Part O – Overheating Calculations
Part O of the Building Regulations (introduced in 2021) aims to limit overheating in new residential buildings across England. It ensures that homes are designed to reduce the risk of excessive indoor temperatures during warm weather, protecting occupant comfort and health.
Why are the calculations needed?
Overheating calculations demonstrate that a building’s design can stay comfortable without relying heavily on air conditioning. With warmer summers becoming more common, well-insulated homes can trap heat, creating unsafe conditions, especially for vulnerable occupants. Assessing overheating at the design stage helps identify risks early so passive measures like shading, glazing control, and ventilation can be built in to improve comfort and energy efficiency.
Part O offers two compliance routes:
- Simplified Method – suitable for typical dwellings and layouts.
- Dynamic Thermal Modelling Method – used for complex or highly glazed designs.
Simplified Method
The simplified method checks compliance using limits for glazing and ventilation, based on risk level, orientation, and shading.
Key principles:
- Different rules apply for moderate and high-risk locations (e.g. London).
- Restricts glazing to reduce solar gain; shading or larger openings can increase limits.
Typical steps:
- Identify risk category.
- Record glazing orientation and area.
- Note shading and ventilation openings.
- Compare with tables in Approved Document O.
Example limits (indicative):
- Moderate risk, no shading – glazing ≤ 21% of floor area.
- High risk, with shading – glazing ≤ 13%.
If limits are exceeded, mitigation or dynamic modelling is required.
Dynamic Thermal Modelling
Used for complex designs, this route simulates indoor temperatures over a year using validated software (e.g. IESVE, TAS, Design Builder) and CIBSE DSY1 2020 weather data.
Compliance is assessed under CIBSE TM59, ensuring:
- Living spaces and bedrooms stay within temperature limits.
- Night-time comfort is maintained for healthy sleep.
A report must summarise inputs, assumptions, weather data, results, and recommended improvements.
Mitigation Strategies
- External shading (balconies, shutters, overhangs).
- Increased natural ventilation or dual-aspect layouts.
- Reflective finishes to reduce heat gain.
- MVHR systems with summer bypass.
- Layout design to minimise west-facing glazing.
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Frequently Asked Questions
To make things easier for you guys, we have a couple of common questions that we receive from clients.
Part O of the Building Regulations is designed to prevent overheating in new residential buildings. It ensures that homes are designed to stay comfortable during warmer weather, reducing reliance on energy-intensive cooling and improving occupant safety and wellbeing.
Overheating calculations assess whether a property is at risk of high indoor temperatures based on glazing, ventilation, shading and layout. They are required at the design stage of most new residential developments in England to prove compliance with Part O.
Part O offers two methods: the Simplified Method for typical layouts and risk levels, and Dynamic Thermal Modelling for complex, highly glazed or high-risk buildings. If a design cannot meet the simplified limits, modelling is usually needed.
Not always. Many standard homes can comply via the Simplified Method using limits for glazing and ventilation. More unusual designs, or properties in higher-risk locations, often require Dynamic Thermal Modelling to demonstrate compliance.
Common solutions include external shading, larger ventilation openings, dual-aspect layouts, reflective finishes, and MVHR with summer bypass. These passive strategies can significantly reduce solar gain and improve indoor comfort without increasing energy use.