Managing Overheating Risk in Energy-Efficient Homes
Highly insulated, airtight FHS homes face increased overheating risk. Part O of the Building Regulations addresses this through the simplified and dynamic assessment methods. The government is planning a full technical review of Approved Document O.
Part O Background
Part O was introduced in December 2021 as a new requirement to mitigate overheating risk in new residential buildings. It sets limits on internal temperatures: living rooms must not exceed 28°C for more than 3% of occupied hours, and bedrooms must not exceed 26°C for more than 1% of occupied hours. Two compliance routes exist: a simplified method and a dynamic thermal modelling method.
Simplified Method Issues
Consultation feedback identified the simplified method as simultaneously too complex and too restrictive. It does not adequately account for thermal mass, passive ventilation options, internal shading, or the cumulative effect of multiple passive cooling strategies. Many respondents reported that developments failed using the simplified method but passed with dynamic modelling without any design changes.
Dynamic Modelling Challenges
Concerns were raised about inconsistencies in results across different assessors and software packages. Passive cooling strategies and natural ventilation are not well modelled. Several respondents called for alignment with CIBSE TM59 guidance, integration with HEM, and standardised reporting requirements. The interaction between noise limitations and window opening assumptions was also flagged.
Conflicts with Other Regulations
Part O requirements can conflict with other Building Regulation requirements: Part Q (window security) on ground floor bedrooms, Part K (guarding heights) for openable windows, Part B (fire escape window sizes), and Part F (purge ventilation). The government acknowledges these conflicts and will address them in the planned technical review.
Planned Technical Review
The government will proceed with a full technical review of Approved Document O, considering: adoption of updated CIBSE TM59 for the dynamic method, improvements to the simplified method, review of noise and security guidance, overlaps with other Building Regulations, weather file updates, and potential extension to MCU dwellings. The Building Safety Regulator is also considering actions to address common issues.
Window Guarding Research
HSE research concluded that a guarding height of 1100mm is protective against tipping risks for practically the entire adult British population, whereas 800mm protects less than 1%. This evidence supports the Part O requirements but has raised concerns about impacts on building aesthetics, wheelchair access to external views, and design flexibility.