Richard Tibenham, business development manager at global climate tech firm IES, discusses overheating, how it is becoming more commonplace, and CIBSE TM59’s role in dealing with it
After a summer of regular heatwaves across the UK, overheating remains firmly on the agenda for developers, housing providers, local authorities, and building owners. While individual years will inevitably vary, the long-term trend is clear: average summer temperatures are rising, and periods of prolonged hot weather are becoming more common than they were when much of today’s housing stock was originally designed.
The result is a growing need to ensure that buildings remain comfortable, healthy and resilient under a wider range of operating conditions. The updated CIBSE TM59 methodology reflects a growing focus on designing homes that remain comfortable in a warmer climate.
This presents an important challenge for the built environment sector. Homes and residential buildings being designed today are expected to remain in service for at least the next 60 years and, in many cases, substantially longer. Decisions made during design and retrofit projects today will therefore influence occupant comfort, energy use and resilience for generations of building users. As future climate conditions continue to evolve, overheating assessment is increasingly viewed as a fundamental building performance consideration rather than simply a compliance exercise.
For the social housing sector, the importance of designing and maintaining homes that remain resilient to future summer temperatures continues to grow. Housing providers are already facing increased scrutiny around the quality, safety and condition of homes, and there is a growing recognition across the sector that overheating can have significant implications for resident health, wellbeing and building performance. As regulatory expectations continue to evolve, many landlords are recognising the value of proactively understanding and mitigating overheating risks as part of their long-term asset management and investment strategies.
Against this backdrop, the publication of CIBSE TM59:2026: Overheating Risk in Dwellings represents the most significant update to the UK’s residential overheating assessment methodology since the original standard was introduced in 2017. Developed in response to industry feedback, accumulated operational evidence and evolving climate projections, the updated standard aims to improve how overheating risk is assessed and mitigated in residential buildings
TM59 remains the industry’s principal methodology for assessing overheating risk through dynamic thermal simulation and continues to provide the framework that underpins overheating assessments for new homes and major residential retrofit projects. The guidance is intended to support the design of homes that remain comfortable in increasingly warm summer conditions while prioritising passive design approaches and minimising reliance on mechanical cooling
What are the Key Changes to the TM59 Guidance?
While the overall objective remains unchanged, TM59:2026 introduces several important updates. Most notably, the standard includes a revised assessment methodology for bedrooms during sleeping hours. Based on new research into sleep quality and thermal comfort, the updated criterion now evaluates the number of nights where mean bedroom temperatures exceed defined thresholds, replacing the previous focus on individual hourly exceedances. The underlying research found that acceptable sleeping temperatures are generally higher than those assumed within the 2017 methodology
Other key updates include:
- Updated weather file requirements aligned with the latest UK climate change projections.
- A new three-stage assessment methodology that reinforces a passive-first design philosophy.
- Clarified treatment of projects where natural ventilation is constrained by noise, security or air-quality considerations.
- Explicit recognition of ceiling fans as a low-energy overheating mitigation strategy.
- New modelling assumptions and internal gain profiles for home office spaces.
- Clearer guidance on assessment criteria for mechanically ventilated and communal spaces.
Collectively, these changes provide a more robust and realistic assessment of overheating risk while reflecting the way residential buildings are designed and occupied today.
Why is TM59:2026 important?
Overheating is emerging as one of the most important building performance challenges facing the residential sector. Rising summer temperatures, increasingly airtight construction, highly glazed façades and urban heat island effects all increase the risk of excessive indoor temperatures.
TM59:2026 provides designers with a structured methodology to identify overheating risk during design rather than after occupation, allowing cost-effective mitigation measures to be incorporated before construction is complete. By undertaking analysis at an earlier stage, project teams can evaluate alternative design solutions and understand the implications of key decisions before they become costly or difficult to change
Importantly, the revised standard extends beyond new-build developments. TM59:2026 specifically highlights its application to major retrofit projects and recommends overheating assessments as a means of ensuring energy-efficiency upgrades do not inadvertently worsen thermal comfort. The guidance also supports pre- and post-intervention assessments, enabling design teams to understand how retrofit measures affect occupant comfort and resilience
The methodology continues to reinforce best practice through its emphasis on passive design measures such as shading, natural ventilation, thermal mass and façade optimisation before active cooling solutions are considered. This aligns overheating mitigation with wider energy efficiency, net zero and resilience objectives
Regulatory considerations
It’s important to note that the current UK Building Regulations framework, including Approved Document O in England and equivalent overheating guidance elsewhere in the UK, continues to reference TM59:2017 as the recognised dynamic thermal modelling methodology. While TM59:2026 has now been published, there has been no formal confirmation regarding whether, when, or how the updated methodology may be adopted within future revisions to Building Regulations. Industry commentary from CIBSE has indicated that any future regulatory adoption of TM59:2026 will be a matter for government and regulatory authorities rather than CIBSE itself. Project teams should therefore continue to confirm the applicable compliance methodology with planning authorities, building control bodies and clients on a project-by-project basis
Beyond Compliance: designing for future performance
While compliance with Approved Document O remains a statutory requirement, many clients are increasingly looking beyond minimum regulatory requirements and focusing on how buildings will perform throughout their operational life.
The current Part O framework was developed around the requirements and assumptions available at the time of publication and continues to reference TM59:2017 as the recognised dynamic thermal modelling methodology. By contrast, TM59:2026 incorporates updated climate projections, revised comfort criteria, a clearer assessment methodology for constrained ventilation scenarios, and a more structured approach to passive-first design
As a result, TM59:2026 can provide project teams with a more comprehensive understanding of overheating risk and the effectiveness of mitigation measures under future operating conditions. The methodology places greater emphasis on evaluating passive design solutions, understanding the impact of real-world constraints, and reducing unnecessary reliance on mechanical cooling systems
For developers seeking to deliver resilient, low-energy buildings that remain comfortable for decades to come, adoption of TM59:2026 can provide valuable insight beyond regulatory compliance. The methodology supports a broader building performance perspective, helping design teams balance thermal comfort, overheating resilience, energy demand and occupant wellbeing from the earliest stages of design
Implications for Design Teams
One of the most significant practical changes is the introduction of the new three-stage assessment process.
The methodology now requires designers to:
- Demonstrate compliance using passive measures only.
- Assess the impact of real-world constraints such as noise, security and air-quality limitations on natural ventilation.
- Introduce enhanced mechanical ventilation and, where necessary, cooling strategies if compliance cannot be achieved through passive measures alone.
This approach provides a more rigorous and realistic understanding of building performance but may also require additional modelling iterations, greater coordination between disciplines and increased simulation effort compared with assessments undertaken using TM59:2017. Design teams should therefore consider the programme and resource implications when planning projects using the new standard.
However, this additional effort should be viewed positively. The enhanced methodology provides a more thorough assessment of how buildings will perform under real operating conditions, helping project teams make better-informed design decisions and reducing the likelihood of costly overheating mitigation measures being required once in use.
