We used dynamic thermal modelling to help a detached bungalow in Somerset achieve Part O compliance, effectively addressing overheating risks and providing regulatory peace of mind.
Updated July 2026: This assessment was carried out under TM59:2017. CIBSE’s new TM59:2026 methodology introduces 2050s climate data and a more realistic, partially constrained ventilation approach – both directly relevant to projects like this one, where security concerns limit night-time window opening. Read our full breakdown of what’s changed in TM59:2026.
Introduced in June 2022, Part O of the Building Regulations focuses on minimising overheating risks in new dwellings, ensuring comfortable living spaces. Compliance can be particularly challenging for unique or complex projects where standard assessment methods may fall short.
This case study highlights a challenge faced by a regional housebuilder with a development in Somerset. While most of the development was made up of 4-bedroom houses which passed the simplified Part O assessment, one 3-bedroom bungalow constructed with a timber frame and its own heating system failed to comply. Dynamic thermal modelling using CIBSE TM59:2017 was key to resolving this issue.
The Challenge
Bungalows present unique challenges when it comes to Part O compliance due to their design constraints. Since all bedrooms typically qualify as “easily accessible,” night-time window opening for ventilation can raise security concerns. This often results in failing the simplified Part O assessment, even if the design itself effectively mitigates overheating risk, as was the case here.
Our Solution: Dynamic Thermal Modelling
We recommended dynamic thermal modelling, an approach that involves creating a digital model of the building. This method considers various factors that influence thermal performance, including:
- Location
- Orientation
- Construction materials
- Occupancy patterns
This detailed analysis provides a more nuanced assessment of overheating risk compared to the simplified assessment method.
The Dynamic Thermal Modelling Process
We used DesignBuilder software to stimulate the bungalow’s thermal performance, incorporating:
- Occupancy, equipment, and lighting gains were applied based on TM59:2017 protocols, the recognised industry standard for modelling thermal comfort at the time of assessment. These gains reflect typical usage patterns to ensure accuracy.
- The house aimed for an air permeability of 4m³/hr/m²@50Pa, ensuring minimal uncontrolled air leakage and aligning with Approved Document F (2021) guidelines.
- Background ventilation rates were set following Approved Document F guidelines, ensuring good indoor air quality and occupant comfort.
- Window opening patterns were set to reflect the security concerns, limiting night-time window opening. No blinds or curtains were included in the initial assessment.
- Weather data aligned with TM59 guidance, utilising the CIBSE DSY1 weather file for Cardiff, incorporating climate data specific to the area, including temperature, humidity, and solar radiation.
- Surrounding structures providing shading to the bungalow were also modelled. This ensures a comprehensive assessment of the shading effect from nearby buildings on the overall thermal performance of the dwelling.
Initial Results and Mitigation Strategies
Our initial analysis identified overheating risks in the bedrooms, kitchen, and living room. To address these, we collaborated with the client to explore mitigation options:
- Initially, we investigated lowering the window G-value (solar heat gain coefficient). While this helped some rooms pass the overheating risk criteria, the kitchen remained at risk.
- To avoid compromising aesthetics and the SAP rating, we implemented strategic shading solutions:
- 0.5m overhang and side fins were added to the kitchen and living room bi-folding doors to effectively block sunlight during peak hours, reducing solar heat gain and mitigating the risk of overheating.
- Deeper window reveals at 100mm were incorporated to provide additional shading for the bedroom windows. This passive shading technique helps prevent direct sunlight from entering, thereby reducing heat gain and improving comfort.
The client favoured these shading measures for their minimal impact on the building’s overall design and cost-effectiveness compared to altering window designs or introducing mechanical ventilation.
Final Outcomes
With these solutions in place, all rooms met Part O criteria, ensuring year-round comfort for future occupants. Key outcomes included:
- Compliance with Part O: Avoiding potential delays or penalties, providing peace of mind, and meeting building standards.
- Long-term Thermal Performance: Ensuring sustainable comfort through dynamic modelling.
Advantages of Dynamic Thermal Modelling for Complex Projects
This project highlights the benefits of dynamic thermal modelling for complex projects:
- Enhanced Accuracy: Overcomes the limitations of the simplified method, especially when security limits night-time ventilation.
- Detailed Risk Assessment: Offers a nuanced understanding of overheating risks within the dwelling.
- Informed Mitigation Strategies: Enables exploration of effective solutions without compromising design or energy efficiency.
Our Lead Sustainability Consultant shares his insights on why this approach was essential:
Dynamic Thermal modeling provides a powerful tool to ensure that the homes we built today are not only energy efficient but are also prepared to deal with the future challenges of changing climates. By using advanced software to allow for design flexibility, and the use of glazing properties and shading that is not used in the simplified overheating method.
Richard Millard, Lead Sustainability Consultant at Building Energy Experts
Would This Project Pass Under TM59:2026?
Bungalows like this one – where night-time security concerns restrict window opening – sit exactly in the territory TM59:2026 was written to address more realistically. Under the old methodology, ventilation was effectively assumed fully open or fully closed; under TM59:2026’s partially constrained approach, windows can now be assumed open 50% of the time under these restrictions, alongside future 2050s climate data. Projects of this type assessed today may need re-testing to confirm whether the same mitigation measures still hold up.
What’s Next?
Don’t let overheating risks delay your project. Whether you’re an architect planning a new development or a house builder seeking to ensure compliance, our team is here to offer tailored insights to ensure compliance and comfort – under TM59:2017 or the new TM59:2026 methodology. Contact us today to discuss your project’s unique requirements and find out if dynamic thermal modelling can benefit your next development.
