on the percentage of core credits achieved, multiplied by the Environmental Weighting Factor. The council’s building achieved the maximum EPI score of 10. Future plans for the building include purchasing electricity from renewable sources; installing photovoltaic cells on the roof; reducing carparking spaces by 10% by 2011 and the installation of a biomass boiler.

77

thermal mass.indd 77

03/01/2007 16:52:47

Appendix A7 – Vodafone Headquarters

Vodafone Headquarters – UK (2004) Client: Vodafone Group plc Architect: Fletcher Priest Architects Structural engineer: Buro Happold Services engineer: Cundall Johnston & Partners Contractor: Bovis Lend Lease Case study material: Cundall Johnston & Partners

Figure 45

Vodafone’s UK headquarters building in Newbury uses mixed-mode ventilation, with supplementary chilled beams for additional cooling to meet high predicted internal heat gains. The decision to opt for a low-energy solution was influenced by the client’s own desire to build an environmentally friendly building, and by local planning considerations. The choice of a green field site outside of Newbury was contentious in planning terms. It was therefore necessary to demonstrate that there would be a significant benefit to developing the site. Vodafone employees had previously been accommodated in a number of ageing office buildings located around Newbury. The energy performance of these buildings was assessed and compared with the predicted energy performance for a new low-energy office development. It was shown that a significant saving would result by relocating the staff into the proposed new buildings. The energy-saving potential was a critical element in persuading the local authority to grant planning permission. It meant that, from the outset, Vodafone were committed to building innovative, low-energy offices rather than conventional air-conditioned units. The potential to use the thermal mass for cooling was identified by the architect before the structural or building-services engineers were appointed. The geometry of the building and its layout were determined by the architect based on the site topology and access provisions, but also with some awareness of the features necessary to maximise the cooling potential from the thermal mass. In this respect, the architect was able to draw on previous experience with other high thermal mass buildings. When the building-services engineers were appointed, an early task was to assess the FES cooling potential and to recommend any necessary changes to the design that might improve cooling performance. It was found that a worthwhile degree of cooling was possible with minimal change to the building layout. The finished buildings comprise 51,600m2 (43,000m2 of office space) and are intended to accommodate around 3,000 employees. There are seven independent buildings, each with two rectangular sections joined along their full length by central atriums. Each floor is open to the central atrium and the office floor plates are approximately 15m wide with a floor-to-ceiling height of 3.5m. Supplementary cooling in the form of chilled beams fed from a chiller, combined with displacement ventilation, enable the buildings to cope with maximum predicted heat gains of around 110W/m2. Pre-cooled fresh air is supplied to the raised floor space at a temperature of 17–18°C to deal with localised heat gains. Extraction is by natural means, the air being exhausted through high-level windows in the atrium.

78

thermal mass.indd 78

03/01/2007 16:52:47

Appendix A7 – Vodafone Headquarters

The performance of the chilled beam/displacement ventilation system was subjected to physical testing by BSRIA. A mock-up of a typical office unit was constructed and artificial heat gains were introduced to simulate the predicted loads that would occur in the real building. Temperatures and air-movement patterns were measured and recorded to demonstrate that, even when no significant cooling was available from the thermal mass, the chilled-beam system would still maintain temperatures within the design limits. Night cooling of the thermal mass is achieved by the automatic opening of high-level windows on each floor. Using the stack effect of the building, cool night air is drawn into the building, travels across the floor space, before exiting at a high level via the atrium. The performance of natural ventilation, and hence likely output from the thermal mass cooling, were estimated using thermal modelling and CFD. This was able to predict the likely night-time air movement patterns through the building, and the effects of varying window-opening areas on each floor. The aim was to achieve a negative pressure on each floor so that air would be drawn in through windows on the lower floors and exit through high-level windows in the atrium. If summertime night cooling by natural ventilation is insufficient to bring down internal temperatures, the displacement ventilation system is enabled to provide additional pre-cooling to the top side of the slabs. Control of the night ventilation strategy is by means of internal and external temperature sensors. To ensure the finished concrete surface was of an acceptable standard, the formwork was arranged in regular panels with a finer-than-normal tolerance on steps and bleeding at joints. Casting lines in the concrete were mainly hidden by carefully aligning them above the chilled beams. The monotony of the exposed slab is also broken up by the chilled beams themselves, the design of which was heavily influenced by the architect. The client requested openable windows so that occupants would gain some feeling of control over their environment and to take advantage of free cooling on cooler days. To deal with the potential input of warm moist air into the building, the control system had to be configured such that the chilled beams could operate without the risk of condensation. The controls were set up so the building would default to operate in the lowest energy operating mode available with the prevailing external conditions. Figure 46

