CIBSE published TM59:2026 – Overheating Risk in Dwellings: A Design Stage Methodology, the first major revision since the original 2017 edition. The update replaces a single-pass compliance check with a three-stage modelling strategy covering unconstrained, constrained and cooling assessments. The regulatory position remains unresolved: Approved Document O still cites TM59:2017, and no transitional arrangements or adoption timetable have been published.
Table of Contents
- What is TM59 and why does it matter for UK homes?
- What are the three stages of the TM59:2026 methodology?
- Which weather files and zones must assessments use in 2026?
- How have the bedroom and living room criteria changed?
- What else is new — communal spaces, ceiling fans and home offices?
- Can TM59:2026 be used for Part O compliance yet?
- How does a TM59:2026 assessment work in practice?
- What should designers and developers do now for 2026 projects?
- Key Takeaways
- References
What is TM59 and why does it matter for UK homes?
TM59 is CIBSE’s design-stage methodology for assessing overheating risk in homes, first published in May 2017 with ISBN 9781912034185 across 17 pages. Corrections were issued in June 2017 to Table 2, Figure 1 and Figure 7, according to CIBSE.
The methodology applies to dwellings, care homes and student residences. It underpins overheating analysis required by Part O of the Building Regulations and London Plan requirements for new residential buildings, as noted by TM59 Expert.
The complex dynamic route is needed where the Simplified Method is insufficient. High glazing ratios, acoustic constraints or complex layouts typically trigger the need for dynamic thermal modelling rather than the simplified approach.
Higher insulation levels, improved airtightness, large glazing areas and limited natural ventilation can improve energy efficiency but increase overheating risk if not considered at design stage, according to Elmhurst Energy.
What are the three stages of the TM59:2026 methodology?
TM59:2026 introduces a three-stage modelling strategy, according to IES. Stage 1 is an unconstrained assessment that assumes windows can be opened freely with no noise or security restrictions.
Stage 2 accounts for real-world limitations including noise, security, air quality and restricted window openings. Mechanical ventilation is permitted as the primary mitigation measure at this stage.
Stage 3 allows cooling systems such as split AC, VRF or fan coil units only as a last resort. The structure enforces a passive-first design sequence rather than allowing cooling to be specified early in the design process.
This staged approach means designers must demonstrate that passive measures and constrained ventilation strategies have been exhausted before mechanical cooling can be justified, according to Elmhurst Energy.
Which weather files and zones must assessments use in 2026?
Assessments must use, as a minimum, the DSY1 High 2050s weather file, with heatwave testing recommended, according to IES.
Weather file locations have moved from individual weather stations to a 28-zone system covering England, Wales, Scotland and Northern Ireland. This represents a significant change in how assessors select climate data for a project.
TM59:2026 uses Design Summer Year files from CIBSE’s 2025 dataset, covering the 2030s (2019–2039), 2050s (2039–2059) and 2080s (2069–2089). The DSY1 file is defined as a moderate year containing heat events with a return period of seven years.
DSY2 contains the year with the most heat events, while DSY3 contains the year with the longest heat events. The minimum requirement to use DSY1 High 2050s means projects are tested against a warmer future climate rather than historical conditions.
How have the bedroom and living room criteria changed?
The bedroom criterion now uses mean night-time temperature rather than counting hourly exceedances, reflecting research into sleep quality, according to IES and Elmhurst Energy.
For Category II dwellings, the mean night-time temperature must stay below 27°C, with no more than four exceedance nights allowed between May and September. This is a shift from the previous approach of counting individual hourly exceedances.
Window control now uses Top (Operative Temperature) rather than Ta (Air Temperature). Operative temperature combines air temperature and radiant temperature, providing a more representative measure of occupant comfort.
Patio doors have their own specific requirements under the updated methodology. Internal doors are now modelled as closed in fire door situations, which affects airflow calculations and overheating predictions.
What else is new — communal spaces, ceiling fans and home offices?
Overheating assessment of communal spaces is now mandatory and feeds directly into the overall compliance outcome, according to IES. This means corridors, lobbies and shared amenity areas can no longer be excluded from the analysis.
