TM59:2026 Explained: 10 Questions About the New CIBSE Overheating Guidance
As UK summers get hotter and periods of extreme heat increase in frequency, overheating is becoming one of the most important design considerations in residential developments. Homes are expected to be highly insulated and energy efficient, but they must also remain comfortable, healthy and resilient during warmer weather.
In response to this growing challenge, CIBSE has published TM59:2026 – Overheating Risk in Dwellings: A Design Stage Methodology, representing the first major update to the residential overheating assessment methodology since its original release in 2017. The revised guidance introduces a new assessment framework, updated climate data, revised thermal comfort criteria and a stronger emphasis on passive design solutions.
For architects, developers, planners, building services engineers and sustainability consultants, understanding these changes will be essential for designing compliant residential projects.
Below are the answers to the ten questions you may be asking:
1. Why Has TM59 Been Updated?
Since TM59 was first introduced in 2017, overheating assessments have become a standard part of residential design, particularly following the introduction of Approved Document O in England and Wales and equivalent requirements in Scotland. During this period, our understanding of overheating risk, occupant behaviour and climate change impacts has significantly improved.
TM59:2026 responds to:
2. What Are the Key Changes in TM59:2026?
TM59:2026 introduces several updates, which include:
3. How Has the Bedroom Overheating Criterion Changed?
Based on recent sleep research by Loughborough University, bedrooms will now be assessed on whether or not they exceed the relevant average temperature threshold on more than four nights between May 1st and September 30th. This replaces the previous methodology which counted the number of hours where a bedroom’s temperature exceeds 26°C.
Now, if the average temperature is above 26°C for Category 1 dwellings (temporary accommodation) or 27°C for Category 2 dwellings (resident accommodation) for more than four nights a year, the overheating assessment will fail.
4. What Is the New Three-Stage Assessment Methodology?
Stage 1: Assessment Using Passive Measures
The development must be assessed using passive measures only, without considering site-specific constraints like noise or security.
Stage 2: Assessment Incorporating Ventilation Constraints
The site-specific constraints on window openings are introduced and ceiling fans and mechanical ventilation may be considered, if necessary.
Stage 3: Assessment Including Cooling Systems
Mechanical cooling may finally be introduced but only if compliance cannot be achieved at the previous two stages.
5. Which Weather Files are Required for TM59:2026?
TM59:2026 methodology now adopts weather data representing future climate conditions based on the 2050s climate scenario, reflecting the expectation that homes designed today will remain in use for many decades and must be resilient to a warmer future climate.
This represents a notable shift from assessing overheating risk using primarily current climate conditions and places greater emphasis on future-proofing residential developments against climate change.
For designers and developers, achieving TM59 compliance may become more challenging, particularly for highly glazed developments, single-aspect apartments and schemes with constrained natural ventilation. Early consideration of shading, façade design, glazing specifications and passive cooling strategies is therefore becoming increasingly important.
6. Are Ceiling Fans Now Permitted Under TM59:2026?
Yes, TM59:2026 now formally recognises ceiling fans as a legitimate overheating mitigation strategy when installed as part of the base build or major retrofit.
The guidance allows temperature threshold adjustments under certain criteria where sufficient air movement is achieved. Ceiling fans can therefore provide a low-energy alternative to active cooling while improving occupant comfort.
As overheating risks increase, we expect ceiling fans to become a more common feature of residential developments, particularly where cooling systems are undesirable or impractical.
7. How Does TM59:2026 Assess Noise, Security and Air Quality Constraints?
TM59:2026 provides much clearer guidance on constrained ventilation. Where windows or ventilation devices can operate for at least 50% of occupied hours between May and September, spaces may still be assessed using the natural ventilation criteria. Where constraints prevent ventilation openings from being used for 50% or more of occupied hours, the mechanically ventilated criteria must be applied instead.
This is particularly important for developments located near:
8. How are Home Offices Included?
TM59:2026 introduces dedicated occupancy profiles and assessment criteria for home offices.
This reflects the long-term shift towards hybrid working and ensures spaces intended for home working are appropriately assessed for overheating risk.
For many residential developments, this may require additional consideration during the thermal modelling process and could influence early-stage design decisions.
9. How are Corridors and Communal Spaces Now Assessed?
Corridors and communal areas are now assessed against a dedicated overheating criterion (Criterion D), ensuring that thermal comfort is considered beyond the dwelling itself. This is particularly relevant for apartment developments, build-to-rent schemes, student accommodation and care facilities.
For high-density residential developments, designers may need to give greater consideration to corridor ventilation strategies and heat build-up within communal areas at an earlier stage of the design process.
10. How Does TM59:2026 Relate to Approved Document O?
At the time of writing, Approved Document O continues to reference TM59:2017. While TM59:2026 provides the latest CIBSE methodology and industry best practice, further guidance is expected regarding future adoption within the regulatory framework.
For projects currently at planning or design stage, there may be value in reviewing performance against both TM59:2017 and TM59:2026. This is particularly relevant for larger or longer-term developments, where evolving industry expectations may become increasingly important during the project lifecycle.
Need Help with TM59 Assessments?
Whether you’re designing a single residential development or a large multi-unit scheme, our team are here to discuss how TM59:2026 could impact your development and how overheating risks can be addressed through effective design, modelling and compliance strategies.