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Higher temperatures and more frequent and intense heat waves are inevitable as the UK climate warms. High indoor summertime temperatures result in diminished health and well-being and can be fatal, especially for people with underlying health problems. In 2026 the Climate Change Committee elevated the ‘Risks to buildings and communities from heat’ into the highest risk category - ‘Critical Action needed’.
TM59 provides the industry blueprint for assessing overheating risk in new dwellings and major refurbishments using dynamic thermal models. It explains how models should be used, the assumptions to be made and a defines a three-stage overheating assessment strategy. It is applicable to houses, apartments, sheltered accommodation, care homes, student accommodation, prisons, and bedrooms in hotels and hospitals, etc.
Drawing on a decade of real-world application and recent academic insights, this new version of TM59 supports the delivery of homes that are not only future-ready in terms of climate resilience but also aligned with the drive to reduce operational carbon and energy costs. Key elements of this update are a new bedroom overheating criterion, backed by recent Loughborough University research and updated ceiling fan modelling that highlights them as an effective, passive comfort solution.
By mandating that all homes be tested under unconstrained conditions using 2050s weather data, the updated TM59 guidance places a strong emphasis on optimizing passive design from the outset. This shifts the focus towards inherently resilient homes that can maintain summer comfort into the future without relying on active cooling
Contents
Introduction
1.1 Background
1.2 About the TM59 methodology
1.3 Linked documents
1.4 Existing dwellings and retrofit
1.5 Passive summer temperature control
1.6 Basis of the overheating criteria
1.7 Supporting information
2 The methodology
2.1 Selecting spaces for analysis
2.2 Modelling the sample spaces
2.3 The TM59 modelling strategy
2.4 Assessment criteria
2.5 Reporting and implementation
3 Guidance on modelling
3.1 Weather files
3.2 Geometry and construction
3.3 Solar gain control and shading
3.4 Natural ventilation, window and door openings
3.5 Infiltration, mechanical ventilation and cooling
3.6 Ceiling fans
3.7 Internal heat gains and profiles
3.8 Noise, security and air quality constraints
Appendix A: Further reading
Appendix B: Summary of the modelling strategy: opening constraints, ventilation and cooling assumptions, applicable overheating criteria and reporting
Appendix C: Reporting requirements checklist
Appendix D: Modelling ventilation openings
Appendix E: Heat gain profiles
References
Acknowledgements
Authors: Susie Diamond (Inkling) (Lead Author), Becci Taylor (Arup), Antonietta Canta (Arup)
Executive editor: Kevin Lomas (Loughborough University)
Peer reviewers: Ben Abel (Hilson Moran), Anna Mavrogianni (UCL), Marguerita Chorafa (Introba), Jack Harvie-Clark (Apex Acoustics Ltd)
The Institution is grateful to the following for testing the methodology on actual projects:
David Heppenstall (Applied Energy), Antonietta Canta (Arup), Joe Aston (Arup), Rafal Wiewior
(Atelier 10), Antonino Saporito (Atkins), Joel Callow (Beyond Carbon), William O’Reilly (Buro Happold),
Vojtech Panek (Design Builder), Andrew Hilmy (EDSL), Leon Tatlock (Etude), Vicky Kontopoulou
(Etude), James Crowley (Hilson Moran), Colin Rees (IES), Ahmed Usaamah (IES), Lewis Goldberg (IES),
Marcus Haydon (Inkling), Susie Diamond (Inkling), Claire Das Bhaumik (Inkling), Marguerita Chorafa
(Introba), Zeina Krayim (Introba), Giuseppe Borgia (Introba), Ben Roberts (Loughborough University),
Kostas Mourkos (Loughborough University), Marc Seligmann (Maccreanor Lavington), Matt Berry
(Max Fordham), Cameron Forsyth (Max Fordham), Stavroula Koutroumpi (Max Fordham), WeiWen
Low (Mott MacDonald), Stephen Porritt (Porritt Associates), Chris Michael Armstrong (QuinnRoss).
The Institution is also grateful to the following for reviewing the text prior to publication:
Luis Pereira (Apex Acoustics), Andrew Mackay (Arup), Michael Edwards (Arup), Alex Smith (Arup),
Joe Aston (Arup), Zoe De Grussa (CIBSE), Suzanne Kimman (Haringey Council), Nikhil Doshi
(Hodkinson Consultancy), Marcus Haydon (Inkling LLP), Gwilym Still (Max Fordham), Chris Michael
Armstrong (QuinnRoss)