Part O: Understanding the new regulations on overheating

Part O building regulations overheating

The government has introduced Part O of the Building Regulations to prevent homes overheating in summer. Anastasia Mylona, head of research at CIBSE, explains what is in the new regulation and why it was needed

Improvements in the energy efficiency standards for both new and refurbished homes means that dwellings are now better insulated and more airtight than ever. However, a regulatory focus exclusively on the prevention of heat losses in winter has failed to address the growing numbers of homes overheating in summer, particularly in towns and cities where ambient temperatures can be significantly higher.

Overheating was an issue the Chartered Institution of Building Services Engineers (CIBSE) had become increasingly concerned about. CIBSE had therefore produced evidence and contributed to various government steering groups and projects considering the issue over the past decade. More recently, CIBSE was part of the Building Regulations Advisory Committee that advised government on the various ways in which overheating could be addressed in the Building Regulations.

This advice now forms the basis of the new Part O Building Regulation, published on 15 December 2021 which came into effect on 15 June this year.

Approved Document O: Overheating provides guidance on how to comply with the new requirements. Alongside the prevention of overheating in domestic dwellings, the new Part O also applies to overheating in properties, such as care homes and student accommodation.

Designers have two routes to demonstrate compliance with Part O:

  • A simplified method, limiting solar gains and maximising natural ventilation potential through window sizing or window design, and
  • A dynamic thermal modelling route, based on CIBSE’s TM59 Design methodology for the assessment of over­heating document.

Part O

What is the simplified method?

The simplified method is based on the principle that overheating is exacerbated by excess solar radiation and insufficient ventilation.

The methodology sets out to limit the amount of glazing on the south, west and east facades while ensuring there is sufficient openable window area to prevent overheating. To do this, the simplified method requires details about the glazed area and opening area of every single window in the dwelling to be calculated in order to check:

  • That the total glazed area within the dwelling does not exceed a limit based on the floor area and orientation of the most glazed facade.
  • And that the total area in the most glazed room does not exceed a percentage limit, based on the floor area of that room.

Maximum allowable glazed area varies depending on whether a dwelling has windows on opposite sides to allow cross-ventilation. As might be expected, the allowable glazed area is reduced for buildings without cross-ventilation.

Having established the maximum glazed area, the simplified method then requires designers to check the area of openable windows is sufficient to remove excess heat. The minimum allowable free area is calculated based on a percentage of floor area or glazing area, whichever is the larger. The minimum allowable free area is higher for buildings without cross-ventilation.

Approved Document O also makes a distinction between buildings in high-risk urban locations, such as central London, and moderate-risk locations when establishing the minimum free area for ventilation.

It’s early days but feedback from industry is that the application of the simplified method is anything but simple. That said, I assume that it will be the compliance method of choice for architects or housebuilders designing a one-off home in low-moderate risk areas.

What is dynamic thermal modelling?

Dynamic thermal modelling is expected to be the route to compliance followed by most of the industry because it provides more flexibility in how a design can be developed to prevent overheating.

The dynamic thermal modelling route is based on the methodology developed by CIBSE in TM59. This involves the creation of a model of the dwelling, or development, using dynamic thermal modelling software and using the standard set of data inputs from TM59. Based on these inputs and using climate modelling from CIBSE 2020s Design Summer Year (representative of 2010-2040), the model can be used to show whether a scheme is compliant.

For those familiar with building simulation tools, the modelling route will be relatively straightforward and CIBSE’s TM59 provides standardised input for internal gains, occupancy profiles etc.

To demonstrate compliance, the model will need to show that the internal temperature within a space does not exceed a specified temperature for a certain number of hours, with an added requirement for bedrooms than for other areas of the dwelling.

When carrying out an assessment, the designer should try to identify and check all dwellings in a development that are at risk of overheating. For an apartment block, these are likely to be those dwellings with large, glazed areas; on the top floors; having insufficient or no shading; having large south, west or east-facing windows; being single aspect; or having limited opening windows.

The Good Homes Alliance has produced a helpful tool to identify key factors contributing to overheating risks and suggesting possible mitigation measures. The tool is intended to be referenced in future updates to TM59 to help identify dwellings most at risk at the early design stages.

The dynamic thermal modelling approach could be particularly advantageous to a developer because it will allow them to target solutions for specific rooms or dwellings where problems have been identified, rather than having to introduce more generic solutions across the entire development, potentially allowing for more creative solutions.

A dwelling should be constructed to meet the requirements of Part O using passive means as far as reasonably practical. Mechanical cooling is not totally excluded but should only be used when all other practical options have been explored.

CIBSE TM59 was introduced in 2017. Since then, it has been used extensively by industry, including as a means of demonstrating compliance with the London Plan, with positive feedback.

That said, it is impossible to say that a compliant scheme will never overheat because there is a lot of uncertainty. Weather patterns are not completely predictable – and neither is the way people use their homes. Nevertheless, with the introduction of Part O, CIBSE can take comfort from the fact that designers will now have to demonstrate to building control bodies that they have taken all necessary steps to address the overheating risk in a dwelling at the design stage.


Anastasia Mylona

Head of research


Tel: +44 (0)20 8675 5211






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