Tackling overheating in UK homes: a guide to Approved Document O
Approved Document O, effective from 15 June 2022, sets standards to limit overheating in new UK homes through passive cooling, controlled solar gain and ventilation, with compliance via the Simplified Method or Dynamic Thermal Modelling.
UK homes are getting hotter, and the problem is not only comfort. A house that overheats disrupts sleep, and sustained poor sleep has real health consequences for the people living there. Approved Document O came into force on 15 June 2022 and sets standards intended to keep new homes below that point.
Design requirements
Approved Document O asks you to limit the heat coming in through the windows and to provide enough ventilation to get the excess back out. It treats high-risk and moderate locations differently. Two constraints shape most designs:
- Noise and air quality: ventilation solutions must account for night-time noise and local air pollution, so windows can be opened without compromising comfort or health.
- Security and safety: measures must address potential security risks and safety concerns such as falls or entrapment.
Mechanical cooling
The regulations allow mechanical cooling, but only after the passive options have been shown to fall short. Ventilation and shading come first; cooling plant is what you justify once those cannot do the job on their own.
Early design decisions
Overheating risk is best assessed at the earliest design stages. Building orientation, window size and placement, and shading are all far easier to change on paper than on site.
Compliance pathways
There are two main compliance options:
- Simplified Method: provides specific design limits set out in Approved Document O.
- Dynamic Thermal Modelling: based on CIBSE TM59, this offers more flexibility but requires advanced software and expertise.
Strategies for passive cooling
Keeping a house cool without plant comes down to two jobs: letting less heat in, and getting the rest back out.
- Less heat in: orientation, glazing area, low-gain glass, deeper window reveals, external shading such as awnings or shutters, and whatever the trees and neighbouring buildings already shade for you.
- More heat out: natural cross-ventilation, and controlling the heat the building makes itself, which matters most where communal heating pipework runs through the dwellings.
Compliance and reporting
A passing result on its own is not enough. Building control also expects the paperwork behind it:
- Each new home has to pass the overheating assessment, reported to local building control using the checklist at Appendix B.
- The route you chose and the reasoning behind it must be recorded, because building control will ask how the decision was made.
- The homeowner has to be given instructions on operating and maintaining whatever keeps the house cool.