Assessing the potential for overheating in homes
Why does overheating in homes happen? And how can the construction and energy efficiency sector help to prevent it becoming more commonplace in the coming decades as the climate changes?
These may seem like odd questions to ask on a cold, January day, but the housebuilding industry was invited to answer such questions in a survey carried out by the Zero Carbon Hub and Sustainable Homes in 2014..
In general terms, by overheating, we mean the phenomenon of excessive or prolonged high temperatures in homes, resulting from internal or external heat gains, which may have adverse effects on the comfort, health or productivity of the occupants.
Exposure to excess heat in homes, especially over prolonged periods during warmer months, can have consequences for the health of the people living there. Fortunately, similar to the problem of cold homes, simple changes to the design of dwellings, certain measures, and advice for occupants can help to prevent the problem occurring. Certain solutions can even create a win-win by simultaneously making homes more energy efficient and helping to keep them cool. For example, low energy lighting reduces energy use and should also lower the level of internal heat gains.
One important weapon in our arsenal to prevent a home overheating, is to carry out an assessment of the potential for overheating to occur before anyone moves in. The logic being that such assessments can flag up risky design choices. We then have the chance to put things right early on if needed.
We already know that, at a national level the majority of homes do not overheat, but we need to catch the ones that are very likely to. Research suggests that up to 20% of homes in England may already be overheating, Risk assessments can help us decide where to focus solutions.
Focusing on prevention of overheating is not only expected to be less costly in the long run, but more effective as an approach. Designers have the unenviable job of balancing lots of different requirements and requests. Perhaps, for example, meeting consumer demand for large, south-facing windows whilst also limiting high solar gain. It is often easier to find complementary solutions before plans are firmed up.
There are a number of methodologies building designers currently use to assess the potential for dwellings to overheat.
The Chartered Institution for Building Services Engineer’s (CIBSE) Guide A – Environmental Design (2015) is a good example. The guide advises that bedrooms and living rooms within a dwelling should stay within certain temperatures for specified periods of time. In the 2015 edition of the guidance, these comfort thresholds are allowed to vary depending on recent outdoor temperatures and the ability of the occupants to adapt their surroundings to stay cool - the “Adaptive Comfort Model”. An exception is made for bedrooms where an absolute threshold temperature of 26°C remains.
Another basic “overheating check” used for new homes is contained in the Government’s Standard Assessment Procedure – “Appendix P”. It requires energy assessors, when carrying out the SAP assessment for a property, to calculate the propensity of the building to overheat in June, July and August. If the average internal temperature (over day and night) is calculated to be above 23.5°C, it is determined to have a high risk of overheating. However, building designers use SAP Appendix P with some caution as it is not intended to inform design decisions.
In our Overheating Survey we asked Housing Providers whether they are making use of such methods. 59% of the 74 Housing Providers who answered this question reported having a form of assessment process in place intended to identify properties at risk of overheating. 36% did not, and the remainder did not know.
Interestingly, a number of Housing Providers later interviewed by the Zero Carbon Hub described how their technical teams are using their experience and knowledge of overheating risk factors to identify sub-sets of properties or designs which have characteristics making them more prone to overheating before carrying out a formal modelling exercise. A form of ‘first pass’.
These ‘higher risk’ properties were then subjected to detailed checks and, if found to fall short of the chosen overheating criteria, measures would be installed or design modifications made to reduce the potential for overheating. Having good risk assessment processes therefore allowed them to narrow down mitigation efforts to the properties that needed it most.
Does your organisation have responsibility for shaping the performance of new homes? Is the assessment of the risk of overheating something which is routinely carried out?
To see the Zero Carbon Hub’s Big Picture report go to http://www.zerocarbonhub.org/recent-publications