Claire Stone, Sustainability Consultant at AES Sustainability explains what has changed in the newly revised edition of CIBSE TM59 methodology for tackling overheating risk.
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.