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.

Background

At present there is no rigorous definition of what constitutes overheating in dwellings.  However, there is documented evidence that temperatures currently being reached in some existing dwellings are harmful to occupant health and well-being.  It is also evident that some dwelling designs currently passing planning and building control, exhibit the same design and construction characteristics as existing dwellings where high temperatures are known to have caused harm to health and well-being.  An accepted definition of what constitutes overheating is needed, so that designers, assessors and surveyors can predict and mitigate against the risk of overheating.  It would also alert and inform policy makers and the whole construction industry to the importance of this issue.

The factors that contribute to overheating in dwellings include urbanisation, occupant behaviour and interventions, orientation, aspect, glazing, internal gains, thermal mass, changes in building design (including the drive for energy efficiency, leading to highly insulated and airtight dwellings), pollution, noise and security.  Many previous assessments and existing research into the subject have tended to make the assumption that window opening is an available option when occupants become hot but, particularly in urban locations, window opening is not always appropriate or safe for occupants, or is limited in its effectiveness.  The Government’s Standard assessment procedure (SAP) currently makes simplistic assumptions about ventilation, and is applied uniformly across the housing stock and throughout the UK.  Alternative means of assessing overheating risk are required in order to capture factors such as a building’s microclimate and actual occupant use of the means of ventilation provided.

At the present time, reported cases of overheating in existing dwellings are still relatively low in the context of the size of the housing stock, although three major factors are contributing to widespread concerns that the problem of overheating will increase in future years.

Firstly, the energy efficiency and zero carbon agendas have resulted in a drive for more highly insulated and airtight dwellings, in both new build and retrofit.  Historically in the UK the use of heavyweight construction materials and limited amounts of thermal insulation, allied with high levels of infiltration through gaps in the building fabric, have contributed to minimise overheating.  Highly insulated and airtight low and zero carbon homes, often designed with large areas of glazing, mechanical ventilation and/or communal heating systems have the potential to overheat throughout the year, not just in the summer months or in heatwaves.  In rural and suburban locations it may be easy to use natural ventilation (e.g. window opening) to help cool dwellings, but in urban and deep-urban locations it is often not possible to do this.  For example, in some cases windows cannot be opened to a great enough extent or occupants are not willing to do so for any length of time on the grounds of noise, pollution or security concerns.

Secondly, it is widely predicted that climate change will lead to further increases in temperatures in the UK, this being especially the case in the South East and in urban locations.  Furthermore temperature maxima are expected to be higher and more frequent, as are heatwaves in which high temperatures persist for several days and nights.  These meteorological factors will exacerbate the risk of overheating, especially in urban settings where other factors such as the urban heat island (UHI) effect come into play.  Recent heatwaves have demonstrated very graphically the risk of overheating for some sections of the population.  For example, Donaldson noted that in England and Wales estimated excess mortality associated with the 1995 heatwave (30 July to 3 August) to be 619 deaths (an 8.9% increase).  In Northern France in August 2003 unprecedentedly high day and night-time temperatures for a period of three weeks resulted in 15,000 excess deaths.  The IPCC reported that across Europe as a whole it was estimated that the number of excess deaths was in the range of 35,000 in 2003.  In addition to heatwaves, it is expected that higher ultraviolet radiation levels and more days with dangerous ozone levels in the air will encourage more people to spend more time indoors.

Thirdly, increased urbanisation and an ageing population are expected to contribute to the impact of possible future scenarios.  In the past 50 years there has been a 30% increase in the ratio of those living in urban areas to those in rural areas.  Due to the UHI effect and the unique microclimate generated by cities, and changes in dwelling design, more people may be affected by adverse health outcomes from rising temperatures.  Population demographics are also changing globally, and particularly within the UK.  As the UK population ages a greater proportion of the population will be exposed to higher indoor temperatures since the elderly tend to spend more of their time indoors.

