CIBSE TM59 2026 – Overheating Update
As UK summers become warmer and climate resilience moves higher up the industry agenda, overheating risk is an increasingly important consideration in residential design. In response, CIBSE has published TM59:2026, introducing several significant updates to how overheating assessments are carried out and how homes are designed for future occupants.
The CBC Sustainability team can guide clients on what this means for projects but in short, the revised guidance places greater emphasis on occupant comfort, future climate conditions, and recognises the shift in the way people use their homes. For developers, architects, and design teams, understanding these changes will be essential for achieving compliance and delivering resilient, comfortable buildings.
New Bedroom Overheating Criteria
For Category II dwellings, the revised guidance now states that mean bedroom temperature must remain below 27°C, with no more than four exceedance nights allowed between May and September. Previous criterion counted hourly exceedance.
This shift reflects growing recognition of the impact overheating can have on sleep quality, health, and overall wellbeing. As a result, strategies that support effective night-time cooling are likely to become increasingly important during the design process.
Communal Areas Are Now Included
Overheating assessments for communal spaces are no longer optional. Shared areas such as corridors, lobbies, and internal circulation spaces must now be assessed, and their performance contributes directly to overall TM59 compliance.
Developments will therefore need to demonstrate that both private and communal spaces can remain comfortable during warm weather conditions.
Assessing Buildings Against Future Climate Scenarios
TM59:2026 now requires assessments to be carried out using projected 2050s climate data (DSY1 high-emissions scenarios) rather than historical weather files.
This move towards future-proofing residential developments will encourage greater adoption of passive design measures such as solar shading, optimised glazing, improved ventilation strategies, and careful building orientation.
Formal Recognition of Ceiling Fans
For the first time, ceiling fans are acknowledged within TM59. The guidance allows comfort thresholds to be adjusted where increased air movement from ceiling fans improves occupants’ thermal comfort.
As the industry strives to reduce operational carbon, ceiling fans may become a more common feature within residential projects.
A Standardised Home Office Profile
With hybrid and remote working now firmly established, TM59:2026 introduces a standard occupancy profile for home office spaces.
The new standard creates greater consistency and reflects the additional heat gains generated by occupants and IT equipment during the working day.
Greater Focus on Occupant Experience
Another important update is the move from using air temperature to operative temperature when assessing window controls.
Operative temperature considers both the air temperature and heat radiated from surrounding surfaces. This approach will enable design for thermal comfort rather than simply meeting temperature-based metrics.
Looking Ahead
TM59:2026 reflects the growing need for homes that are comfortable, resilient, and capable of performing in a changing climate. By introducing future weather scenarios, strengthening assessment criteria, and recognising modern living patterns, the guidance encourages a more holistic approach to overheating mitigation.
At Cudd Bentley, we will factor TM59:2026 early in the design process as part of our collaboration with clients. Thorough overheating assessments not only support compliance but help create healthier, more sustainable homes that remain comfortable for years to come.