Overheating has become an increasingly important issue for homes. Dr Anastasia Mylona from CIBSE explains the issue further and outlines some potential solutions
It feels odd to be talking about the impact of climate change after the UK has just experienced the coolest summer since 2015. In one sense, 2024’s summer was noticeable precisely because it bucked the trend. But it was not cooler everywhere on the planet, this year Central and Southern Europe and North America all experienced heatwaves.
In future, there may be UK summers that are cooler too, but not many; the long-term global trend is of rising temperatures and a warming climate. It was, after all, only two years ago that temperatures broke the 40°C mark for the first time in the UK as a series of heatwaves hit the nation and the Met Office issued its first-ever red warning for extreme heat. The prediction is that parts of the UK should expect temperatures over 45°C by 2050.
Increased external temperatures will result in higher internal temperatures in existing homes and flats, which could result in many of them overheating. This will impact the health of occupants; those expected to suffer most are the elderly and those with pre-existing health problems, which should be of concern to a nation with an ageing population.
According to the Environment Audit Committee’s (EAC) report 2024 ‘Heat resilience and sustainable cooling’, UK heat-related deaths “are believed to have exceeded 4,500 in 2022”. The committee says this figure “could rise to 10,000 annually without concerted action to adapt to the warming climate.”
Overheating does not only affect the elderly, it has broader wellbeing effects on the population at large, including through sleep deprivation. The EAC says economic costs from overheating are likely to “run into the billions.”
Overheating is the accumulation of warmth within a building to the extent where it causes discomfort to the occupants. Until Part O of the Building Regulations came into effect in 2022, there was no requirement for the explicit assessment of overheating to be considered in a home’s design. Consequently, there are thousands of homes built relatively recently with increased levels of insulation and airtightness to keep them warm in winter (in compliance with Part L), but which may be at increased risk of overheating in summer.
Alongside increased external temperatures, factors that can contribute to overheating include: solar gains from unshaded windows, heat from people and equipment and, in communal apartments, heat from uninsulated domestic hot water pipework in communal areas. There is also the increasing problem of the urban heat island effect.
The type of home most likely to be affected by overheating are apartment buildings in city centres with windows on only one elevation which faces south, east or west, particularly if there is no opportunity for cross ventilation.
Occupant behaviour can also make overheating worse. In the UK people often leave windows open to increase ventilation throughout the day, even when the outdoor temperature is higher than indoors whereas in the Mediterranean occupants are used to closing windows (and shutters) to keep heat out at the warmest time of the day.
As the climate warms, overheating issues are set to increase. Research has shown that by 2050, passive measures such as thermal mass and night-time ventilation may not be effective in providing sufficient thermal comfort in homes and active cooling systems, such as air conditioning, may be necessary in some locations in the UK.
Currently less than 3% of dwellings have air conditioning, but this figure is expected to grow significantly, particularly as the number of people using their home as their main place of work is increasing. The International Energy Agency says that record-breaking temperatures feed the demand for air conditioning which in turn results in surges in demand for electricity – risking a vicious cycle of increased greenhouse gas emissions that in turn make the world even hotter. The cost of running air conditioning is also expected to lead to a doubling of electricity bills for homeowners.
The definition of overheating in Approved Document O of the Building Regulations is based on the Chartered Institution of Building Services Engineers (CIBSE) document TM59. The Approved Document outlines two methods for achieving the performance targets: a simplified method and the more thorough dynamic thermal modelling method embodied in CIBSE TM59.
The Building Regulations apply to new-build homes, but four out of five homes that will exist in 2050 are already built, meaning that the scale of retrofitting required for existing homes to protect them from overheating is vast.
Overheating is exacerbated by excess solar radiation. Modelling shows that external shading is highly effective protection against solar gains because it stops sunlight reaching a window. Overhangs, for example, can be incorporated as part of the building design to provide shade to windows including extended roof eaves, balconies, upper floor external walkways, deep window reveals and the addition of brise soleil are all highly effective at blocking out high-angle summer sun.
External roller blinds and shutters are another option more often seen in continental Europe; shutters can be opened and blinds retracted when shading is not required to maintain views out. CIBSE Technical Memorandum ‘TM37: Design for improved solar shading control,’ outlines options designers have in addressing solar gains. Solar control glazing can also be used to reduce solar gains, although this has the disadvantage that it will also reduce heat gains in winter when they can contribute to meeting space heating demand.
Internal blinds are another option, although less effective. CIBSE’s ‘Technical Memorandum Dynamic thermal modelling of basic blinds (TM69: 2022)’ recognises the role of internal shading in reducing indoor air temperatures along with the impact blinds have on internal daylight levels.
Increased ventilation is another mitigation option for overheating. Opening windows is a cheap and effective way to expel heat from a dwelling, but only when outside temperatures are cooler than those inside, such as early morning. The option to open windows, however, is less than ideal for dwellings in noisy or polluted locations or where safety or security considerations prevent windows being opened wide. Of course, as the climate warms and outside temperatures rise, the effectiveness of natural ventilation in preventing overheating will diminish.
As summer temperatures in the UK continue their inexorable rise, tackling overheating in existing homes will become increasingly important. The EAC, called for Part O to be expanded to cover refurbishments of existing properties as well as material changes of use to residential properties. The committee is also calling for “a comprehensive national heat resilience strategy.”
CIBSE is one of the leading sources of guidance on dealing with overheating. It is ready to work with the Government to make the built environment more climate resilient. For more information
on CIBSE go to www.cibse.org
Dr Anastasia Mylona is technical director at CIBSE