79

thermal mass.indd 79

03/01/2007 16:52:51

Appendix A7 – Vodafone Headquarters

When natural ventilation is possible, the chilled beams are shut off and the windows opened. When external conditions are unsuitable for natural ventilation, the windows are closed and cooling is provided by the chilled beams. When the windows are closed, the chilled beams initially operate at an elevated chilled water temperature until the internal dew point temperature has fallen and condensation is no longer a risk. The chilled water temperature is determined by means of internal dew point temperature sensors. To minimise solar gains, fixed over-hanging external louvres were installed. These cast a permanent shadow across the glazing but do not obstruct the views. The air-tightness of the building was given high priority by the architect. Building interfaces were carefully detailed in order to ensure that air infiltration would be minimised. High-quality glazing was specified with reliable long-life seals. On completion, the building was pressure-tested to ensure that the required air-tightness standard was achieved. To minimise the risk of infiltration when operating in mechanical ventilation mode, the displacement ventilation system maintains the building at a slight positive pressure, with respect to outside conditions. To enable thermal contact between the concrete slab and internal air, there are no false ceilings. Chilled beams with integral ‘direct/indirect‘ luminaires are suspended from the slab. These provide approximately 75% of their output as uplighting and 25% as downlighting. Pipework and electrical cabling for the beams are fed from bulkheads routed along the edge of each floor plate, on either side of the central atrium. Services enter the bulkhead via the services risers within the atrium and are then hidden within the linear chilled-beam units. The finished concrete soffits were painted white to ensure a high reflectance value and thereby maximise the output of the indirect lighting component from the luminaires. Acoustic protection was required within the office areas to prevent excessive sound reverberation, due to the hard, flat concrete soffits. It was considered that the use of sound baffles at high levels would disrupt air-movement patterns around chilled beams and across the slab itself. The chilled beams themselves could not accommodate sufficient acoustic absorption. Therefore, the solution adopted was to provide a whitenoise amplifier. High-level speakers integrated within the chilled beams provide a constant sound-masking tone, which effectively cancels background noise in the office space. Following handover, a period of fine-tuning was required in which the original design team were involved. One aspect of this has involved varying the minimum slab temperature settings (i.e. the slab temperature which signals night-cooling ventilation systems to switch off). Accurate prediction of the appropriate temperature is dependent on actual building usage, therefore some experimentation was necessary post-handover to achieve the optimum value.

80

thermal mass.indd 80

03/01/2007 16:52:56

Appendix A8 – International Building, Royal Holloway

International Building, Royal Holloway, University of London Client: University of London Architects: ECD Architects Services engineer: Fulcrum Consulting Case study material: Fulcrum Consulting and Royal Holloway, University of London

