Energy efficiency: overheating and ventilation in homes

Thermal comfort

16 May 2016

Rob Pannell looks at issues related to overheating and ventilation in homes

We in the housing sector are, sadly, all too aware of the problems that can arise for people living in fuel poverty in homes that are too cold in the winter. The health of vulnerable people, in particular, can start to fail.

Making homes more energy-efficient and thermally comfortable is now a core part of our response to this problem. Each winter, the government, industry bodies and the media seek to raise awareness of where people can get help to keep warm. But there is still a long way to go: an estimated 2.3 million households live in fuel poverty in England and there are tens of thousands of deaths caused by the cold weather each year so efforts to insulate homes and reduce demand for heat must continue.

Given this, it seems somewhat strange to raise the issue of overheating and excess heat inside buildings. By “overheating”, we mean in general terms the phenomenon of excessive or prolonged high domestic temperatures, resulting from internal or external heat gains, which may have adverse effects on the comfort, health or productivity of the occupants.

Overheating has not, historically, been something that building professionals have had much cause to consider. But this is beginning to change as researchers and practitioners see more evidence of the issue, not just for a few days during hot summers but sometimes during cooler summers and for prolonged periods of time.

Driven in part by the increasing density of urban living, new trends in building design and an ageing population who are more vulnerable to the effects of excess heat, up to 20% of dwellings in England may already be overheating to a greater or lesser extent.

Of course, this also means that the vast majority of housing stock is not currently overheating. However, a warming climate unfortunately means that it is very likely that overheating will become more common in future, and, as a result, thermal comfort during summer months as well as the winter is something to which we will all need to give greater attention.


When researchers investigate why some buildings overheat, they usually find combinations of well-known causes: for instance, how well buildings keep out unwanted heat from the sun, or how effectively the occupants manage unwanted heat from internal and external sources. An apartment block may have large, unshaded windows in the south-west-facing elevation, resulting in high solar gains; or a terraced building may have an uninsulated room in the roof, which has little protection from the sun and limited means of ventilation.

The ability to identify such causes gives reason to be optimistic about spotting risky combinations of location, orientation and building design early enough in construction, and even retrofit projects, to allow modifications to be made. In some cases, works to improve the energy efficiency of the building can also help to reduce the risk of overheating, such as the installation of low-energy lightning. In the vast majority of cases, the risk of overheating will be low and no further action will be needed, but it is still good practice to make simple checks.

If unwanted heat builds up inside homes, the occupants might become uncomfortable and find it difficult to sleep, but health issues can develop, particularly in vulnerable groups such as the elderly and young infants. But what level of comfort should we be seeking? To answer this, we must understand and agree as a sector what we mean by overheating.

Defining overheating

Overheating is an umbrella term commonly used to describe anything from a mild case of discomfort to a severe situation where excess internal heat is threatening the health of a building’s occupants. Of course, how well an individual copes with heat is a very subjective matter, so care must be taken when setting generally applicable rules.

Overheating is an umbrella term describing anything from a mild case of discomfort to a situation where excess internal heat threatens the health of occupants

The building industry has responded to this challenge, in part, by developing “thermal comfort” temperature thresholds for use when designing new buildings. The Chartered Institution for Building Services Engineers (CIBSE) Guide A: Environmental Design (2015) is a good example.

The guide advises that bedrooms and living rooms should stay within certain temperatures for specified periods of time. These comfort thresholds are allowed to vary depending on recent outdoor temperatures and occupants’ ability to adapt their surroundings to stay cool, though an exception is made for bedrooms, where an absolute threshold temperature of 26°C remains.

Another basic overheating check used for new homes is contained in Appendix P of the UK government’s Standard Assessment Procedure (SAP). It requires energy assessors to calculate the propensity of a building to overheat in June, July and August when carrying out the SAP for a property. If the average internal temperature – over day and night – is calculated to be above 23.5°C, it is determined to have a high risk of overheating. However, building designers use SAP Appendix P with some caution because it is not intended to inform design decisions.

The government’s Housing Health and Safety Rating System: Guidance for Landlords and Property-Related Professionals helps environmental health officers assess whether a building may be hazardous to occupants’ health. But again, this mechanism, although potentially powerful, is not intended to be an official overheating definition or standard, and as a result is not generally used to guide the design of buildings.

These frameworks form pieces of a jigsaw, but none represent an official, agreed and sector-wide definition of what constitutes overheating.

Criterion 3 of Part L1A of the Building Regulations in England requires developers to demonstrate that “reasonable provision” has been made to “limit heat gains” in new dwellings. It states that “reasonable provision” can be demonstrated by achieving a low or medium risk rating in the SAP overheating check. Criterion 3 is designed to limit the energy used to cool homes, and is not intended to be a thermal comfort standard or definition.

Housing providers

A survey of 75 housing providers carried out by Sustainable Homes and the Zero Carbon Hub in November 2014 suggested that, as would be expected, housing providers are using different definitions of overheating.

Respondents were asked: “How does your organisation define ‘overheating’ in residential properties?” Around two-thirds of the 75 organisations that provided information defined overheating in general terms, and these definitions related to the thermal comfort of occupants.

For example, one organisation described overheating as: “A condition found in a domestic property whereby the indoor temperature is too high and cannot be controlled to provide comfort to the occupants.”

Eight organisations referred to SAP Appendix P as the basis for their definition. The remainder said they defined overheating using quantified criteria, including CIBSE’s previous Guide A: Environmental Design (2006) or criteria that have been developed specifically for Passivhaus designs.

If the goal is to deliver high-quality, energy-efficient homes that are thermally comfortable in winter and in summer, then agreeing what we are aiming for is probably a useful thing. However, we must also be certain that any future requirements are very carefully developed to limit unintended consequences. Protecting the health of potentially vulnerable occupants must also be a consideration.

Research done to date provides a solid foundation for agreeing a sensible and pragmatic way forward, and the Zero Carbon Hub intends to work on coordinating thought on the next steps in the coming months.

In the meantime, it is worth thinking about the different ways to measure and assess overheating and deciding which are most relevant for your organisation. What are the common-sense design solutions you can adopt to help prevent homes overheating?

Rob Pannell is Managing Director at the Zero Carbon Hub

Further information

  • The Zero Carbon Hub closed for business on 31 March 2016
  • Related competencies include Legal/regulatory complianceSustainability
  • This feature is taken from the RICS Building control journal (April/May 2016)