Thermal Mass in Lightweight Domestic Construction by Fusion Building Systems

Research Summary

Thermal mass in lightweight domestic construction

Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact of accepted practice or matters of opinion at the time of publication, TRADA Technology, The Steel Construction Institute, the authors and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use. ÂŠ TRADA Technology Ltd 2013 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form, by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owners. ISBN: 978-1-909594-02-9

Research Summary Thermal mass in lightweight domestic construction

Acknowledgement TRADA Technology Ltd would like to thank the Steel Construction Institute (SCI) for providing the following report. Prepared by Roland Chuter, the report is a summary update of original research work undertaken by Oxford Brookes University and sponsored by Tata Steel, comparing lightweight framed construction with traditional construction in terms of delivering cost-effective housing without a significant risk of overheating.

1 Background High levels of thermal insulation and airtight fabric in modern residential building has raised concerns that recently built dwellings will have a tendency to overheat during periods of hot weather.

ARUP R&D and Reading University) to bring it in line with the UK scenario for climate change in 2050, as outlined in the report Climate change scenarios for the UK (Hulme et al).

4 The building A typical three-bedroom semi-detached/end of terrace house was chosen as a representative example. Figure 1 shows the ground floor and first floor layout of this two-storey building. In choosing the construction technologies and specific materials for comparison it was important to have representative systems. Some studies purporting to show a reduced overheating risk for traditional construction have used unrealistically heavyweight construction materials.

There is further concern that global warming may push up temperatures in the UK over the next 50 years, within the lifetime of the dwellings built today, and that this could exacerbate potential overheating. Finally, there is the impression in some areas of the construction industry that modern methods of construction, including lightweight framing, may perform poorly with respect to their overheating risk when compared with traditional methods of construction. This report summarises research undertaken by Oxford Brookes University demonstrating that the risk of overheating in a house built with lightweight framing is comparable to that of a house built to current standards with brick and lightweight block.

2 Overview of methods To investigate the risk of overheating for different construction types under different climate assumptions, a dynamic thermal simulation model of a typical three-bedroom house was developed using Tas. Tas is a dynamic thermal model from Environmental Design Solutions Limited which allows modelling of the internal temperatures of a building as well as its heating and cooling demands.

3 Climate For the base case model, the CIBSE (Chartered Institute for Building Services Engineers) test reference year (TRY) for London was used. This gives typical weather data to allow comparisons between the performances of different buildings. To account for potential rising temperatures in the UK due to global warming, a second weather set was used. The TRY data in this weather set was adjusted (using a method developed by

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Figure 1 Plan of the ground floor and first floor of the house

Typical brick and block construction in the UK tends to consist of lightweight concrete blocks for internal walls and brick for external walls. These lightweight concrete blocks are typically aerated and have a density of around one-quarter that of

Research Summary Thermal mass in lightweight domestic construction

heavyweight concrete blocks with a thermal conductivity of around one-tenth. Party walls are typically mediumweight concrete block with upper floors and partitions being lightweight timber. The internal finish is usually plasterboard on dabs or battens. Four construction systems were considered in this analysis: lightweight, mediumweight, heavyweight and very heavyweight. The first is considered to represent modern, lightweight, framed construction, the next two represent typical forms of brick and block construction and the fourth is included for completeness but is considered to be unrealistically heavyweight. (This is

because the blocks used are only necessary for high-rise construction and on normal developments would lead to increased health and safety risks due to the weight of the blocks as well as increased embodied energy and carbon.)

5 Construction systems Table 1 and Table 2 show the structural features and other assumptions for the four construction systems considered. Each external wall construction was adjusted to the same U-value of 0.28Wm-2K-1.

Table 1: Structural elements for each construction type 1. Lightweight framed (LW)

2. Mediumweight masonry (MW)

3. Heavyweight masonry (HW)

4. Very heavyweight masonry (VHW)

External wall

Single (12.5mm) plasterboard / insulated frame/ cavity / brick

Single plasterboard (dabs) / lightweight block / insulated cavity / brick

External walls: plaster / mediumweight block / insulated cavity / brick

Plaster / dense concrete block / insulated cavity / brick

Party wall

Double plasterboard (25mm) / insulation in frame cavities / double plasterboard

Double plasterboard / block / insulation / block / double plasterboard

Plaster / block / insulation / block / plaster

Plaster / block / insulation / block / double plaster

Partitions

Dry-lined stud partitions

Plastered dense concrete block partitions

Floors

Solid ground floor; framed upper floor

Solid ground floor (tiled, uncarpeted); concrete upper floor (tiled, uncarpeted)

Roof

Ventilated roof with tiles

Table 2: Other assumptions Assumptions Windows

Double glazed low emissivity glass with timber frames (U = 1.8Wm-2K-1) , approximately 18% on the south façade

