Gurprit Bass is associate director, façade engineer & sustainability lead at façade specialist Wintech
With record-breaking temperatures hitting the UK this week, it is appropriate to consider the issue of overheating. In residential dwellings, overheating has historically not been given serious consideration as an issue. With the advent of more energy-efficient and airtight homes, as well as the effects of climate change, the effects of heat have become problematic.
The Zero Carbon Hub states that potentially up to 20 per cent of the housing stock in England is already affected and the issue is likely to become more prevalent in future. Overheating of homes over prolonged periods of time can have severe consequences on the health of the occupants and in extreme cases it can be a risk to life.
While certain UK planning authorities have specific overheating requirements, a new Approved Document O has been released by the Department for Levelling Up, Housing and Communities to attempt to address this issue for all new residential dwellings. Its aim is to protect the health and welfare of occupants by reducing the occurrence of high indoor temperatures.
To achieve this, the guidance seeks to limit unwanted solar gains in summer and provide adequate means of removing excess heat from the indoor environment. It should be noted that this guidance document does not guarantee the comfort of the building’s occupants.
There are two routes to compliance:
- The simplified method; and
- The dynamic thermal modelling method.
The simplified method bases the risk of overheating on location, and limits unwanted solar gains by using external solar shading and glass selection with low G values (<0.4) and high daylight values (>0.7). The G value is a measure of how much solar heat is allowed in through a particular part of a building.
Cross-ventilation is also encouraged, with mechanical cooling only used where ventilation alone is insufficient in removing heat from the indoor environment.
Dynamic thermal modelling follows the methodology in CIBSE TM59 and may offer flexibility over the simplified methodology for some buildings.
Other performance aspects
In meeting the overheating requirements, sufficient consideration must be given to other performance aspects to prevent their derogation, such as excessive noise at night, minimising air pollution, security, safeguarding against falling and protection from entrapment.
To be able to operate the dwelling as designed and prevent overheating, a home user’s guide should be created, which contains a section on ‘staying cool in hot weather’ to provide the occupant with basic information on how best to maintain a comfortable internal temperature.
Mitigating overheating while also meeting other performance targets may require some serious compromises to be made to the future architectural design of buildings. Those with external shading devices, low G values, proportionally less glazing and larger openable vents are likely to become more commonplace.