Can Homes Stay Comfortable in a Warmer Climate Without Air Conditioning?
CIBSE TM59:2026, Overheating Risk in Dwellings: A Design Stage Methodology, is the updated technical guidance used across the UK building industry to assess overheating risk in residential developments. The updated methodology sends a clear message:
"Passive design should always be the first line of defence against overheating."
As the climate continues to warm, overheating has become one of the key design risks for residential developments. Rather than defaulting to mechanical cooling, TM59:2026 sets out the principle that well-designed buildings using effective passive design measures can, in many cases, maintain comfortable indoor environments through increasingly warm summers, while minimising operational energy use and carbon emissions.
The TM59:2026 Three-Stage Assessment Process
TM59:2026 introduces a three-stage assessment process for evaluating overheating risk:
Stage 1: Passive Design First
Assess the building without site-specific restrictions on natural ventilation (ignoring noise, security and outdoor air quality constraints). The design must first demonstrate that thermal comfort can be achieved through passive measures: solar shading, glazing optimisation, natural ventilation, thermal mass and building layout. Ceiling fans and heat recovery units providing background ventilation may also be considered as a low-energy comfort strategy.
Stage 2: Real-World Constraints
Apply realistic site constraints to window opening. Continue to optimise passive measures and introduce enhanced mechanical ventilation only where passive strategies alone cannot achieve compliance.
Stage 3: Mechanical Cooling
Mechanical cooling is considered only after passive design and ventilation strategies have been fully explored and demonstrated to be insufficient.
The Four TM59:2026 Overheating Assessment Criteria
TM59:2026 also sets out four overheating assessment criteria:
Adaptive comfort in naturally ventilated spaces
Sleeping comfort in bedrooms, using updated sleep research
Mechanically ventilated or cooled spaces
Communal circulation areas within residential buildings
Why This Matters for Design Teams
At reLoad, we see this update as a reminder that overheating assessments should be used to influence design, not simply to demonstrate compliance.
Our Passive Design Consulting Service works alongside design teams and clients from the earliest design stages to reduce overheating risk before it becomes a costly engineering problem. By combining passive design strategies with dynamic thermal modelling, we help project teams:
Optimise building orientation and massing
Refine glazing ratios and façade design
Design effective external shading
Maximise natural ventilation opportunities
Improve resilience to future climate conditions
Minimise reliance on mechanical cooling, reducing operational energy use, carbon emissions and long-term operating costs
The earlier passive design is considered, the greater the opportunity to create buildings that are comfortable, resilient and future-ready.
We're already applying these principles with clients across British Columbia, and we'd welcome the chance to do the same on your next residential project.