What does Part O of the Building Regulations cover?

The new Building Regulation updates came into force in June of this year. They are the next step on the road to Future Homes Standard, which aims to make sure all new homes built from 2025 produce around 75% less carbon emissions than those constructed under current regulations, with final goal of net zero in 2050.

These changes include the new addition of Part O – Overheating. This is a new document looking at the increased possibility of overheating due to more airtight and efficient constructions.   

The aim of the new regulations is to make sure that buildings make reasonable provision to:

• Limit unwanted solar gains in summer
• Provide an adequate means to remove heat from the indoor environment

Improvements in the energy efficiency standards for both new and refurbished homes means that buildings are now better insulated and more airtight than ever. However, up until now, with the building regulations predominantly focusing on the prevention of heat losses in winter, this has led to the failure to address the problem of overheating in summer.

The guidance given in Part O applies to new residential buildings only, which includes both dwelling houses and flats, institutional residential places such as homes, schools, colleges, halls of residence and other student accommodation, plus other similar establishments where people sleep on the premises.

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

How Do You Calculate Overheating Risk?

Two types of measurement are viable to ensure compliance with Part O:

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

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

It sets out to limit the amount of glazing within a building, while ensuring there is sufficient openable window area to prevent overheating.

This requires a detailed plan of all glazed areas and the opening area of every single window in the dwelling to check the following:

• 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.

The maximum allowable glazed area varies depending on whether a dwelling has windows on opposite sides to allow cross-ventilation. The area of openable windows also needs to be checked to make sure it 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.

The location of the development is also important to determine whether it is either ‘moderate risk’ or ‘high risk’ - the latter locations being parts of central London and some parts of Central Manchester, the ‘Moderate Risk’ Locations are all remaining areas in England. The full postcode list is set out in Approved Document O.

The Dynamic Thermal Modelling approach can be used for more complex buildings and where greater accuracy and interpretation is required.

CIBSE Thermal Modelling software (TM59) is used to accurately replicate the form of the building and map it against the potential overheating on a seasonal or hourly basis.

This tool can then be used to work out the potential overheating risk in specific rooms and also at specific times of the day. It is also possible to model different mitigation methods such as glass coatings, shading and shutters or blinds to work out their efficacy.

This approach 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.

Let Achieve Green guide you

As experts in providing professional advice on the design and construction of low energy and sustainable buildings, we can offer an analytical view on the proposed solutions and how these will translate into practical solutions to achieve compliance.

We have a team of professionally trained consultants that offer nationwide coverage, and that can guide you smoothly the processes, from concept to completion. Talk to us today to find out how we can help.