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As overheating risk continues to rise up the building design agenda, CIBSE has introduced a substantially revised second edition of TM59, its standardised methodology for predicting and assessing overheating risk in homes and residential buildings. Now titled Overheating Risk in Dwellings: A Design Stage Methodology, the 2026 edition reflects nearly a decade of industry learning, evolving climate projections and changes to UK building regulations.

The updated guidance represents far more than a routine refresh. It introduces a new assessment framework, more detailed modelling requirements and a stronger emphasis on passive design measures, helping project teams create more resilient and comfortable homes.

A New Passive-First Approach

One of the most fundamental changes within TM59:2026 is the introduction of a formal three-stage assessment methodology. The 2017 edition relied on a relatively straightforward pass/fail assessment against overheating criteria. In contrast, the new edition adopts a structured hierarchy that prioritises passive design solutions before introducing mechanical intervention.

Every dwelling must first be assessed under an unconstrained passive design scenario (Stage 1). Where site-specific issues such as external noise, poor air quality or security concerns restrict natural ventilation, the assessment progresses to Stage 2, which allows enhanced mechanical ventilation strategies. Mechanical cooling is only considered at Stage 3 and is intended as a last resort. This approach aligns closely with the Greater London Authority's Energy Assessment Guidance (2022) and cooling hierarchy to reinforce the industry's move toward low-energy design solutions.

Four Formal Assessment Criteria

The assessment methodology has also been expanded. While the 2017 edition focused primarily on two criteria for naturally ventilated homes, TM59:2026 establishes four distinct criteria covering different building spaces and ventilation strategies.

Criterion A assesses daytime comfort in naturally ventilated living rooms, kitchens and bedrooms, and now home offices, and remains broadly consistent with the adaptive comfort approach used previously. Criterion C formalises the requirements for mechanically ventilated or cooled spaces, introducing a dedicated compliance route with temperature limits that were previously referenced through CIBSE Guide A rather than TM59 itself.

Perhaps most notably, communal circulation areas are now assessed under a new Criterion D. In the 2017 edition, overheating within corridors and communal spaces was highlighted as a potential risk but did not constitute a failure. The new guidance makes this a mandatory element of compliance, requiring communal areas to remain below 28°C for all but 3% of occupied hours.

A Major Revision to Bedroom Assessments

The most significant technical change within TM59:2026 relates to the bedroom night-time assessment. Research published since the original TM59 was released has improved understanding of the relationship between indoor temperatures and sleep quality, leading to a complete revision of Criterion B.

Under the previous methodology, bedrooms failed if temperatures exceeded 26°C for more than 1% of annual sleeping hours. The 2026 edition moves away from this simple hourly exceedance approach and instead evaluates the number of overheated nights during the summer period.

Sleeping hours have been revised from 10pm to 7am to 11pm to 8am, and the assessment is now based on mean operative temperature. Bedrooms are allowed a maximum of four overheated nights between May and September, with temperature thresholds varying depending on occupant category.

This change is intended to provide a more realistic indication of actual sleep disruption and better reflect the latest academic evidence relating to overheating and occupant wellbeing.

Ceiling Fans Become a Recognised Mitigation Strategy

Another notable development is the expanded treatment of ceiling fans. Although fan-assisted cooling was acknowledged briefly within the 2017 guidance, TM59:2026 provides a detailed methodology that formally recognises ceiling fans as an overheating mitigation measure.

Where adequate air movement is achieved, ceiling fans can justify an increase in allowable comfort thresholds within certain assessment criteria. The guidance specifies both the required air speeds and the maximum temperature uplifts that can be applied. However, these benefits are restricted to occupied daytime spaces and mechanically ventilated areas, with no uplift permitted for sleeping bedrooms or communal circulation spaces. The document also includes guidance to prevent double-counting fan benefits within dynamic thermal models and references a dedicated Ceiling Fan Design Guide.

Greater Consideration of Real-World Constraints

The updated guidance places much greater emphasis on practical site constraints that affect ventilation strategies. Noise, security risks and poor outdoor air quality are now addressed in a dedicated section, supported by references to specialist guidance and formal acoustic assessments.

Rather than applying restrictive assumptions throughout the entire year, TM59:2026 encourages designers to adopt a more evidence-based approach. Ventilation limitations should only be applied during specific periods identified by acoustic or environmental assessments, helping to avoid unnecessarily conservative overheating predictions.

More Detailed Guidance for Mechanical Ventilation

Mechanical ventilation and MVHR systems receive considerably more attention in the new edition. The guidance now distinguishes clearly between background ventilation rates and boosted operation, explaining which strategies are compatible with each assessment stage. It also provides specific requirements relating to summer bypass functionality, user controls and acoustic performance.

The result is a much more detailed framework for assessing mechanically ventilated dwellings than was available in the original 2017 publication.

Reflecting Modern Living Patterns

Recognising changes in how homes are used, TM59:2026 introduces a dedicated home office occupancy profile. This is the first entirely new room type added to the methodology and reflects the continued growth of hybrid and remote working.

The profile applies to spaces that may be too small to qualify as bedrooms but are capable of accommodating a desk and chair. Associated heat gains from occupancy and equipment have been developed specifically to reflect typical home working patterns, ensuring overheating assessments more accurately model contemporary lifestyles.

Improved Alignment with Current Regulations

When TM59 was first published in 2017, Approved Document O did not exist. As a result, there was little interaction between overheating assessments and Building Regulations. The 2026 edition addresses this by introducing extensive cross-referencing to both Approved Document O and the Scottish Technical Handbooks.

Throughout the document, CIBSE highlights areas where modelling assumptions may differ from regulatory requirements, particularly regarding ventilation openings and solar shading assumptions. This additional guidance should help project teams navigate both compliance routes more effectively.

Wider Scope and More Comprehensive Reporting

The revised TM59 also broadens its scope beyond traditional houses and apartment buildings. The methodology now explicitly applies to a wider range of residential accommodation, including sheltered housing, care homes, student accommodation, hotels, hospital bedrooms and prisons. Guidance has also been added for retrofit projects, allowing designers to assess overheating risk before and after refurbishment works.

Alongside these changes, reporting requirements have become considerably more detailed. Project teams are now expected to document assessment results for every stage of the process, justify occupant categories, record ventilation and shading assumptions, and provide evidence for any noise, security or air quality constraints used within the model.

Conclusion

The 2026 edition of TM59 marks a significant evolution in overheating assessment methodology. The new passive-first framework, revised bedroom comfort criteria, mandatory communal area assessments and expanded reporting requirements all point towards a more rigorous and evidence-based approach.

The recent updates to CIBSE TM59 represent a significant step forward in ensuring homes remain comfortable, healthy and resilient in a warming climate. For housebuilders, this means a greater emphasis on demonstrating overheating compliance through robust design strategies, careful specification of glazing and shading, and a more holistic approach to ventilation and building performance. While the changes may introduce additional design considerations, they also provide an opportunity to future-proof developments, enhance occupant wellbeing and deliver homes that are better equipped to meet both regulatory expectations and the challenges of climate change. Those who embrace these requirements early will be best placed to create high-quality homes that meet the needs of both today's buyers and future generations.

 At AES Sustainability, we are helping clients navigate the transition to TM59:2026, ensuring overheating assessments align with the latest guidance while supporting practical, cost-effective design solutions.