Overheating in new homes: understanding Approved Document O

What it is

Approved Document O is guidance for meeting a Building Regulations requirement (Part O) that new residential buildings in England must be designed and built so that unreasonable overheating is limited in summer. It applies to new homes and some other residential buildings, and has different rules for other work. It came into force in England in 2022.

The principle

The requirement has two linked aims:

1. Limit unwanted solar heat gains into the building in summer.
2. Provide a means of removing excess heat from the indoor environment.

The guidance also says that designers should give priority to passive measures, rather than relying on mechanical cooling.

It applies to new homes and some other residential buildings, and has different rules for other work.

Ways to comply

Broadly, there are two routes:

• A simplified method, using tables that depend on the location, the amount of glazing, the orientation, the shading and the ventilation openings. It is a practical route for many homes, with limits on what it can be used for.
• A dynamic thermal modelling method, using a recognised calculation to predict indoor temperatures against set criteria. It is often needed for more complex or more highly glazed homes.

What helps

• Shade. External shading, such as overhangs, fins, shutters and trees, is the most effective way to reduce solar gain. See Designing for a hotter future.
• Glazing choices. A suitable amount of glazing, with appropriate glass and orientation. See Windows and glazing.
• Ventilation openings. Large, secure openings that let air through, ideally across the home (cross-ventilation) and at different heights.
• Thermal mass, where it can be used with night cooling.
• Layout. Putting bedrooms where they stay cooler, and avoiding large west-facing glazing.
• Reducing internal heat, such as from pipes and equipment.

Practical issues

• Security and safety. Windows that open for ventilation should be secure, and for upper floors safe. The guidance deals with security and noise, so that people can keep windows open at night.
• Noise and pollution. Where outside noise or air quality prevents open windows, other measures may be needed.
• Mechanical cooling is not the preferred first step, though it may be part of some solutions.

Existing homes

The requirement applies mainly to new residential buildings and certain changes. But the thinking, shade, ventilation and reducing heat gains, applies to every home.

Why it matters

Hotter summers are a growing concern. Designing for them now protects the people who will live in the homes for decades. See also Airtightness and ventilation.

In short

Shade the glass, provide good ventilation, avoid reliance on machines, and check the current rules with a qualified assessor.

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