Overheating Risk Analysis 
for Homes (CIBSE TM59)

Building Regulations have as of 2021 introduced Appproved Document Part O which now includes a requirement to demonstrate mitigation measures taken to reduce the risk of overheating.

With increasing summer temperatures and the Urban Island Heat Effect in large cities, over heating is a major cause of discomfort which contributes towards illness and even mortality to many elderly or vulnerable people each year.  

A lack of consideration and poor design are major factor and especially in new builds the increase in insulation and airtightness, whilst it has been beneficial to help reduce energy usage and lower CO2 emissions, this has however highlighted the lack of design and consideration to the effects of overheating meaning current design practices are the most common factor in overheating. Naturally the Building Regulations looks to avoid the use of mechanical cooling (air-conditioning) and favours the inclusion of passive design measures to mitigate the risk.

For dwellings you are required under Part O of the Building Regulations to demonstrate compliance using one of the following two methods, these are defined as:-

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Simplified Method (Section1)

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The simplified method firstly looks at the location of the dwelling or building to identify whether it is in an area of high or moderate risk of overheating as defined in appendix C of Approved Document O. Depending whether the design offers the potential for cross ventilation and the glazing orientation, this will determine the maximum amount of glazing as a percentage of the rooms floor area. Additionally, measures including external shading devices and overhangs need to be included to glazing between the orientations of North East & North West via South together with setting the minimum requirements for openable free ventilation area within rooms. This obviously presents restrictions on designs given the limited glazing which can be provided using this simplified method. For dwellings and buildings with glazing in excess of those set in Table 1.1 & 1.2 or unable to include the minimum requirements will need to demonstrate compliance using Dynamic Thermal Modelling (see below).

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This approach isn't appropriate for most projects as the limitations usually result in a loss of building design features most designers, builders and occupants expect. Thats why we recommend the following option as this usually provides the ability to include these otherwise prohibited design features whilst incorporating sufficient measures to limit the effects of overheating.

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Dynamic thermal modelling (Section 2)

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Using the 3D dynamic thermal modelling approach offers designers a greater flexibility in design and can take into account greater levels of details that help to mitigate the effects of overheating. These include:-

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• Site specific weather conditions (using geographically closest weather files)

• Shading from surrounding buildings and trees

• Project specific fabric u-values and airtightness

• Inclusion of mechanical ventilation systems

• Complex building shapes

• Bespoke shading devices (Shutters, External blinds, Overhangs & Awnings)

• Window installation positioning (Reveal depths)

• Specific glazing solar transmission values/specification (g values)

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