Part O Overheating Calculations

Part O (Overheating) is a mandatory building regulation in England and Wales designed to protect occupant health and comfort by limiting unwanted solar heat gains and ensuring adequate excess heat removal.

A Part O assessment is required for all new-build residential developments – including single houses, multi-unit apartment blocks, student accommodation, and residential care homes. It is also required for major conversions or changes of use that create new residential dwellings. Assessments should be conducted during early architectural design (RIBA Stages 2-3) to ensure building envelope choices comply before construction begins.

Approved Document O outlines two compliance routes:

– The Simplified Method (“Route A”): A prescriptive, table-based assessment that sets strict limits on maximum glazing areas and minimum openable window areas based on location (High Risk vs. Moderate Risk zones) and ventilation strategy. While quick and low-cost upfront, it is very rigid and often forces unwanted architectural compromises, such as smaller windows or mandatory external shutters.

– Dynamic Thermal Modelling / CIBSE TM59 (“Route B”): A sophisticated 3D software simulation using regional weather data and occupant usage profiles. It accurately predicts room-by-room temperatures throughout the year. While it requires deeper analysis, TM59 provides far greater design flexibility and frequently proves compliance where the Simplified Method fails.

The Simplified Method assumes ideal conditions – meaning windows can be wide open whenever internal temperatures rise. However, Approved Document O explicitly restricts openable windows if there are external constraints:

– Acoustics: Windows cannot be relied upon at night if external noise levels exceed statutory limits.

– Security: Ground-floor and easily accessible windows cannot be left open overnight due to PAS 24 security risks.

– Pollution & Safety: High external air pollution or fall risks limit window openings.

When these acoustic or security constraints apply, the Simplified Method almost always fails. CIBSE TM59 Dynamic Thermal Modelling becomes essential to prove that the building remains safe and comfortable without relying on open windows.

We do not simply issue a failing report. If your model exceeds temperature thresholds, we use our thermal simulation software to evaluate practical, cost-effective mitigation options following a passive-first hierarchy:

1. Solar Gain Reduction: Optimising glazing specifications (lowering solar g-values) or adjusting window geometries.

2. Shading Strategies: Adding fixed external shading, brise soleil or structural overhangs on southern and western facades.

3. Ventilation Optimisation: Improving trickle vent capacities, optimising window opening angles or introducing night-time purge ventilation.

4. Mechanical Intervention: Only if passive measures cannot resolve overheating risk (for example, in high-noise urban apartment blocks) do we model mechanical ventilation or cooling options as a last resort.

While both rely on Dynamic Thermal Simulation (DTS), they apply to different building sectors:

– CIBSE TM59 is designed specifically for residential properties (dwellings, flats, student accommodation) and forms the methodology for Part O Route B compliance.

– CIBSE TM52 is designed for non-domestic commercial properties (offices, schools, care homes, community buildings) and is used for BREEAM and commercial planning applications.

CIBSE published TM59:2026, updating the dynamic thermal modeling framework to account for future climate predictions (2050s weather data), revised bedroom comfort criteria and an updated staged assessment process.

Currently, Approved Document O formally references TM59:2017 as the statutory compliance standard. However, forward-thinking developers and local planning authorities are increasingly looking toward TM59:2026 to future-proof assets against long-term climate change. We can model your scheme against current statutory requirements while providing future-risk sensitivity analysis where required.

To provide an accurate rate card quote and select the correct compliance route, we typically require:

– Architectural floor plans, elevations, and building sections.
– The site location and orientation.
– Any acoustic reports or site constraint notes (to check for noise or security restrictions on openable windows).
– The proposed ventilation approach (natural vs. mechanical extract).