An overheating criterion for bedrooms in temperate climates: Derivation and application

Adequate sleep is crucial to human health and well-being and elevated night-time temperatures can degrade sleep quality. European countries with temperate climates use temperature thresholds between 25°C and 28°C to identify homes that are overheated. The current UK bedroom threshold of 26°C, is based on one small study, which is now over 45 years old. An extensive literature review indicated that with modern summertime bedding and bedwear, which enables body coverage to be easily adjusted, thermal comfort can be achieved for night-time bedroom temperatures up to approximately 29°C. Temperatures measured in 591 bedrooms during England’s hottest ever summer, 2018, are re-analysed. The prevalences of night-time overheating generated by alternative criteria are compared with the prevalences generated by the established adaptive overheating criterion. Comparisons are made for homes with different dwelling and household characteristics. Finally, a new overheating criterion is proposed based on the mean night-time bedroom temperature, with thresholds between 26°C and 29°C depending on the application of the criterion. The allowable exceedance of the chosen threshold is limited to seven nights between May and September. Adopting thresholds of 27°C for vulnerable households and 28°C for others, 23% (5.5 million) of the main bedrooms in English homes were deemed to be overheated in the hot summer of 2018, far fewer than the 69% obtained using the current UK bedroom criterion. Irrespective of the threshold chosen, there were clear, consistent and significant differences in overheating prevalence depending on dwelling and household characteristics. The proposed new overheating criterion is applicable to unconditioned bedrooms in temperate regions. It seeks to strike a balance between the risk that hot bedrooms will be air-conditioned and the risk of temperatures detrimental to a “good nights’ sleep”.

Practical Application

A new overheating criterion is proposed to identify overheated bedrooms. It adopts the familiar format of a temperature threshold and an allowable exceedance. It is applicable in temperate climates when people are asleep in unconditioned bedrooms. The criterion is intended to aid the interpretation of night-time temperatures predicted by dynamic thermal models and temperature measurements in existing bedrooms. It is applicable to individuals of different heat sensitivity, the design of new homes or the refurbishment of existing homes. With further testing and refinement, it offers a credible replacement to the existing UK bedroom criterion given in CIBSE Guide A, TM59 and in other guides. It can thus underpin the new overheating regulations, Part O, for the design of new dwellings in England.