Deliberately Defining Overheating

Wednesday, October 14, 2015


This blog piece is part of a series. We will explore some of the policy themes and solutions currently being developed by the housing and health sectors to address the risk of overheating in homes. This post focuses on definitions of overheating. The next will look at methods for assessing overheating risk.

Policy solutions to overheating in homes: Definitions

“Overheating” in buildings has not, historically, been something the UK housing sector needed to grapple with too much. Our leaky, cold housing stock needed attention, and fixing this was and continues to be high on our agenda. And rightly so.

Overheating as an issue began to emerge in more recent decades, driven in part by increased urban living at high density, new trends in building design, and an ageing population who are more vulnerable to the effects of excess heat. Experts and practitioners began to examine how to design and deliver buildings which are thermally comfortable in the summer, as well as in the winter, and to take steps to identify any potential for overheating to occur and reduce the likelihood of this happening.

In a general sense, overheating refers to the accumulation of unwanted warmth within a building to an extent where it causes discomfort or harm to the occupants.

When researchers have looked into why some buildings overheat, they usually find a combination of well-recognised causes related to how well buildings keep out unwanted heat from the sun, or how effectively the people living there can reject or purge unwanted heat from internal sources. This gives reason to be optimistic about housing providers being able to spot potentially risky combinations of location, orientation and building design early enough in construction projects to allow modifications to be made. In the vast majority of cases, the risk of overheating will be low and no further action will be needed.

But what level of comfort should we be aiming for? To answer this we must get to grips with 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 very severe situation where excess heat inside a building is threatening the health of the occupants. Of course, how well an individual copes with heat is a very subjective thing, so care must be taken when setting generally applicable rules.

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 Engineer’s (CIBSE) Guide A – Environmental Design (2015) is a good example. The guide advises that bedrooms and living rooms within a dwelling should stay within certain temperatures for specified periods of time. In the 2015 edition of the guidance, these comfort thresholds are allowed to vary depending on recent outdoor temperatures and the ability of the occupants to adapt their surroundings to stay cool - the “Adaptive Comfort Model”. 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 the Government’s Standard Assessment Procedure – at “Appendix P”. It requires energy assessors, when carrying out the SAP assessment for a property, to calculate the propensity of the building to overheat in June, July and August. 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 as it is not intended to inform design decisions.

Lastly, the Government’s Housing Health and Safety Rating System (HHSRS) provides guidance for Environmental Health Officers to help them to assess whether a building may be hazardous to the health of the occupants. 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 sector-wide definition on overheating.

Criterion 3 of Part L1A of Building Regulations 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 use energy to cool homes, and is not intended to be a thermal comfort standard or definition.

What definitions are being used by 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 within their organisations.

Respondents were asked “How does your organisation define ‘overheating’ in residential properties?”

Approximately two thirds of the 75 organisations who provided information defined overheating in general terms, and these definitions related to the “thermal comfort” of occupants rather than health-related definitions.

For example, one organisation said overheating is “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 referenced SAP Appendix P as the basis for their definition. The remainder said they defined overheating using quantified criteria including CIBSE’s Guide A Environment Design (2006) or criteria developed specifically for Passivhaus designs.

Going forward

The question is whether an agreed definition or “standard” is needed and what form it would take?

A number of recent reports highlighted some of the consequences that the lack of an official definition can have. For instance, the Zero Carbon Hub’s June 2015 report “Overheating in Homes – The Big Picture” shared industry concerns that:

  • Housing associations and housebuilders lack clarity on what reasonable steps they must take and rules they should apply to safeguard their current and future occupants from overheating; and
  • Professionals tasked with assessing the risk of a property overheating may choose differing criteria to judge the performance of buildings, limiting comparison between them and generating anxiety about what the ‘right’ test to apply is.

If the goal is to deliver high quality, energy efficient homes which 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 avoid creating unintended consequences. Protecting the health of potentially vulnerable occupants must also be a consideration.

Thankfully, research done to date provides a solid foundation for agreeing a sensible and pragmatic way forward, and the Zero Carbon Hub intends to bring together a group in the coming months to coordinate thinking on next steps.

In the meantime, it is worth thinking about the different ways of measuring and assessing overheating and deciding which are most relevant for your organisation. Which allows you to feel confident that you are genuinely taking reasonable steps to limit the risk of overheating in your stock? What are the common sense design solutions you can adopt now to help prevent homes overheating?

To see the Zero Carbon Hub’s Big Picture report go to