Overheating Mitigation - Building Regulations Part O



Building Regulations Part O (Approved Document O ‘Overheating Mitigation’ ) came into force in June 2022 which requires new dwellings (including conservatories) to meet the requirements of Part O.

Part O does not apply to extensions or conservatories added to residential buildings after they are built or to buildings undergoing a change of use.

Compliance can be demonstrated by one of two methods (both of which are available from our in-house accreditation team); Simplified Method and Dynamic Thermal Method.

The Simplified Method:

To carry out the simplified method the location of the development must first be determined as either ‘moderate risk’ or ‘high risk’:

  1. ‘High Risk’ Locations are parts of central London and some parts of Central Manchester. The Approved Document O provides post codes of areas of high risk within Appendix C.

  2. ‘Moderate Risk’ Locations are all the remaining areas in England not considered as ‘High Risk’.

The development must then also be categorised as either having Cross-Ventilation or not, and the simplified method will then require full details of glazing and opening specifications in order to provide a result.

The Dynamic Thermal Modelling Method:

If the simplified method is too prescriptive then the Dynamic Thermal Modelling method can be used as an alternative for demonstrating compliance.

The Dynamic Thermal Model must undergo and satisfy a CIBSE TM59 overheating assessment. It should follow the methodology of CIBSE TM 59 whilst also allowing for some limits set out within the Approved Document. For example, the Approved Document states the internal temperatures that must be used to simulate when a window begins to open or close. It also states the occupied hours to use during the day and night, and gives alternative guidance for ground floor windows that should be closed overnight for security purposes.

Limit Solar Gains

The approved document also provides a list of ways to limit solar gains into a building that can be modelled, such as shutters, external blinds, or overhangs. Other ways it suggests limiting solar gains would be to alter the glazing design (size or G-Value), in the Building Design (Balconies), or through shading provided by surrounding buildings or landscapes.

Remove Excess Heat

To remove excess heat from a building the approved document suggests either opening windows, installing ventilation louvres, a mechanical ventilation system, or a mechanical cooling system. Although, all passive means of removing excess heat from the building must be used as far as practically possible before the inclusion of a mechanical cooling (air-conditioning) system is used.