Figure 47 Photos: Fulcrum Consulting & Royal Holloway, University of London

The 3,300m2, three-storey building comprises academic offices, seminar rooms, social areas and cafes. Precast hollowcore slabs are used in a novel way to provide passive cooling. The architect and building-services engineer were appointed together at the earliest briefing stage of the project. Both were companies that marketed themselves as specialists in sustainable buildings and innovative design. These companies were selected because it was the client’s desire to achieve a low-cost, environmentally friendly building. A high thermal mass building was initially suggested by the building-services engineer. The client already had some awareness of passive cooling techniques and was amenable to this proposal. The main motivators for the client were that an FES system could provide sufficient cooling to meet the main requirements of occupants, it was environmentally friendly and was cheaper than any form of mechanical cooling system. A maximum internal temperature of 28°C was agreed by the client in order to make the proposed solution viable. Precast hollowcore concrete slabs, 260mm in depth, were used for the first and second floors. The planks span the width of the building and the ends are exposed on opposite faces. Under favourable conditions, motorised flaps covering the ends of the cores are opened to allow outside air to pass through the slabs, entering and exiting the building on opposite facades, thereby achieving cooling of the slab (Figure 48).

Figure 48

81

thermal mass.indd 81

03/01/2007 16:52:57

Appendix A8 – International Building, Royal Holloway

Figure 49

As with all buildings utilising passive cooling solutions, high levels of insulation were applied and air-infiltration levels were minimised. The building was videoed using a thermal imaging camera to help identify and rectify any areas of cold bridging and excessive heat loss. In particular, the ends of exposed slabs were examined to ensure that they would not permit excessive heat loss (due to cold bridging) during the heating season. In order to prevent this risk, the ends of the slab and covering flaps were insulated to prevent direct contact with outside air. The building was also pressure tested to ensure that the specified air infiltration rates were being achieved. Problems of workmanship associated with the atrium glazing were identified during construction and rectified. The flaps on each side of the hollowcore slabs are controlled in response to external conditions and by temperature sensors in the slab itself. When slab sensors indicate that cooling is required, and external conditions are suitable, the flaps open to permit winddriven air to pass through the cores. The windows around the building can be opened by occupants, avoiding the need for mechanical ventilation. A security guard checks that all windows are closed at night. External shading at roof level helps to limit solar gain. In addition, sliding external louvres can be pulled across the windows by occupants to further reduce solar gains and glare on sunny days. CFD modelling was used to determine how much wind pressure would be required to overcome the resistance through the cores and ensure a worthwhile degree of cooling to the slabs. Thermal modelling predicted a worst-case internal temperature of 28°C during the hottest periods of summertime.

82

thermal mass.indd 82

03/01/2007 16:53:06

Appendix A8 – International Building, Royal Holloway

The building was constructed on a green field site with no obstructions to the wind in immediate proximity. The main façade of the building faces south-west to coincide with the prevailing wind direction. Weather data recorded at Heathrow Airport (relatively close to the site itself) provided an accurate profile of wind speeds and directions against which the design could be assessed. The concrete soffit is finished with a dense plaster that gives a good appearance without significantly compromising cooling performance. Corridors on each side of the occupied spaces have false ceilings enabling services to be distributed through the building. Trunking from the ceiling void in the corridor supply light fittings in each room. Suspended fluorescent lights hang below the trunking and reflect light from the white plaster soffit. Achieving acceptable acoustic conditions has not been a problem since the rooms are relatively small and are fitted with soft carpets and curtains. Power and structured cabling is distributed within a 100mm-deep false floor with floor outlets. Before handover, the building-services designer held induction sessions with the maintenance team to explain the intended operation of the building. The main maintenance challenge was to understand the control and operation of motorised flaps and to ensure that these remained in good working order. Due to the large number of motorised actuators involved, regular checking and replacement of faulty actuators is required. Independent post-handover monitoring of the building by BSRIA showed that the predicted 28°C limit was being achieved.

83

thermal mass.indd 83

03/01/2007 16:53:10

Appendix A9 – Jubilee Library

Jubilee Library, Brighton (2004) Client: Brighton & Hove City Council Developer: Mill Group Architects: Lomax, Cassidy & Edwards, Bennetts Associates Services engineer: Fulcrum Consulting Structural engineer: SKM Anthony Hunts

