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Baker Homes Blog

Deliberately defining overheating

Posted by Daniel Navarro on 14/10/15 09:00

This blog piece is part of a series by Baker Homes and the Zero Carbon Hub.  We will explore some of the policies and solutions being developed to address overheating in homes.  The following post focuses on definitions of overheating; the next will look at how to assess the risk of overheating.

Policy solutions to overheating in homes: Definitions

“Overheating” in buildings has not, historically, been something the UK housing sector has 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.

As an issue, overheating 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.

The second part of this article will examine the approaches taken by housing providers to define and reduce overheating in homes.


Topics: Social housing, Housing Associations, Overheating, Housing

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