TM59:2026 formally credits ceiling fans, allowing comfort threshold uplifts based on increased air speed for certain criteria. This gives designers a recognised passive cooling measure that can help meet compliance without mechanical cooling.
A standardised home office occupancy profile has been added, recognising post-pandemic working patterns. Homes with dedicated workspaces now have a defined occupancy and equipment heat gain profile for assessment purposes.
A new TM59 Compliance Checklist has been introduced to support transparency and consistency across assessments. The checklist provides a standardised format for documenting modelling inputs, assumptions and results.
Can TM59:2026 be used for Part O compliance yet?
The current version of Approved Document O still references TM59:2017 as the dynamic thermal modelling route for demonstrating compliance, according to Elmhurst Energy.
It is unresolved whether Part O compliance should continue under TM59:2017, whether TM59:2026 can be used voluntarily, or whether transitional arrangements will be introduced. No formal guidance has been published on these questions.
No source states when software providers will incorporate the updated methodology, or when training courses and competency schemes will be updated. Assessors and designers face a period of uncertainty while the industry adapts.
Government is undertaking a full review of Approved Document O following industry feedback from the Future Homes Standard consultation. No implementation timetable has been published for this review.
How does a TM59:2026 assessment work in practice?
The complex route uses the NHBC/Elmhurst Part O report template and Elmhurst Overheating Scheme accreditation, according to TM59 Expert. This provides a standardised reporting format accepted by building control bodies.
A Newbury Racecourse Apartments case study involved a 4-storey L-shaped block of 1 and 2-bedroom flats using the Swindon DSY1 file. Noise-affected facades limited night-time window opening at a 30 dB LAeq,8hr limit, with Mechanical Extract Ventilation at 50 l/s specified for failing bedrooms.
A Sawtons, Cambridge case study covered a detached dwelling with double-height spaces and extensive south-facing glazing, using the London Gatwick weather file. The design used low g-value glazing (0.4), 600mm–1000mm overhangs, water-cooled air conditioning in the high-gain kitchen/living area, and bedroom fans.
CCA Environmental delivers TM59 overheating assessments and Part O compliance modelling. The practical stages described in these case studies map directly to the assessment services the company provides for residential developments across the UK.
What should designers and developers do now for 2026 projects?
Treat TM59:2026 as the design benchmark even where TM59:2017 remains the regulatory reference. The three-stage strategy and DSY1 High 2050s minimum will shape future compliance, according to IES.
Build in passive cooling early. Ceiling fans, solar shading and natural ventilation now have clearer recognition in the methodology, according to Elmhurst Energy.
Address noise, security and air quality constraints at concept stage. These factors determine whether Stage 2 mitigation is viable and whether a project can avoid mechanical cooling.
Plan for mandatory communal space assessment and the new compliance checklist in project programmes. These additional requirements will affect modelling scope, programme time and consultant input.
Key Takeaways
- CIBSE published TM59:2026 – Overheating Risk in Dwellings: A Design Stage Methodology, the first major update since the 2017 original.
- TM59:2026 introduces a three-stage strategy: unconstrained assessment, constrained assessment, then cooling as a last resort.
- Assessments must use at least the DSY1 High 2050s weather file, drawn from CIBSE’s 2025 dataset across a new 28-zone system.
- The bedroom criterion now uses mean night-time temperature below 27°C for Category II dwellings, with a maximum of four exceedance nights from May to September.
- Communal space assessment is now mandatory and counts towards the overall compliance outcome.
- Approved Document O still references TM59:2017, and no transitional arrangements or adoption timetable have been published.
- Ceiling fans are formally credited, a standardised home office profile has been added, and a new TM59 Compliance Checklist supports consistency.
References
- TM59:2026 is here. What does it mean for Overheating Assessments? – IES – 2026-07-24
- TM59 Design methodology for the assessment of overheating risk in homes (2017) – CIBSE – May 2017
- CIBSE Publishes Updated TM59 Overheating Methodology – Elmhurst Energy – 2026-07-10
- TM59 Explained: Do You Need It? (2026 Update) – Building Energy Experts
- TM59 Overheating Assessment UK | Part O Compliance | CIBSE – TM59 Expert