Figure 50 Photos: Termodeck

The Jubilee Library in Brighton has received numerous accolades for its approach to design and its ‘green’ performance. Winner of the South East England RIBA Award 2005; 2005 PFI Awards – Operational project with the best design; Named BSJ/CIBSE Major Project of the Year. Commended as an ‘exemplary piece of urban development’ by CABE. And on the shortlist for both the Prime Minister’s Better Public Building Award and the RIBA Stirling Prize. The building’s blueprint won a BREEAM ‘Excellent’ rating even before construction work had begun. The building incorporates a combination of solid cast in-situ concrete slabs in the central area, and precast hollow core slabs in the perimeter rooms, heated and cooled by mechanical ventilation; a system typically referred to by the trade name ‘Termodeck’ (Figure 52). Built as a PFI initiative, long-term energy and running costs were important to both the client and developer. The architects appointed had previous experience of designing aesthetically pleasing, functional, high thermal mass concrete buildings that took advantage of FES cooling solutions. From the outset, the form, orientation and structure of the building were considered with the intention of designing out as many engineering services as possible. The building-services engineers were appointed at an early stage to assist in this process. The design involves a four-storey building, with reading rooms, meeting rooms and staff accommodation situated either side of a central, double-height atrium, itself built on two floors. The central space is constructed of a C40 concrete table, supported by a series of eight tree-like columns with fins that support an exposed ceiling soffit (Figure 51). The thermal mass provided by the exposed concrete is a vital part of the building’s passive ventilation system. In summer, fresh air is passed directly through the slab cores of the Termodeck system, which cools the air during the day and is, in turn, cooled at night as a result of the diurnal temperature swing. Having cooled the perimeter rooms, air from the Termodeck system then passes freely in to the central area of the library where it comes into contact with the solid concrete floor slabs. A large free area around the perimeter of the slabs and openings in the centre allows the air to travel upwards to three wind towers on the building’s roof, where it is exhausted. In winter, the thermal mass continues to help provide a stable and comfortable internal temperature, aided by a high-efficiency heatrecovery system that captures heat from lighting, PCs and people, recycling it back into the ventilation supply.

84

thermal mass.indd 84

03/01/2007 16:53:10

Appendix A9 – Jubilee Library

Figure 51

Heating bills during winter months are further reduced by the library’s magnificent south-facing glass facade, with louvres specially angled to allow in winter sun but deflect it in the summer. There is a conventional air-conditioning chiller unit in the library’s plant room to cap the internal temperature at 25°C in summertime. But this is not expected to be required often, as the overall cooling effect achieved by passive means is put at 4–5° lower than the ambient external temperature. Walls in the library are timber-clad, giving good sound absorbency. The main area of the library is filled with shelves of books which themselves provide excellent noise attenuation.

Figure 52

To avoid unnecessary operation of lights and therefore minimise heat gains, lighting is switched on by daylight sensors. Luminaires are generally the spotlight type, which are focused on the main working areas, desks, etc. Uplighters are fitted to concrete columns, shinning across the concrete soffits to create a pleasing aesthetic effect around the splayed ribs. The concrete slabs are painted white to optimise reflectance of the uplighting. It was expected that there would be a significant energy reduction in both summer and winter from the exposed concrete’s thermal mass when compared with a conventional air-conditioned building. There was also a significant saving in the capital cost of the plant, comprising two air-handling units with heat recovery and 50kW gas-heater batteries. This is only 20% of a more typical 600kW boiler installation that would normally be used for a similar-sized building. The M&E component of the build was around 16–17% of Jubilee Library’s total cost, compared to a conventional heating and cooling system of between 20 and 25%.

85

thermal mass.indd 85

03/01/2007 16:53:12

Appendix A9 – Jubilee Library

The estimated total energy consumption of the building is only 35kWh/m2/y. Fine-tuning of the heating and cooling system is still taking place, but heating energy figures of just 29kWh/m2/y have already been recorded in the short period that the library has been open. Summer performance has also been very good; even during hot summer days, the majority of rooms stay below 25°C, the temperature above which the chiller units come into operation.

