How big is the problem of overheating in homes and what can we do to mitigate the effects? Andrew Nash, Nuaire divisional manager, takes a look
2024, 2023, 2022 and 2014 are the UK’s warmest years on record, according to the Met Office’s analysis. Hot temperatures are not just uncomfortable but have serious health implications and, 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.
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 26C 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 homes 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.
Low-income households and those living in social housing bear the brunt of overheating.
Why are our homes overheating?
Global warming is certainly a major part of the problem, but it isn’t the entire story. Homes built to modern Building Regulations are better insulated and designed to be more airtight 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 into their homes is not always an option due to noise, pollution and security 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.
While 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 calls 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.
Mechanical cooling
Mechanical cooling 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 new build homes, these will be able to meet the extract ventilation rates required to help mitigate overheating in the summer if equipped with a third speed to provide a Part O purge.
However, many housing developments have problem areas when it comes to overheating, such as corner apartments, which may have more glazing area relative to their internal floor area, and that get sunshine for longer.
MVHR is unlikely to succeed in keeping temperatures within the TM59 guidelines in these circumstances and installing air conditioning just to these specific apartments is not a practical solution due to the logistics of the refrigerant pipework and associated cost, not to mention the necessity for centralised cooling plant.
In these instances, a hybrid cooling system is a good option. These new hybrid systems integrate with the MVHR system and operate in conjunction with it. It combines the heat-exchanger coolth recovery of an MVHR system with the cooling effect provided by a DX coil which significantly lowers the temperature of the fresh-air supply.
The MVHR is pre-programmed to target a comfortable temperature, maximising free-cooling through its bypass when the external air is cooler than internal, and coolth recovery when hotter external air can be cooled by extracted stale air from wet-rooms. The cooling module activates automatically to lower supply air temperatures when required.
Time for a national strategy?
The introduction of Part O represents a significant step in recognising and addressing overheating but increasingly, organisations in the housing sector are calling for a national strategy that is more ambitious than the current collection of policy and initiatives, and that introduces urgency into combatting and mitigating overheating.
Ultimately, we will have to wait to see how seriously the government really takes this issue.
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