Latest updated CIBSE TM59 guidance & what residential design teams should now check

Overheating is now closely tied to planning, Building Regulations and homes performance once people move in. Decisions made during conception and design development can make a big difference between a scheme that works through passive design and one that incurs expensive changes later.

The updated guidance spotlights a simple point – homes designed today need to cope with warmer summers over their lifetime, which means overheating should be tested while the design can still be changed.

What is CIBSE TM59?

CIBSE TM59 is the UK’s recognised method for assessing overheating risk in homes using dynamic thermal modelling.

It gives project teams a consistent way to predict indoor temperatures, assess comfort and check whether a proposed design is likely to overheat under realistic conditions. It is widely used for apartments, housing schemes, student accommodation, care homes and residential refurbishment projects.

TM59 also supports many planning requirements and is one of the main routes used to demonstrate compliance with Approved Document O. The 2026 edition updates the methodology to reflect current research and future climate expectations.

Further information is available from the CIBSE TM59 Knowledge Portal.

What the 2026 update means for Part O

Approved Document O was introduced to reduce overheating risk in new homes in England and sets the regulatory framework. TM59 gives many project teams the detailed assessment route, particularly where dynamic thermal modelling is needed.

The updated guidance reinforces what many designers are already seeing on their projects – you can’t deal with overheating in isolation. Window sizes, glazing, orientation, shading, ventilation, insulation and façade design all have an impact on thermal comfort. Changing elements like these once technical design is complete can be costly and may also affect planning, energy performance or appearance.

Approved Document O is available via GOV.UK.

Overheating risk starts with design decisions

Good overheating strategies begin during RIBA Stages 1 to 3, (not just before Building Regulations approval). Early modelling allows project teams the time to test options before layouts and specifications are fixed. For example:

• ✓ Adjustment to glazing ratios
• ✓ External shading worked into the architectural design
• ✓ Ventilation strategies coordinated with Part F
• ✓ Façade materials reviewed alongside Part L targets
• ✓ Apartment layouts refined to reduce higher-risk single-aspect homes.

This joined-up approach helps reduce redesign and makes it easier to balance different regulatory requirements.

enevo sits its overheating assessment process within a wider building physics team that also support SAP calculations, thermal modelling, daylighting, ventilation and acoustic assessments. Bringing these areas together gives significant oversight capability and, employed early helps clients avoid conflicting advice later in the project.

Future climate assumptions are becoming harder to ignore

One of the main themes in the 2026 update is the greater focus on future weather conditions.

Homes completed today are likely to be occupied well beyond the 2050s. Designing around today’s climate can leave residents with homes that are harder to keep comfortable as global temperatures rise. The updated guidance gives greater weight to future weather files when assessing overheating risk.

Rather than treating future climate as a minor sensitivity check, design teams should view it as part of designing homes that remain comfortable over the long term. Recent research from Loughborough University also points to the need to account for hotter summers when designing new homes.

Bedrooms, comfort and real occupant use

Bedrooms remain one of the most sensitive parts of an overheating assessment.

We all know how poor night-time comfort can affect sleep, health and general wellbeing. Bedrooms with large west-facing glazing, limited natural ventilation or single-aspect layouts often need deeper review.

The updated guidance continues to encourage realistic assumptions about how people actually use their homes, rather than relying on perfect scenario or ideal conditions that may not happen in practice.

This can be even more challenging with refurbishment projects, where existing façades, planning limits or listed building status may restrict shading, ventilation or layout changes. Again, early modelling helps identify those issues before the design route narrows.

Ceiling fans and passive comfort measures

The updated TM59 guidance also provides clearer consideration to ceiling fans as a low-energy way to improve thermal comfort. Ceiling fans don’t offer a quick get-out for poor design, but they can be part of a wider passive cooling strategy where suitable.

For most schemes, the first step should still be to reduce overheating through design:

• ✓ Optimise orientation and glazing
• ✓ Provide effective external shading
• ✓ Maximize natural ventilation where practical
• ✓ Reduce unwanted solar gain
• ✓ Improve façade performance.

Once these options have been reviewed, project teams can then decide whether extra measures are needed, helping to reduce reliance on mechanical cooling and supports lower operational energy use.

What project teams should review now

Design teams should consider whether they are:

• ✓ Assessing overheating before glazing, shading and ventilation strategies are fixed
• ✓ Reviewing layouts for vulnerable bedrooms and single-aspect dwellings
• ✓ Testing future weather scenarios, not just current climate data
• ✓ Coordinating overheating assessments with Part L, Part F, SAP, daylighting and acoustic requirements
• ✓ Considering passive cooling before moving to mechanical cooling
• ✓ Modelling refurbishment projects early enough to work around existing constraints.

Projects that assess these points early usually face fewer late-stage design changes and a smoother route through Building Regulations.

How enevo can support overheating and building physics assessments

enevo offers expert, experienced in-house building physics consultancy across residential and mixed-use developments, supporting clients from early concept through to completion. Our seasoned Building Compliance team delivers overheating assessments, dynamic thermal modelling, SAP calculations, ventilation advice, daylighting studies and wider environmental compliance services as part of a coordinated design process.

By bringing a wide variety of service offerings together, we are able to help project teams identify overheating risks early, balance different regulatory requirements and develop practical design responses before redesign becomes expensive.

Whether you’re delivering a new residential scheme or refurbishing existing homes, an early overheating assessment can support compliance, reduce programme risk and help create homes that remain comfortable for years to come.

If you want to discuss a project, then give us a call and speak to a member of our team about overheating calculations and Part O assessments.

We can also review overheating risk alongside SAP calculations, thermal modelling, ventilation, daylighting and wider building physics requirements.