Overheating

Overheating in homes

Homes in the UK have not historically been associated with overheating. This is probably due to a combination of the heavyweight materials from which they were constructed, a low level of thermal insulation and plenty of unplanned ventilation through minor gaps in the fabric. Added to that, sash windows provided high- and low-level ventilation that would rapidly purge excess heat on a hot day if the home had been closed up all day.

Contrast that with very highly insulated low and zero carbon homes, which have been built and tested to high standards of airtightness, with double-glazed windows that have coatings specifically designed to trap the sun’s heat. Add in some communal heating pipework and additional services that permanently stay warm and it’s hardly surprising that there is growing concern about overheating in new homes.

Although the number of incidences reported so far is thankfully low, it would seem fair to assume that the risk of overheating will increase as we make progress towards the zero carbon homes standard. Also, if as expected, climate change leads to a further increase in summer temperatures, then overheating will become even more of an issue and one that cannot be ignored. A heat wave in Northern France in 2003 that lasted three weeks and is estimated to have resulted in 15,000 excess deaths, the majority of which were older people, serves to remind us of how pressing an issue this is for us to deal with.

What is overheating?

The term overheating is used to describe when conditions in a building make occupants feel uncomfortable or heat stressed. The cause of overheating is complex and not a simple measure of maximum temperatures. Long continuous periods of above-average indoor temperatures in homes are thought to be a significant factor affecting people’s health.

Current design guidance aims to minimise heat discomfort in buildings. For homes, the CIBSE Environmental Design, Guide A (under review) sets out temperature thresholds (28˚C for living rooms and 26˚C for bedrooms) and defines overheating as when these temperatures are exceeded for more than 1% of the time. Looking at temperatures recorded in existing homes we know that overheating can significantly exceed these limits during an episode like the 2003 heat wave. In some studies ‘degree hours’ are used to define the extent of overheating observed. Overheating at these kinds of levels (for example, over the course of nine days) would make the homes very uncomfortable for most people.

What can be done to minimise the potential for overheating in homes?

There are many features that designers can use to minimise overheating in homes. Approaches include:

  • Thermal mass
  • Solar shading and shutters
  • Cooling ventilation strategies
  • Heat reflective finishes
  • Planting

Further reading about overheating can be found in:

  • Overheating in homes – An introduction for planners, designers and property owners
  • Overheating in new homes – A review of the evidence (NF46)
  • Understanding overheating – where to start: An introduction for house builders and designers (NF44)