What is an Overheating Risk Assessment?

What is an Overheating Risk Assessment?

With the introduction of the new Part O: Overheating in the Building Regulations from this June, you may be asking yourself what exactly is overheating, and how is it addressed in the Building Regulations? Firstly, we must discuss what exactly overheating is, and then we will look at the methods used to carry out Overheating Risk Assessments as detailed in the new Approved Document O.

What is overheating?

A simple definition of overheating is the accumulation of heat within a building, causing discomfort to occupants. Defining exactly what overheating and assessments of these falls into three main categories, as outlined by the Zero Carbon Hub in their Defining Overheating: Evidence Review[1]:

  1. Thermal comfort;
  2. Health; or
  3. Productivity

Of these definitions, the most widely applied in relation to the design of buildings is thermal comfort, and as such is the area we will focus on. BS EN ISO 7730 defines thermal comfort as ‘that condition of mind which expresses satisfaction with the thermal environment’.

Discomfort caused by overheating is subjective, with the point at which ‘hot’ becomes ‘too hot’ varying from person to person depending upon a variety of factors[2] such as age, gender, state of health, clothing and activity levels. Differences in thermal comfort between the genders is illustrated by research conducted by Karjalainen (2012)[3], which found that under the same thermal conditions women are 50% more often dissatisfied with the indoor climate than men.

 For example, take two identical people, one wearing thick winter clothes and the other in casual summer wear and imagine putting them through a temperature range of 0-30°C. Clearly, the temperatures at which they experience thermal comfort/discomfort will differ based on the clothing they are wearing! However, methods of determining the risk of overheating in a particular building must prescribe a quantitative definition of when thermal discomfort will be experienced by the occupants in a way that allows consistent comparison between varied buildings. This is where Overheating Risk Assessments come in.


What is an Overheating Risk Assessment?

An Overheating Risk Assessment is a desk-based analysis of the potential for overheating of a building. Under the new Approved Document O: Overheating, there are two methods of determining the overheating risk of a new building, a Simplified Method and the Dynamic Thermal Modelling method.

Simplified method

Approved Document O details a Simplified method that can be used to provide a different way to assess the likelihood of overheating so that a more complex and expensive TM59 assessment doesn’t have to be done where it is deemed unnecessary. These focus on two areas, limiting solar gains and removing excess heat. The requirements to achieve compliance with these two areas differs depending on the location of the building and whether or not it has cross-ventilation. The simplified method looks at the area of glazing in proportion to the floor area, and prescribes a minimum free area (the geometric open area of a ventilation opening) that must be achieved. This is called a ‘Simplified Method’ for a reason, and is not an appropriate method of Overheating Risk Assessment in all buildings. Where this method is insufficient, Approved Document O prescribes the use of more complex modelling via the Dynamic Thermal Modelling method.

Dynamic Thermal Modelling: CIBSE TM59

CIBSE’s TM59: Design methodology for the assessment of overheating risk in homes is the backbone of the Dynamic Thermal Modelling method, and it defines the criteria for when overheating will/will not occur as below:

Criteria for homes predominantly naturally ventilated

Compliance is based on passing both of the following two criteria

  1. For living rooms, kitchens and bedrooms: the number of hours during which the temperature is greater than or equal to 1°C greater than the comfort temperature shall not be more than 3% of occupied hours
  2. For bedrooms only: the operative temperature in the bedroom from 10pm to 7am shall not exceed 26°C for more than 1% of annual hours

Criteria for homes predominantly mechanically ventilated

  • All occupied rooms should not exceed an operative temperature of 26°C for more than 3% of the annual occupied hours. The calculation is done separately for each room.

TM59 provides standardised details for different areas where assumptions must be made to ensure consistency across overheating calculations, such as occupancy and equipment heat gains and lighting. Dynamic simulation software must be used, and thus the cost to the developers is greater if this method is used. However, it will offer the designer additional design flexibility over the simplified method.


Other details of Approved Document O

The designers must also consider other factors that may mean the standards of the simplified method cannot be met, such as:

  • Noise
  • Pollution
  • Security
  • Protection from falling
  • Protection from entrapment

In such situations where these will prevent the use of openings as expected in the simplified method, the Dynamic Thermal Modelling method must be used to demonstrate compliance with Part O.


Overheating is a newcomer to the Building Regulations, but its arrival is no surprise. With the ever increasing fabric performance and higher levels of airtightness in new buildings being driven by Part L and the UK Government’s aim of achieving net zero by 2050, overheating is an unintended consequence that must be addressed now so that costly and energy-intensive cooling solutions do not have to be installed after the fact.

 In addressing the performance of buildings during the winter, their performance during the summer has been neglected, and throwing in the effects of the urban heat island and climate change, addressing these issues now becomes even more important.

[1] Defining Overheating: Evidence Review. Zero Carbon Hub. (March 2015). Link.

[2] CIBSE TM59: Design methodology for the assessment of overheating risk in homes. CIBSE. (2017)

[3] Karjalainen, S. (2012). Thermal comfort and gender: a literature review. Indoor Air (22:2). Link.

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Article published 16/05/22

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