86

thermal mass.indd 86

03/01/2007 16:53:18

Appendix A10 – Bamford Library – Harper Adams University

Bamford Library – Harper Adams University (2003) Services engineer: Faber Maunsell Structural engineer: Faber Maunsell Case study material: Faber Maunsell

Figure 53 Photos: Faber Maunsell

Harper Adams University’s new learning resource and engineering design centre (Sponsored by JCB) is a landmark building that reflects the standing of one of the top agricultural teaching establishments in the country. Sustainability was a key theme of the brief with a requirement that every element was considered for its environmental, social and economic qualities – all within an extremely tight budget. To meet the exacting brief, the design team adopted a mainly timber structure which reflects the traditional agricultural ‘barn’ architecture, and used locally sourced materials to construct the two-storey building that sits in harmony with its surroundings. Unfinished surfaces were utilised, where appropriate, with the structure exposed throughout. For example, the reinforced concrete flat slab on the first floor is supported on concrete columns and has a high-quality exposed soffit to the ground floor. This method of construction is extremely economical to build as there is no need for expensive downstand beam shuttering and the majority of the reinforcement can be laid as a simple mesh. The second-floor timber frame rises from 3m height at the eaves to 8m height at the apex and was designed using traditional timber principles and then constructed using mortice and tenon joints. It provides a pleasingly light, airy space. A stressed skin timber roof, constructed from a double layer of plywood separated by purlins 300mm deep, is supported on timber Y columns at 6m centres. The roof itself is clad in traditional terracotta tiles.

Figure 54

The internal environment is the key to the building’s sustainable design. Typically, the high IT loads in a learning resource centre lead to a mechanically ventilated building. For Harper Adams, the design team decided to challenge this because mechanical ventilation is expensive, takes valuable space and is less sustainable than natural ventilation which provides the same comfort conditions. Using the latest modelling techniques, the Faber Maunsell design team succeeded in making a totally naturally ventilated building. The exposed concrete soffit of the ground floor provides thermal mass to assist the cooling while the high-pitched ceilings on the first floor enable the air to stratify and use stack effects to expel the warm air. On extremely hot days there is a small fan in each extract duct powered by a roof-mounted photovoltaic panel. Continuing the sustainable brief, a rainwater-harvesting scheme provides water to toilets and urinals. The whole building was delivered for a cost of approximately £980/m2, which is significantly lower than the average of such buildings, which is nearer £1,750/m2. The result is a landmark building that embodies a modern feel with a high-quality internal environment while still reflecting the university’s rural heritage. A final touch to the process was that the library desk was built by the university students using timber sourced from the university’s renewable woodlands.

87

thermal mass.indd 87

03/01/2007 16:53:18

Appendix A11 – Design Studios for Gifford Engineers

Figure 55

Design Studios for Gifford Engineers, Southampton, Hampshire (2003) Client: Gifford Architect: Design Engine Structural engineer: Gifford & Partners Services engineer: Gifford & Partners Contractor: Leonard Field Case study material: Gifford & Partners

Photos: Gifford & Partners

The 1,600m2 two-storey design studios for Gifford and Partners uses mixed-mode ventilation to cool both sides of its concrete floor slabs, supplemented by a chilled water underfloor cooling. The client for the building were consulting engineers, Gifford and Partners, who had their own in-house building-services design expertise and were therefore able to contribute fully to the briefing and carry out the design of the thermal mass solution themselves. Gifford place a strong emphasis on the environmental design of projects – an approach that they call ‘Building Sciences’ – and they wanted a building that would be representative of the type of work they specialise in. They therefore appointed an architect to undertake the office design, but very much within a collaborative design process. This was a case of the building-services engineer being involved from the very outset and with significant control over the design decision-making process. It was recognised that the level of thermal insulation and quality of fenestration were critical to the success of any low-energy cooling solution. Whereas the architect might normally compromise insulation and external glazing levels in favour of aesthetic appeal, the client brief set very clear limits on these issues which the architect had to work within. As a result, the walls, floor and roof are super-insulated to minimise heat gains and losses. The walls are constructed with an external cedar rain screen cladding and a sandwich of Panelvent panels on the outside, Stirling board on the inside and a 300mm-deep filling of Warmcell achieving a U-value of 0.1W/m2°C. The building’s steel frame is encased entirely within the walls. Window area was also minimised to limit heat gains and losses. Long, thin ‘pillar box’ style windows are situated at seated eye level on the ground floor to provide natural light at occupants’ desks and visual interest to the open-plan office. The largest area of glazing on the ground floor is on the south-side of the building where the window extends the full height of the wall. However, this is protected from direct sunlight by a large overhang which puts the window in permanent shade. The outdoor space covered by the overhang is used as a seating area. The windows are high-quality, argon-filled, low-emissivity, double-glazed units giving U-values of around 1.3W/m2K. Most of the ground floor is made up from an in-situ concrete slab, whilst the first-floor slab comprises a hybrid slab, i.e. precast concrete planks with a post-tensioned in-situ topping. The post-tensioning cables were fed through ducts in the wet concrete and then pulled tight so that the concrete sets in compression. As a result, 9.6m spacing of columns was possible.