Airtightness

A value for air infiltration of 0.35 air changes per hour was used, conforming in this case to an air permeability of 7.5m3h-1m-2

Internal gains

The house is occupied by two adults and two children (out during weekdays) who are assumed to use lighting and appliances in the living room and kitchen during scheduled occupancy

Heating

Gas condensing boiler with radiator system and thermostatic radiator valves (22oC for living areas, 18oC for bedrooms), heating is on a timed schedule (off at nights)

Occupation

Living room: 17:00–23:00 weekdays, 07:00–23:00 weekends Bedrooms: 23:00–07:00 weekdays and weekends

Ventilation

Weekdays: living room windows start to open after 17:00 when temperature exceeds 24oC, and are fully open (30% open area) when the temperature reaches 26oC. Closed at 23:00. Bedroom windows start to open after 17:00 when the temperature exceeds 18oC and are fully open (30% open area) when temperature reaches 20oC, remaining open all night until 07:00 Weekends: living room windows open 07:00–23:00 (temperature control as above). Bedroom windows open 24 hours (temperature control as above)

Research Summary Thermal mass in lightweight domestic construction

6 Results Heating demand Figure 2 shows annual heating loads for current weather and for predicted 2050 weather. It is clear that the warming assumed for 2050 leads to a significant reduction in heating demand. Across construction types there is very little difference, with heavyweight construction tending to have slightly higher heat demand. This is because thermally lightweight construction responds more quickly to the intermittent heating schedule (morning and afternoon), leading to lower heat demand. In other words, when the heating switches on it initially needs to supply more energy to the heavier-weight cases to bring the thermally heavy construction elements up to temperature than it does in the lightweight case.

Figure 3 Percentage of occupied hours in bedroom 1 spent over a range of temperatures across different construction types

Figure 4 Percentage of occupied hours in bedroom 1 spent over a range of temperatures across different construction types for 2050 climate Figure 2 Heating demand for the whole house across different construction types

Overheating There are many definitions of overheating. The approach taken by this study was to consider temperatures during occupied hours, which were assumed to be 23:00–07:00 daily for bedrooms and 17:00–23:00 weekdays, 07:00–23:00 weekends for living rooms. One way to consider overheating is to look at a frequency plot of exceeded temperature for a certain room. Figure 3 below shows the percentage of occupied hours for which certain temperatures are exceeded for the master bedroom (bedroom 1). Findings indicate that there is very little difference in overheating risk in this case between lightweight, mediumweight or heavyweight construction solutions. This pattern is maintained when considering the predicted 2050 climate, albeit with a slightly higher temperature distribution across the board (Figure 4).

The risk of overheating in the bedroom is identical for lightweight, mediumweight and heavyweight constructions because although the lightweight bedroom experiences slightly higher peak temperatures during the day, it cools down far more quickly in the evening. This is demonstrated by Figure 5, which shows the lightweight and mediumweight buildings reaching a temperature that is slightly higher than the heavyweight case but cooling off quickly in the evening. The overnight temperatures are very similar, with the lightweight case resulting in the coolest temperature at the end of the night. The ability of the lightweight construction to lose heat at night over these hot periods is so effective that, when considered across the whole year, overheating in the lightweight bedroom is lower than the heavyweight constructions. Figure 6 also demonstrates that the risk of overheating in the living room (fraction of occupied hours above 27°C) is very low (less than 1%) for all forms of construction. In the lightweight case this is primarily because lightweight construction responds very well to window opening, allowing cooling.

Research Summary Thermal mass in lightweight domestic construction

Importance of ventilation and shading The results of this study show that thermal mass in domestic buildings has a negligible effect on overheating during occupied hours. However it has also highlighted that if the climate warms in the future, the risk of overheating may well increase. For this reason it is sensible to consider strategies that will mitigate this risk and hopefully avoid the need for air conditioning.

Figure 5 Temperature profiles over two summer days for bedroom 1 across construction types

One of the difficulties of naturally cooling a domestic dwelling is in providing secure ventilation. When residents are in the building this can be achieved through window opening but it is unlikely that windows can be left open while the building is unoccupied. Also, this strategy relies on natural ventilation, which can only achieve so many air changes per hour. A possible alternative is to include some mechanical ventilation which, with low fan power, will be substantially less energy intensive than air conditioning but can make an impact on high temperatures. Another strategy is to provide solar shading. For example this could be in the form of overhangs or brise soleil, which will cut down heat gains from the sun. This has the benefit of requiring no operational energy and prevents heat from entering the building, rather than trying to remove it later.

Figure 6 Overheating risk for bedroom 1 and living room across different construction types for current weather

This finding is demonstrated in Figure 7 which shows a very small increase in percentage of occupied hours between 22–25°C (lightweight and mediumweight compared to heavyweight), which all but disappears when considering temperatures above 25°C.

