HEAT RECOVERY How do we address overheating?
Andrew Nash, Nuaire divisional manager investigates the issue of overheating in homes, how big the problem is, and what we can do to mitigate the effects
T
he years 2024, 2023, 2022 and 2014 are the UK’s warmest years on record, according to the Met Office’s analysis1. In fact, the 10 warmest years in the UK since records began in 1884 have all
occurred since 2000. The chances of a summer matching the 2022 heat wave in this country could, according to the Met Office, be greater than 50% by 2050.
How does overheating impact people?
Hot temperatures are not just uncomfortable, but have serious health implications, especially for older people and those with existing health conditions. High temperatures raise blood pressure and heart rate, impacting the circulatory, nervous, respiratory and renal systems, which can lead to dehydration, heat exhaustion and heatstroke. In extreme cases it can lead to death: the UK Health Security Agency estimates that there were up to 3,712 deaths attributed to heat in summer 2022, the highest number in any given year2. Mental health is also impacted by hot weather as it can exacerbate the symptoms of psychiatric illnesses. Even for those people whose health is not
directly affected, hot temperatures often prevent us from getting the sleep we need and therefore have consequences for productivity. The Office for National Statistics estimates that productivity in Great Britain was reduced by an incredible £5.3 billion in 2020 due to heat3.
Are our homes overheating?
Under the Chartered Institution of Building Services Engineers’ (CIBSE) ‘TM59 Design methodology for the assessment of overheating
risk in homes’, overheating is defined as when the internal temperature threshold of 26°C is surpassed for over 3% of the annual occupied hours, for predominantly naturally ventilated dwellings. Using the TM59 standard, a 2019 government study on overheating in homes4 showed that out of eight modelled house and flat typologies, not a single one met the acceptable risk criteria for overheating. Apartments are particularly susceptible to
overheating, with living rooms and bedrooms being the rooms most affected. Not everyone is impacted by overheating equally. Low income households and those living in social housing bear the brunt of overheating.
Why are our homes overheating?
The Met Office is categorical about the reason behind our increased temperatures: “UK mean temperatures have been shifting over the decades as a result of human-induced climate change”. But global warming isn’t the entire story when it comes to increasing temperatures in our homes. Homes built to modern Building Regulations are better insulated and designed to be more air tight and therefore energy efficient. However, this can also lead to raised indoor temperatures. And then there’s the location of our new build homes, the majority of which are in cities. For many residents in built up urban environments, opening windows to allow cooler air in to their homes is not always an option due to noise, pollution and safety concerns.
What can we do to address overheating in homes?
There are wide ranging means by which we can reduce heat levels in our homes. Ensuring cities have green spaces, especially in urban areas, is an important factor as these are known to have significant cooling effects. The ‘Urban heat mitigation by green and blue infrastructure’5 international study has shown that botanical gardens can cool city air by 5°C, with parks and wetlands also significantly reducing air temperatures. Whilst this level of intervention needs to be done at town planning level, housebuilders also have a role to play by adhering to Building Regulations, Approved Document O. This provides guidance on mitigating overheating in residential buildings including optimising glazing, solar shading and natural ventilation. The House of Commons’ Environmental Audit Committee’s fifth report of session 2023–24 on heat resilience and sustainable cooling would like to see Building Regulations Part O be expanded to cover refurbishments of existing properties as well as material changes of use to residential. It is also calling for post-occupancy evaluation to be introduced to ascertain the real-world performance of mitigation measures taken under Part O, within the first year of installation. Approved Document O rightly stipulates that mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it. Where this is the case, a cooling hierarchy needs to be followed, which includes acoustic façade vents and mechanical ventilation through to mechanical cooling.
The latter has historically been in the form of air conditioning, but this would need to be on a site-wide basis. Mechanical ventilation in the form of Mechanical ventilation systems with heat recovery (MVHR) are a further option. For many
6 BUILDING SERVICES & ENVIRONMENTAL ENGINEER MAY 2025 Read the latest at:
www.bsee.co.uk
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