Overheating is therefore an important issue which needs to be both understood and dealt with across the UK housing stock.  The causes of overheating in dwellings and the effects that it may have on occupants, especially those in vulnerable groups who might be worst affected, need to be better understood.  Excess heat is already identified as a significant health problem (e.g. see the Housing Health and Safety Rating System, HHSRS guidance publication).  Although not on the scale of excess cold, which is thought to be responsible for approximately 25,000 deaths per year in the UK, excess heat due to high outdoor temperatures is already thought to currently cause around 2000 deaths in the UK per year.  Projections by climate change scientists indicate that this figure may increase markedly over the next century, and increased overheating in dwellings is very likely to be a major contributory factor in such an increase.

Crucially, although a reasonable body of research exists on outdoor heat and its effect on human beings, there is little published data on the way in which dwellings modify external temperatures and on measurements of coincidental outdoor/ indoor temperatures in dwellings.  As well as a need for research into the effects of overheating on building occupants it is imperative that policy makers, planners, designers, builders and dwelling occupants are made more aware of the risks associated with overheating, and that measures are put in place to reduce them.

Introduction

This report is aimed primarily at policy makers and stakeholders in the house-building industry.  It is based on the findings of a review project conducted by BRE for the NHBC Foundation.  The review project was initiated following increasing anecdotal evidence of cases of overheating in the UK, and a series of investigations into dwellings where overheating had been reported – a significant number of them being of very recent construction.

The review project involved a desk study of the causes of overheating, a literature review on the effects of excess heat on human health, review of past BRE investigations into cases of overheating, and consultation with a range of stakeholders.  In order to gauge the views of stakeholders, two interactive workshops were held in 2011.  The first, held at the Health Protection Agency (HPA) in London, concentrated on the medical aspects and involved delegates from central and local government, chartered institutions, consultancies and academia.  The 22 attendees were asked to consider the definition of overheating, causes and effects (including emphasis on vulnerable population groups), methods for mitigation or adaptation, and thresholds for intervention.  The second workshop was held at BRE in Watford and was attended by 55 delegates from the house-building sector, housing association sector, central and local government, chartered institutions, consultancies and academia.  The interactive sessions looked at definitions, causes and effects, thresholds for intervention, treatment or adaptation of existing dwellings, and design of future dwellings.  At both workshops the delegates were asked to identify current knowledge gaps.

It is evident that a crucial starting point is to define exactly what the effects of overheating are in medical terms, since without a workable definition designs cannot be properly evaluated and robust benchmarks cannot be determined.  It is also recognised that as evidence of overheating in existing dwellings increases, there is a need for a standard definition for use by assessors to determine whether a dwelling is overheating, or whether there is a risk of overheating occurring in future.  Based on such a definition, robust thresholds for various levels of intervention should then be established.

Therefore Section 3 of the report initially considers the current definitions of overheating and presents the findings of a literature review of the medical effects of heat and overheating on human beings.  The literature review was led by an HPA medical registrar in association with BRE and it is summarised here; the full review is available on the HPA website. In addition, based on this review, Anderson et al have discussed the definition of indoor heat thresholds for health in a separate paper.

Section 4 considers the causes of overheating in existing dwellings based on basic building physics principles by considering aspects of building design that may contribute to the problem, and by reference to case studies of overheating investigations carried out by BRE.  This section of the report is informed by the desk study, BRE case studies from overheating investigations, and stakeholder consultations.

Section 5 discusses the ways in which overheating may be reduced, again based on the desk study, findings from BRE case studies and consultation with stakeholders.  Key to this is the consideration of lessons which can be learnt from planning, design and construction practices which have contributed to overheating.  It is important to look forward and find ways in which all parts of the house-building supply chain can lessen the risk of building dwellings that have the potential to overheat and cause harm to occupants.  The particular challenges presented by occupant behaviour, climate change, the low/zero carbon agenda and urban locations are considered in detail.

Section 6 of the report presents the main conclusions drawn from the review project and provides recommendations for areas of future research.