88

thermal mass.indd 88

03/01/2007 16:53:21

Appendix A11 – Design Studios for Gifford Engineers

Figure 56

Where steel columns meet the underside of the concrete slab, a conical shear head (‘witch’s hat’) feature in the steel helps to distribute the weight of the concrete and avoids the need for downstand beam sections. The top floor has a saw-tooth roof with north-facing windows on the vertical faces. These windows are motorised and are therefore openable by the occupants using wall-mounted switches. When open, the windows provide natural ventilation of the building. There is a raised floor on the ground-floor and first-floor levels incorporating a displacement ventilation system supplied by fresh air from outside. Low-pressure fans draw air in through external walls and into the floor, which in turn supplies floor-mounted swirl diffusers. The total power consumed by the fans for the entire building is only 200W. The concrete floor and exposed concrete soffit allow night cooling of the thermal mass. Cooling is achieved by operating the underfloor ventilation system overnight. The aim is to purge the building down to 18°C. If the night air is too warm to provide useful cooling, the fans are switched off. During daytime, the coolth stored in the floor slab tempers the incoming fresh air. In winter the fans are speed controlled by an air-quality (carbon dioxide) sensor which helps to deliver the necessary amount of air with minimal heating load. The underfloor heating/cooling system means that there is no need for a central air-handling unit. The raised floor also houses a piped cooling system to supplement the passive effects of the thermal mass; brackets fixed to the top of the floor pedestals support pre-formed insulation channels carrying plastic pipe circuits fed from zone manifolds. Floor tiles rest on the pedestals and are in direct contact with the pipes. In summer, water is circulated at 15°C through the underfloor system delivering 30–40W/m2 of cooling. The same system is supplied with warm water at 50°C in winter to provide heating. The system is served by an air-to-water heat pump which can operate in a heating or cooling mode depending on the requirements of the space. A ground-source heat pump was considered, and would have been more efficient. However, since the requirement for heating/cooling is so small, the pay-back period would be too long. While the majority of the floor space forms open-plan offices, a small area is given over to cellular offices that can also be used as meeting rooms. Each of these has its own radiator and openable window to allow occupants control over the environment. A separate boiler serves radiators located in the cellular offices and in the main reception area. Although the ground floor has a flat exposed concrete soffit, no provision has been made for acoustic treatment. The area works effectively in acoustic terms due to the soft furnishings within the space, including half-height partitions between work spaces and soft carpeting. One exception to this is a ground-floor meeting room, which has acoustic panels on one of the walls to help to deaden reflected noise. Linear light fittings are supported from the ceiling, providing uplighting against the concrete soffit. The surfaces are painted white to maximise the degree of reflected light and compensate for the minimal window area.

89

thermal mass.indd 89

03/01/2007 16:53:23

thermal mass.indd 90

03/01/2007 16:53:25

thermal mass.indd 91

03/01/2007 16:53:25

thermal mass.indd 92

03/01/2007 16:53:25