Trial / Demo
CIBSE has just published TM59:2026 – Overheating Risk in Dwellings: A Design Stage Methodology, and it's the biggest shake-up to residential overheating assessments since the original came out in 2017. The update draws on new research, evolving climate resilience requirements, and lessons learned from over a decade of real-world use.
So what's new? A three-stage modelling strategy, revised bedroom criteria, mandatory communal space assessment, updated weather files, formal recognition of ceiling fan modelling, new home office profiles, and beefed-up reporting requirements. This is in addition to window control now utilising Top (Operative Temperature) rather than Ta (Air Temperature), so the assessment is focused on the temperature the occupants experience, rather than just the air temperature in the room. Furthermore, patio doors also have their own specific requirements, and internal doors are now modelled as closed in fire door situations.
Designing for a future climate, not just today's
Perhaps the biggest mindset shift is this: TM59:2026 no longer just checks how a building performs now, it looks ahead. Assessments must use as a minimum the DSY1 High 2050s weather file, and recommends testing for heatwaves too, so developments are stress-tested against the overheating risks we're likely to face in future. Weather file locations have also moved away from individual weather stations to a 28-zone system, designed to give more representative results.
The three-stage Modelling Strategy
This is the headline change. Modellers now need to show, step by step, how overheating risk is tackled – starting with passive design and only moving to mechanical intervention if needed.
Stage 1 – Unconstrained Assessment
This looks at the building's inherent resilience under ideal conditions, assuming windows can be opened freely with no noise or security restrictions.
Stage 2 – Constrained Assessment
Here, real-world limitations come into play: noise, security, air quality, restricted window openings. Where natural ventilation can't do the job alone, mechanical ventilation can step in as the primary mitigation.
Stage 3 – Cooling Assessment
If passive design and ventilation still aren't enough, Stage 3 allows for cooling systems such as split AC, VRF, or fan coil units – but only as a last resort, in keeping with the passive-first philosophy running through the whole methodology.
Weather Files
Selecting the correct data and stressing the building performance is an acute need when considering heatwave events the UK has already experienced across the 2026 summer. The latest TM59 overheating assessments empoly the Design Summer Year (DSY) released within CIBSE's 2025 dataset, for the 2030s (2019-2039), 2050s (2039-2059), and 2080s (2069-2089) DSY files cover hot weather events:
DSY1: a moderate year containing heat events with a return period of seven years
DSY2: the year containing the most heat events
DSY3: the year containing the longest heat events
These cover the 28 zones representing England, Wales, Scotland and Northern Ireland. IES are an official reseller and you can purchase these comprehensive weather files here.
Bedroom Assessment gets a rework
Rather than counting hourly exceedances, the bedroom criterion now looks at mean night-time temperatures. For Category II dwellings, the average must stay below 27°C, with no more than four exceedance nights allowed between May and September.
Communal areas are no longer optional
Overheating assessment of communal spaces is now mandatory, and it feeds directly into the overall compliance outcome.
Ceiling fans get formal recognition
TM59:2026 now formally credits ceiling fans, allowing comfort threshold uplifts based on increased air speed for certain criteria.
A standard profile for home offices
With home working now the norm for many, a standardised home office occupancy profile has been added, taking the guesswork out of modelling this space.
Better reporting = better quality assurance
Reporting requirements are more structured than before, and a new TM59 Compliance Checklist helps bring more transparency and consistency to assessments across the board.
What this means for IES users
In practice, this means demonstrating a clearer design progression, so expect greater transparancy on the modelling inputs, a wider range of simulation scenarios and a higher level of reporting detail to evidence performance and justify design recommendations.
How we can help
If you're getting to grips with the new methodology, IES Consultancy is well placed to help. We support clients through Stage 1–3 assessments, passive strategy evaluation, ventilation studies, and compliance reporting – all aligned with the latest methodology and weather data. Get in touch to find out more.
