What This Requirement Covers
Approved Document O (Overheating) came into force on 15 June 2022 alongside the 2021 Part L. It addresses the risk of excessive heat in new residential buildings, which has become a growing concern due to climate change, improved airtightness, and larger glazing areas. Part O applies to all new dwellings in England.
Key Requirements
Compliance Routes
There are two compliance routes
- Simplified method: Using the limiting parameters in Tables O1 and O2 for glazing, shading, and ventilation
- Dynamic thermal modelling: Using approved software (e.g., CIBSE TM59 methodology) to demonstrate that the dwelling will not overheat
Simplified Method
The simplified method limits
- Glazing area as a proportion of floor area for each room (Table O1): Varies by orientation, with south and west-facing rooms having lower permitted glazing ratios
- Shading: Fixed external shading devices, overhangs, or recessed windows reduce the effective solar gain
- Cross-ventilation: All bedrooms must have purge ventilation on at least two sides, or have equivalent mechanical cooling capacity
- Free area of opening windows: Minimum free areas are specified for each room type and orientation
Dynamic Thermal Modelling (TM59)
- The CIBSE TM59 methodology assesses overheating risk using hourly weather data for the building's location
- Two criteria must be met:
- The assessment must use the Design Summer Year weather file (DSY1) for the appropriate location
Noise Considerations
- In locations with high external noise (near roads, railways, or airports), opening windows for ventilation may not be practicable
- Part O requires that the overheating assessment accounts for noise constraints
- Where windows cannot be opened due to noise, mechanical ventilation or cooling must be provided
Practical Compliance Tips
- Assess overheating risk at the earliest design stage; changing window sizes, orientation, or shading after planning approval is difficult
- South and west-facing glazing is the primary driver of overheating; limit glazing ratios on these elevations
- External shading (brise-soleil, overhangs, external blinds) is far more effective than internal blinds
- Cross-ventilation is critical; single-aspect dwellings are much harder to keep cool
- Do not rely on mechanical cooling (air conditioning) as the first option; Part O favours passive design measures
- Consider the impact of climate change; current design weather files may underestimate future overheating risk
- Coordinate Part O compliance with Part L (energy efficiency); large windows improve daylighting and solar gain in winter but increase overheating risk in summer