Building Regulations Part O (Overheating) Explained

The 2022 Building Regulation updates to Approved Document Part O were intended to help reduce energy consumption in line with Government pledges to reduce emissions for climate change.

Modern house design and construction is now focused on energy efficiency. Houses are designed to be as airtight as possible, providing adequate ventilation and minimising solar gains.

Houses must be able to efficiently retain heat as needed whilst also limiting and removing excess heat in Summer.

What is Approved Document Part O of Building Regulations?

The updates to Building Regulations Approved Document Part O came into effect in June 2022 to address overheating in buildings.

‘Overheating’ is approached on a basis of prevention and remedy:

• Prevention: by limiting the amount of direct sunlight through windows and glazing
• Remedy: with a provision to quickly remove excess heat from inside a room when it gets too hot

Part O is also concerned with excess heat caused by heating pipes and not having control of heating - usually for apartment blocks with between floor pipes and communal heating.

To achieve Part O regulations, windows and glazed areas are limited to a maximum size depending on:

• The floor area of the room
• The direction they face (south, north, east, west)
• Cross-ventilation inside the building (opening windows on opposite facing walls)
• If the building is in a high-risk area (London postcodes being the highest risk)

In high-risk areas, the use of external shading will be required (explained below).

Under Part O, what is considered as ‘Overheating’ is calculated by two methods:

• The Simplified Method
• Dynamic Thermal Modelling

Note that Approved Document Part O only applies to new build domestic buildings. Extensions and conservatories added to a property after it has been built do not come under the requirements of Part O.

Replacement windows and conservatories as an addition to existing houses are not affected by Part O.

Part O: Limiting Solar Gains

Solar gain is the rise in temperature that is achieved from shortwave radiation.

Direct sunlight produces shortwave infrared radiation which can be absorbed by glass and re-radiated into a room as longwave radiation. Longwaves can’t be absorbed by the glass so the energy becomes trapped in the room - known as the greenhouse effect.

G-values measure the amount of radiation that can pass through the glass. Special coatings and laminate layers can limit the amount of infrared that pass through and are used for windows that face south or south west.

Solar gain becomes an issue especially for large expanses of glass facing south. And this is where the Part O regulations are focused, to reduce the size of windows and glazing that are exposed to direct sun.

Part O regulations state that windows must be limited in size by the direction they face in ratio to the floor area of the room they are installed in. Windows are also restricted for the amount of glazing used in the individual room that has the most glazing (usually a living room).

Windows are restricted in size based on:

• The overall glazing area for the entire property calculated as a percentage of the total floor area
• ‘The glazing area of the room with the most glazing calculated as a percentage of the floor area of that room

The Simplified Method

The requirements for Part O would usually be calculated using the simplified method, unless there is a reason to use Dynamic Thermal Modelling (see below).

The location of the property is taken into account as a moderate or high risk (a list of high risk postcodes is provided in the full Part O document):

• ‘Moderate risk’ location – England, excluding high risk parts of London
• ‘High risk’ location – urban and some suburban parts of London

Cross ventilation is also a key factor, defined as “openings on opposite façades” but not “having openings on façades that are not opposite” such as having windows in two adjoining walls of a corner flat.

This means a room or the building has facing windows that can open to allow a flow of air. Facing open windows creates air pressure as the wind will draw air out of one window, the pressure created draws air in through the other window.

Read more: How to ventilate your home...

The outlines for the Simplified Method are included in the following tables.

Note: The simplified method is not suitable for buildings with more than one residential unit which use a communal heating or hot water system with significant amounts of horizontal heating or hot water distribution pipework. Main distribution routes should be through vertical risers to minimise heat gains into common spaces.

Note that Building Regs stipulate you must be able to demonstrate that passive cooling (such as cross ventilation) is possible before the use of mechanical cooling. You must also take into consideration Document Part F ventilation for any methods applied.

Removing Excess Heat

Purge ventilation to remove excess heat is defined in approved document Part F (Ventilation). This is achieved by allowing minimum opening areas for windows, or by mechanical extraction.

Purge ventilation must be installed in all habitable rooms to meet Part F Building Regulations.

This means that windows must open by a certain amount which is calculated by the total of the floor area of the entire house and the glazed area.

Under Part O, the removal of excess heat by window free opening areas is also divided into rooms with cross-ventilation and those without:

The total free area is for the entire house including bedrooms.

Dynamic Thermal Modelling

The simplified method is the most straightforward and easiest method of calculation for Part O. However, not all builds and projects are straightforward, so in some cases, you must use dynamic thermal modelling.

In Part O Building Regulations, dynamic thermal modelling is defined as the “method of building modelling that predicts the internal conditions and energy demands of a building at short time intervals using weather data and building characteristics.”

Dynamic thermal modelling has the flexibility to calculate on a seasonal or hourly basis for tackling more complex build situations and offers greater design flexibility. Such as:

• Residential buildings with very high levels of insulation and airtightness
• Residential buildings with specific site conditions that mean the building is not well represented by the two locations
• Residential buildings that are highly shaded by neighbouring properties, structures or landscape

To demonstrate compliance using the dynamic thermal modelling method, all of the following guidance should be followed.

• CIBSE’s TM59 methodology for predicting overheating risk
• The limits on the use of CIBSE’s TM59 methodology
• The acceptable strategies for reducing overheating risk

For the full requirements of dynamic thermal modelling, we recommend reading Approved Document Part O.

What Does Part O Mean for Windows?

The most important part to note is that Part O only applies to new buildings. So when you replace your existing windows or add a new conservatory, you are not affected by this rule.

At this time, new extensions will not be impacted, so you can still have large windows in an extension to an existing property. However, it would be worth considering the regulations when designing your extension to create the most comfortable and energy-efficient living space.

For example, building an extension with large expanses of glass facing south and south-west could create an uninhabitable space in the summer months. Is this the best way to construct a living space?

However, in northern climates, the use of solar gain could be more beneficial to warm the property in cooler months balanced against the minimal possibility of overheating in summer.

Whatever your location in the UK, Part O is going to have an impact on the design of new houses. New building design will have to be more considerate of the size and position of windows.