The Chartered Institution of Building Services Engineers (CIBSE) has released the 2026 update to TM59, marking the most significant revision to the UK’s residential overheating assessment methodology since its original publication in 2017. With climate warming accelerating and overheating risk now a mainstream design concern, TM59:2026 brings greater clarity, stronger evidence‑based criteria, and a more structured modelling strategy to support passive-first design.
As CIBSE notes, “The methodology updates the previous version of TM59 (CIBSE, 2017) based on feedback from its use in practice and the latest research.”
This article breaks down the key changes, the revised methodology, and what they mean for designers, assessors, and developers.
1. Why TM59 Needed Updating
Overheating in UK homes has become a critical issue. Modern dwellings — particularly lightweight, single‑aspect apartments with large glazing areas — are more vulnerable to solar gains and restricted ventilation. TM59:2026 responds to:
- New climate projections
- Evidence from monitored homes
- The introduction of Approved Document O
- Industry feedback on modelling inconsistencies
- Academic research into sleep disruption caused by heat
The foreword captures the urgency: “We are living through a period of rapid climate warming… Now more than ever we should revisit traditional UK and continental cooling strategies and prioritise passive approaches.”
2. The Five Major Changes in TM59:2026
TM59:2026 introduces several important updates. The document summarises them clearly:
“A revised overheating criterion for assessing bedrooms… The use of ceiling fans… updated weather files… clarified modelling strategy when window opening is constrained… and a new home office gains profile.”
Here’s what each means in practice:
2.1 Revised Bedroom Criterion (Criterion b)
This is the most impactful change. The previous hourly threshold has been replaced with a night‑time mean temperature limit, based on new sleep research (Lomas & Li, 2023).
- Category I: 26°C
- Category II: 27°C
- Max 4 nights above threshold (May–Sept)
This better reflects how heat disrupts sleep and aligns modelling with real‑world comfort.
2.2 Ceiling Fans Now Formally Included
TM59 now allows temperature threshold uplifts when ceiling fans are installed as part of the base build or major retrofit.
- Up to +1.2°C for naturally ventilated spaces
- Up to +2.1°C for mechanically ventilated spaces
This is a major shift — recognising fans as a legitimate, low‑energy comfort strategy.
2.3 Updated Weather Files
TM59:2026 mandates the new Design Summer Year (DSY) files from Overheating risk in dwellings: weather file requirements (2026a). These reflect the latest UKCP climate projections and ensure assessments consider warmer future summers.
2.4 Clarified Window‑Opening Constraints
The new three‑stage modelling strategy (Stage 1 passive, Stage 2 constrained ventilation, Stage 3 cooling) formalises how to handle noise, air quality, and security constraints.
2.5 Home Office Gains Profile Added
With hybrid working now the norm, TM59 includes a dedicated heat‑gain profile for home offices — essential for accurate modelling in smaller dwellings.
3. The New Three‑Stage Modelling Strategy
TM59:2026 introduces a structured, passive‑first modelling sequence:
Stage 1 — Unconstrained Natural Ventilation
- No noise/security/air‑quality constraints
- Passive measures only (plus background MVHR/MEV)
- Ceiling fans allowed
- Must pass Criterion a (daytime) and Criterion b (night‑time)
If a dwelling passes Stage 1, no further testing is required.
Stage 2 — Apply Site‑Specific Constraints
- Noise, pollution, and security restrictions applied
- Additional passive measures allowed
- Mechanical ventilation allowed (but no cooling)
- Spaces assessed using Criterion a + b or Criterion c + b, depending on openability
Stage 3 — Mechanical Cooling
Only used if Stage 2 fails.
- Cooling added to specific spaces
- Must pass Criterion c + b
This structured approach ensures cooling is only used when absolutely necessary — reinforcing the passive-first design hierarchy.
4. Updated Overheating Criteria
TM59:2026 retains four criteria but refines their application:
Criterion a — Naturally Ventilated Spaces
3% of occupied hours above adaptive threshold.
Criterion b — Bedrooms (Sleeping)
Max 4 nights above fixed threshold.
Criterion c — Mechanically Ventilated/Cooled Spaces
3% of occupied hours above 26°C.
Criterion d — Communal Areas
3% of occupied hours above 28°C.
The document states: “Bedrooms should be designed to pass Criterion b and either Criterion a… or Criterion c.”
5. Implications for Designers and Developers
5.1 Passive Design Is Now Non‑Negotiable
TM59 reinforces shading, ventilation, glazing control, and thermal mass as primary tools.
5.2 Window Design Must Be More Sophisticated
Tilt‑and‑turn, inward‑opening windows, secure night‑vent positions, and acoustically attenuated vents will become more common.
5.3 Ceiling Fans Will Rise in Popularity
Expect more developers to specify fans to avoid mechanical cooling.
5.4 Home Office Modelling Will Affect Layouts
Small rooms may now fail overheating criteria unless properly ventilated and shaded.
5.5 Communal Areas Are Now Fully Regulated
Criterion d formalises overheating limits for corridors, lobbies, and stairwells — critical in tall buildings with HIU/DHW heat gains.
6. Why TM59:2026 Matters
This update is more than a technical refinement — it’s a cultural shift in UK residential design. TM59:2026:
- Aligns modelling with real human comfort
- Reflects modern living patterns
- Responds to climate change
- Reduces reliance on mechanical cooling
- Improves consistency across the industry
It sets a clearer, more evidence‑based standard for designing homes that remain comfortable in increasingly extreme summers.
7. Closing Thoughts
TM59:2026 is a welcome evolution. It strengthens the methodology, clarifies ambiguity, and reinforces passive-first design — all while acknowledging the realities of modern living and climate change.