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Until recently, a dwelling was deemed to have an unacceptable overheating risk if habitable rooms exceed threshold temperatures of 25oC in bedrooms and 28oC in living rooms for more than 1% of occupied hours per year.

The new Chartered Institute of Building Services Engineers (CIBSE) TM52 guidance provides a much more pragmatic approach to assessing summer overheating in homes. CIBSE has published a comprehensive set of criteria that move away from the over-simplistic method of only assessing buildings for hours of exceedance above a fixed temperature, towards a set of criteria which account for external temperatures and severity of internal temperatures.

There are several reasons why the old method of assessing thermal comfort against a static threshold temperature is no longer deemed sufficient:

  1. There is considerable evidence demonstrating that comfortable indoor temperatures ‘track’ external temperatures, meaning that an indoor operative temperature of 29oC is more likely to be acceptable when the external temperature is 35oC than when it is 10o
  2. Single thresholds do not reflect the severity of overheating. For example, there is no way to differentiate between a dwelling that is in exceedance by only 0.5oC for 2% of occupied hours, and one that is in exceedance by 4oC for the same number of hours.
  3. Taking annual hours of exceedance as a single metric fails to identify any extreme.