Figure 7 Percentage of occupied hours spent over certain temperatures across different construction types for the living room with 1990s weather

To investigate these strategies, increased ventilation and solar shading were applied to the domestic dwelling used in this study (Figure 8). Figure 8 shows that both strategies lead to a significant reduction in internal temperatures. It is interesting to note that solar shading tends to reduce the temperatures during the day (as it reduces the solar gain during daytime), whereas increased ventilation gives a more marked reduction during the night when cooler ambient air is available.

Figure 8 Temperature profile of bedroom 1 over a two-day period for various shading and ventilation strategies

Research Summary Thermal mass in lightweight domestic construction

This suggests that a good strategy may be solar shading for the living room, which is more likely to be occupied during daytime, and increased ventilation in the bedrooms for nighttime comfort.

7 Summary Lightweight framed construction can deliver cost-effective houses without a significant increased risk of overheating compared to traditional construction. However, future climate changes and increased levels of thermal insulation and airtightness mean that cooling strategies should be considered as part of the design to avoid the need for mechanical cooling. These strategies could include: •

•

Secure ventilation – effective, controllable and secure ventilation could be provided through secure openings, stack ventilation systems or even mechanical ventilation. This is of particular importance for night-time comfort. Solar shading – this could include overhangs or brise soleil and should be external to the building, enabling the reduction of solar gain. Solar control glazing – this can also be used to reduce solar gains through the windows by including a reflective coating on the external side of the glass to reduce solar gains. Energy efficient equipment – low energy appliances and lighting can cut down on internal heat gains, thus mitigating overheating risk.

8 TRADA Technology comments Global temperatures are predicted to rise over the next few decades as climate change increasingly asserts its influence. This is therefore both an interesting and a timely study which provides a direct comparison of resistance to overheating from different construction systems over a range of thermal mass. As climate change progresses over the next few decades it will remain critical that the dwellings produced have both a low carbon impact and remain comfortable in what is likely to become an increasingly severe climate. The findings of this study are therefore reassuring in that lightweight frame construction has the capacity to meet both these needs in a cost-effective manner.

References Barnard, N. et al., Modelling the performance of thermal mass, BRE Information Paper IP6/01, Building Research Establishment, 2001 BS EN 15265:2007 Energy performance of buildings: Calculation of energy needs for space heating and cooling using dynamic methods – General criteria and validation, BSI Guide A. Environmental design, 7th edition, ISBN 1903287669, CIBSE, 2006 Guide C: Reference data, ISBN 9781903287804, CIBSE, 2007 Davies, M. G., The thermal admittance of layered walls, Building Science, Vol 8(3), pp 207–220, 1973 EN ISO 6946:2007 Building components and building elements – Thermal resistance and thermal transmittance – Calculation method, European Committee for Standardisation EN ISO 13786:2007 Thermal performance of building components – Dynamic thermal characteristics – Calculation methods, European Committee for Standardisation EN ISO 13790:2008 Thermal performance of buildings – Calculation of energy use for space heating and cooling, European Committee for Standardisation Hulme, L. & Turnpenny, J. et al., Climate change scenarios for the United Kingdom, The UKCIP02 scientific report, Tyndall Centre, School of Environmental Sciences, 2002 England and Wales Building Regulations: Approved Document L1A: Conservation of fuel and power (New dwellings), NBS, 2010, available at www.planningportal.gov.uk SAP 2009: The Government’s Standard Assessment. Procedure for energy rating of dwellings, BRE Xu, P. & Li, Y., Thermal mass design in buildings – heavy or light? International Journal of Ventilation, Volume 5(1), 2006

Further reading Hislop, P. with O’Leary, P., External solar shading with wood: a design guide for architects, ISBN 978-1-900510-86-8, TRADA Technology Ltd, 2012 Pitts, G. and Lancashire, R., Low energy timber frame buildings: designing for high performance, 2nd edition, ISBN 978-1900510806, TRADA Technology Ltd, 2011

Research Summary Thermal mass in lightweight domestic construction

About the authors Roland Chuter Since completing his Masters in Renewable Energy and the Environment in 2006, Roland has been working in the field of buildings and energy, developing his knowledge base and expertise in sustainable buildings, energy efficiency, building physics and building integrated renewable energy, most recently in the Sustainability Division at SCI.

Christopher Kendrick Christopher Kendrick studied Mechanical Engineering at the University of Salford, and gained an MSc in Energy and the Environment at Cranfield University in 1995. Following a period working in industry, he joined Oxford Brookes University in 1996 and has helped develop the Building Physics research and consultancy portfolio within the School of Architecture. He has written many conference papers and journal articles relating to the thermal performance of buildings.

TRADA Technology Ltd is contracted by the Timber Research and Development Association to prepare and publish all of its publications.

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