6 THE CLIMATE CHALLENGE A hot topic
Dr Anastasia Mylona of The Chartered Institution of Building Services Engineers (CIBSE) gives the lowdown on the major challenge for housebuilders that is overheating, explaining why it is likely to be incorporated into the Building Regulations.
Institution of Building Services Engineers (CIBSE) document TM59. New homes need to meet two criteria: • Criterion A applies to living rooms, kitchens and bedrooms. It requires that the internal temperature does not exceed a defined comfort temperature by 1℃ for more than 3 per cent of occupied hours over the summer • Criterion B applies to bedrooms between the hours of 10 pm to 7 am, and requires that the internal tempera- ture does not exceed 26℃ for more than 1 per cent of the time.
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pells of very hot weather in recent summers have highlighted the fact that too many homes –
particularly newly built homes – become uncomfortably hot. Building Regulations are moving
towards achieving low or zero carbon designs for homes, and are likely to focus on reducing the risk of overheating as well as increasing energy efficiency. To do this, there needs to be a greater understanding of what constitutes overheating, how it can be addressed, and the costs and benefits of doing so. In September, the Government
published the results of its research into overheating in new homes. The project involved modelling eight dwelling types in five locations across the UK. The findings are damning, saying that “in each case the dwelling has failed the TM59 overheating compliance criteria.” The research was undertaken in
response to recommendations from the Committee on Climate Change, which called for a better understanding of the overheating risk in new dwellings in England and for options to help the housebuilding industry mitigate this risk. The definition of overheating used for the project was from the Chartered
The summer temperatures used in the modelling were taken from CIBSE Weather Files (2016 release) which represent anticipated climate conditions 2010-2039. The risk of overheating was predicted using Dynamic Thermal Simulation modelling. A mix of dwelling types were modelled
including houses and flats with single or dual aspects, and with different forms of construction and different ventilation strategies. The modelled homes were located in Inner London, Outer London, Southampton, Nottingham and Leeds and all complied with Part L of the current Building Regulations (2013). The modelling showed that all
dwelling types in all five locations failed to comply with CIBSE TM59 overheating criteria. The finding of most concern was that flats in London exceeded the allow- able number of overheating hours by up to 600 per cent. Worse still, the modelling assumed
occupants were able to open their windows. In inner city locations, a combination of concerns over security, noise and air pollution concerns means that opening windows may not be an option. For this type of property the document "expects" extensive mitigation to be needed. By contrast, houses in the midlands
and north of England were shown to exceed allowable overheating hours by around 10 per cent. The research
suggests that these homes would require “relatively simple and low-cost measures” to reduce overheating risks. Phase 2 of the research consisted of a
cost-benefit analysis of strategies for these dwellings to reduce overheating risk to an acceptable level. Only three dwelling typologies were
evaluated in this phase: a single aspect apartment, a dual aspect apartment, and a semi-detached house. The dwellings were evaluated for three locations: Nottingham, London Heathrow, and Southampton.
Five risk mitigation packages were considered to limit the risk of overheating: • Internal blinds
• Lower g-value glazing or reduced glazing ratios in flats
• Fixed external shading with internal opening windows/reduced glazing ratios in flats/additional openable windows on third aspect for semi-detached homes
• As above, but with masonry construction • As above, but with a reversible heat pump to provide cooling
The impact on internal temperatures was modelled once again using CIBSE TM59 for each case. The mitigation package used for cost-benefit analysis was the one which met the CIBSE TM59 overheating criteria at minimum capital cost. To understand the potential benefit of
implementing these risk mitigation measures, the document then goes on to consider ways of calculating the cost of overheating in terms of lost productivity, impact on human health, and increased mortality. It concludes that there is a net benefit
of incorporating measures to mitigate the risk of overheating in new homes in the south of England over their life. It says there is a net benefit of incorporat- ing risk mitigation measures in flats in the north of England, but not in low-rise housing. The Government is considering
legislation as a potential route to bring- ing about change in current design and construction practices to reduce overheating risk in new build homes. The Future Homes Standard Consultation Document, published in October, says the Government has committed to consult on a method for reducing overheating risk in new homes and on proposals to reduce the risk. The consultation was due to be published “in early 2020" – you have been warned.
Dr Anastasia Mylona is head of research at The Chartered Institution of Building Services Engineers (CIBSE)
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