Delivering CIBSE TM59 dynamic thermal modelling to secure Part O compliance where security concerns ruled out standard night-time ventilation.
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.
Project Overview
- Client: Undisclosed (regional housebuilder)
- Sector: Private Residential – New Build
- Location: Somerset
- Services Provided: Part O Overheating Compliance, CIBSE TM59 Dynamic Thermal Modelling
- Scale: Single 3-bedroom timber-frame bungalow, part of a wider housing development
- Target: Approved Document Part O compliance (assessed under TM59:2017)
The Challenge
Bungalows present a specific problem for Part O compliance: because every bedroom typically counts as “easily accessible” from ground level, opening windows at night for ventilation raises real security concerns. On this Somerset development, the wider scheme of four-bedroom houses passed the simplified Part O assessment without issue – but one 3-bedroom, timber-frame bungalow with its own independent heating system failed outright. The design itself wasn’t the problem; the simplified method’s inability to account for restricted night-time ventilation was.
The Solution
CIBSE TM59 Dynamic Thermal Modelling
Rather than accept the simplified method’s verdict, we built a detailed DesignBuilder model of the bungalow, incorporating its location, orientation, construction, and realistic occupancy patterns – modelling window opening restricted to reflect the genuine security constraint, rather than assuming free ventilation the client wouldn’t actually use. Initial results confirmed overheating risk in the bedrooms, kitchen, and living room. Lowering the glazing G-value resolved some of the risk, but the kitchen remained a problem – and rather than compromise the design or the SAP rating to fix it, we turned to passive shading instead: a modest overhang and side fins on the kitchen and living room bi-folding doors to block peak-hour sun, and deeper window reveals on the bedrooms to reduce solar gain without altering the windows themselves. The client preferred this route precisely because it left the building’s appearance and cost largely untouched, compared with redesigning the glazing or adding mechanical ventilation.
The Outcome
With shading measures in place, every room in the bungalow met Part O criteria – turning a design that had failed a blanket assessment into one with genuine, evidenced year-round comfort for its future occupants. It’s a clear example of what dynamic modelling can do that the simplified method can’t: distinguish between a design that’s genuinely at risk of overheating and one that simply doesn’t fit the simplified method’s assumptions about how residents will actually use their windows.
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
July 2026 Update: 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.
