Building Regulations 2022 – Part O and overheating in homes

What is Part O?

Introduced in June 2022, Part O sets out the requirements of the Building Regulations for the mitigation of overheating in new residential buildings. Its purpose is to safeguard the welfare of residents as Britain continues to adapt to higher temperatures, by reducing the likelihood of high internal temperatures and providing means to shed excess heat.

Why do we need Part O?

Our national focus has previously been on increasing thermal efficiency of buildings through Part L of the Building Regulations (conservation of fuel and power). These efforts have helped to create homes that perform significantly better during the winter months, increasing comfort and health for occupants while reducing heating bills and energy consumption as the UK heads towards zero carbon. Now that the changing climate is causing heat waves at a greater frequency and severity, these thermally efficient buildings are becoming more susceptible to overheating during the summer months. This is a significant risk to health and comfort and Part O is a response to this growing concern. Attending to the issue of overheating at a design level can also help to avoid introducing retrofit measures such as mechanical air conditioning that would increase the embodied carbon and operational energy loads of buildings.

How can we comply with Part O?

Approved Document O provides guidance on meeting the requirements of Part O, and gives us a simplified method and a Dynamic Thermal Modelling method for mitigating overheating during design.

The simplified approach requires that all rooms have windows with a minimum openable area while simultaneously defining a maximum allowable area of glazing in order to limit solar gain. The requirements are further mitigated by the availability of cross ventilation within the building. The specific requirements can be found starting from Section 1 of the Approved Document O.

The Dynamic Thermal Modelling method is more esoteric and allows for more design flexibility by taking into account factors such as external shuttering and windows that may not be opened due to contextual constraints such as local noise or pollution. The requirements state that passive measures must be implemented before any mechanical methods are applied.

The guidance also specifies areas of higher risk for overheating; these are typically central London postcodes but local authorities may require their areas be treated as high risk even when not listed in the minimum guidance.

Part O is currently only applicable to new residential builds.