As new homes are built with more thermal insulation and to improved standards of airtightness, concerns are emerging of an increased risk of overheating.  Alongside the key issue of indoor air quality, overheating is a risk that needs to be managed carefully as we move further towards the aim of zero carbon new homes and now ranks among our greatest concerns that need to be addressed as a priority.

Earlier this year, NHBC Foundation published Understanding overheating – where to start: An introduction for house builders and designers, providing a guide to the key factors that can lead to high indoor temperatures, and describing some of the measures which should be considered during planning and design in order to mitigate against overheating.

This new report documents a wide-ranging review of existing information, evidence and case studies on overheating, supplemented by outcomes from two industry workshops.  The report discusses what parameters might be used in the definition of overheating, and gives guidance on reducing overheating.  It concludes that there is an urgent need to develop a universally accepted definition of overheating in dwellings, and that the development of a robust threshold for use by planners, designers, builders and authorities is vital for dealing with overheating.

Executive Summary

This report presents and reviews evidence of overheating in dwellings, its causes and the consequences of overheating for the health of occupants.  A number of recent BRE investigations into overheating in dwellings are presented and analysed as case studies.  The report also discusses what parameters might be used in the definition of overheating including possible threshold temperature levels.  Guidance on the requirements for reducing overheating is also presented and discussed.

There is increasing evidence that new and refurbished properties are at risk of overheating, especially small dwellings and flats and predominantly single-sided properties where cross ventilation is not possible.  However, there is also evidence that prototype houses built to zero carbon standards are suffering from overheating, which shows that overheating may also become an issue where cross ventilation is not achievable in lightweight, airtight houses with little or no solar shading.

In many cases the lack of ability to reject the heat build-up from normal occupant activities means that a risk of overheating exists in summer.  However, in some instances the gains are such that the overheating occurs for most of the year and is therefore independent of the external temperature.

A review of existing overheating criteria suggests that they are based on the upper limit of thermal comfort, rather than the threshold for long-term temperatures that may cause serious health problems for vulnerable groups.  The medical evidence shows that although the health effects of exposure to excessive heat can be mild, if left untreated symptoms have the potential to develop quickly into severe, often fatal heat illness.  With global climate change, increasing episodes of extremes in heat, an ageing population and urbanisation, this risk is expected to increase.  However, at present the evidence base with which to inform policy and guidance is limited.

A significant risk factor is night-time temperature because higher night-time temperatures are thought to increase the risk to health due to the inability to recover from daytime heat stress and the interruption to sleep.  Some of the case study evidence shows that the ability to cool dwellings down overnight is severely limited in some urban locations and property types.

The mechanisms of heat gain within buildings are all very well understood.  However, existing guidance and modelling tools appear unable to predict overheating in all cases, and more work is needed to develop them based on robust practically proven research.  This should include a practical assessment of the effectiveness of inclusion of thermal mass and night-time ventilation in new dwellings.  Robust solutions are also needed for minimising heat gains in all future designs, and adaptation of such solutions for application to the existing stock, including reducing heat gains from communal heating systems.

The report concludes that there is still a pressing need to develop a universally accepted definition of overheating in dwellings, and that the development of robust national thresholds for use by planners, designers, builders and authorities is vital for dealing with overheating.  The extent to which such thresholds could or should be regulated, for instance through the Building Regulations, is also a key issue for debate and action.  At a more detailed level, agreement is needed on whether to base temperature criteria entirely on health or simply base them on thermal preferences (as has often been the case historically).  Further research is also needed on how thresholds should take account of a changing outdoor climate, human adaptation and the effect of minimum night-time temperatures and diurnal variation.

NHBC