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Understanding overheating – where to start: an introduction for house builders and designers (NF44)


Traditionally, overheating has not been a problem in housing due to low levels of insulation and gaps in building fabric that allow air to flow in and out. In having to meet more demanding thermal and airtightness standards, new homes may be more predisposed to overheating.

This illustrated guide is an introduction to the causes of overheating and a range of design measures that can reduce it. Seven case studies demonstrate how overheating can occur, including the effect of location, inappropriate design or the way in which the home is used.

More detailed evidence for overheating and how we currently define it is included in NF 46 (Overheating in new homes – A review of the evidence).

Summary of content

Overheating is generally understood to be the accumulation of warmth within a building to an extent where it causes discomfort to the occupants. There is no clear definition of the term ‘overheating’ or the specific conditions under which this can be said to occur. Nor is there any statutory maximum internal temperature in UK Building Regulations or current health and safety guidance.

Work by CIBSE and Arup suggests that most people begin to feel ‘warm’ at 25ºC and ‘hot’ at 28ºC. Their report also defines 35ºC as the internal temperature above which there is a significant danger of heat stress. However, overheating is not just a function of high temperature, other factors such as lack of air movement and sustained periods of high temperatures will also affect people’s comfort levels. Summer overheating is a ‘dynamic’ phenomenon and all the contributing factors and their interactions are difficult to model with steady state tools. Furthermore, some factors may only manifest themselves in particular geographical areas or at a “micro-scale”, such as the ‘heat island’ effect, which is particular to dense urban areas and is often identified by reduced variations in daily external temperatures when compared to the daily temperature differences observed in rural surroundings.

The health impacts of overheating can include an increased risk of illness from respiratory and cardiovascular disease, and the consequences of extreme temperatures sustained over a period are significant: the summer heat wave of 2003 resulted in more than 2,000 extra deaths in the UK. There are also instances in urban areas where serious health issues caused by overheating have led to legal action being taken against the landlord.