CIBSE Publishes Updated TM59 Overheating Methodology
CIBSE’s updated TM59 methodology reinforces the need to design homes that stay comfortable, healthy and resilient during periods of extreme heat.
The Chartered Institution of Building Services Engineers (CIBSE) has published TM59:2026 – Overheating Risk in Dwellings: A Design Stage Methodology, the first major update to the industry-standard overheating assessment methodology since it was first released in 2017.
TM59 provides a standardised approach for using dynamic thermal modelling (DTM) to assess whether a dwelling is likely to experience excessive indoor temperatures during warmer weather.
The update comes at an important time for the built environment. As the UK experiences more frequent periods of high temperatures, overheating is becoming a growing consideration for homes, occupants and the wider housing sector. Recent heatwaves have shown how quickly indoor conditions can become uncomfortable, and in some cases unhealthy, particularly where homes are not designed to manage heat effectively. As the climate continues to change, designing homes that remain comfortable in summer is becoming just as important as ensuring they stay warm and energy efficient in winter.
Why Overheating Matters
Overheating is no longer just a future risk. Longer periods of high temperatures, combined with warmer nights, are already affecting homes, public services, infrastructure and economic activity across the UK. For occupants, prolonged exposure to high indoor temperatures can have a significant impact on comfort, wellbeing, productivity and sleep quality. This is particularly challenging for vulnerable people, who may face greater health risks during periods of excess heat.
The issue is especially relevant in modern homes. Higher levels of insulation, improved airtightness, large areas of glazing and limited opportunities for natural ventilation can all help improve energy efficiency, but also increase overheating risk if not considered properly at the design stage.
This is where dynamic thermal modelling and the CIBSE TM59 methodology play a vital role. By simulating how homes are likely to perform during warmer weather conditions, assessors can identify overheating risks before construction and support developers in putting appropriate mitigation measures in place.
What Has Changed in TM59:2026?
The updated methodology introduces several important changes shaped by industry feedback, practical experience and recent research.
New Bedroom Assessment Criteria
One of the most notable changes in TM59:2026 relates to how overheating is assessed in bedrooms.
Under the previous methodology, bedroom overheating was assessed based on the number of hours the room exceeded a specified temperature during sleeping hours. TM59:2026 introduces a new approach based on mean night-time temperatures, reflecting the latest research into sleep quality and the impact of heat on occupants overnight.
CIBSE states that this change is intended to provide a more realistic assessment of sleeping comfort and how people experience overheating during the night.
New Three-Stage Assessment Process
TM59:2026 introduces a formal three-stage process for assessing overheating risk:
Stage 1 – Considers passive design measures only
Stage 2 – Takes account of local constraints, such as noise, security and air quality restrictions
Stage 3 – Allows for Mechanical cooling where required
This structure reinforces the principle that passive measures should be considered first, with mechanical cooling used only where it is necessary.
Updated Weather Files
TM59:2026 now references a dedicated weather file framework based on the latest climate projections.
This helps modernise the assessment process and places greater focus on how homes are likely to perform in future climate conditions, rather than relying only on today’s weather patterns.
More Detailed Real-World Modelling
TM59:2026 provides expanded guidance on practical issues that can affect overheating risk in real homes, including:
- Noise restrictions
- Security concerns
- Air quality limitations
- Window opening constraints
These factors can have a significant influence on ventilation strategies and how occupants are able to use their homes during periods of hot weather. By recognising these real-world constraints, the updated methodology supports a more practical and realistic assessment of overheating risk.
Recognition of Passive Cooling Measures
TM59:2026 provides clearer recognition to measures that can improve thermal comfort without significantly increasing energy use.
This includes updated guidance on the use of ceiling fans, alongside continued emphasis on passive cooling measures such as solar shading and natural ventilation.
As the industry works to reduce overheating risk while also limiting operational energy consumption, these low-energy solutions are likely to play an increasingly important role in future home design.
What This Means for the Industry
The publication of TM59:2026 provides an important update to the industry’s approach to assessing overheating risk in dwellings. However, there are still some key questions for the sector, particularly around how and when the updated methodology may be adopted for regulatory compliance.
At the time of writing, there is no formal guidance on:
- Whether Part O compliance should continue to be demonstrated using TM59:2017
- Whether TM59:2026 can be used voluntarily for compliance assessments
- Whether transitional arrangements will be introduced
- How quickly software providers will incorporate the updated methodology
- When training courses and competency schemes will be updated to reflect the changes
We do know that Government is currently undertaking a full review of Approved Document O, following industry feedback received through the Future Homes Standard consultation. It is likely that many of the changes introduced in TM59:2026 will help inform the future direction of Part O, but no implementation timetable has yet been published.
What This Means for Assessors
For assessors, the main question is how this affects compliance. The current version of Approved Document O references TM59:2017 as the dynamic thermal modelling route for demonstrating compliance. Until further clarification is provided, assessors should continue to follow existing regulatory requirements while taking time to familiarise themselves with the updates introduced in TM59:2026.
Read the Updated TM59:2026 Methodology
TM59:2026 – Overheating Risk in Dwellings: A Design Stage Methodology, sets out the updated approach to assessing overheating risk in homes using dynamic thermal modelling.
Assessors, designers and housing professionals are encouraged to review the updated methodology and familiarise themselves with the key changes.
Looking Ahead
What is clear is that overheating is becoming an increasingly important consideration in the design of new homes. Recent heatwaves have shown that the risks associated with excess heat are no longer theoretical; they are already affecting occupant comfort, health, and wellbeing across the UK.
TM59:2026 represents the next step in the industry’s understanding of overheating risk and reflects the growing need to design homes that are resilient to a warming climate. While questions remain around implementation and regulatory adoption, the update signals a continued shift in how overheating is assessed and managed.
As heatwaves become more frequent and intense, the focus is moving beyond whether overheating should be considered, to how homes can be designed to remain comfortable, healthy and resilient for years to come.
Expand your Skills into Overheating Risk Assessment
Elmhurst’s Overheating Risk Assessment training supports assessors looking to carry out assessments for new build properties using the detailed route for demonstrating compliance with Part O.
With training routes available for both IES and DesignBuilder, the course helps you build the knowledge needed to assess overheating risk, support clients with compliance, and offer this service alongside SAP calculations.
FIND OUT MORE
Further Information and Sources:
https://www.gov.uk/government/publications/overheating-approved-document-o