IN Brief:
- CIBSE has updated TM59, its design-stage methodology for assessing overheating risk in dwellings.
- The revised guidance includes future 2050s weather files, updated bedroom criteria, and revised modelling for ceiling fans.
- The update strengthens the link between residential design, Part O compliance, passive cooling, and climate resilience.
CIBSE has launched an updated version of TM59, its design-stage methodology for assessing overheating risk in new homes and major residential refurbishments.
The revised guidance, developed in collaboration with Arup, Loughborough University, and Inkling, updates CIBSE TM59: Overheating risk in dwellings to reflect new research, industry experience, and the growing challenge of keeping homes comfortable in a warming climate.
TM59 is referenced within Part O of the Building Regulations and is widely used by designers, engineers, developers, and consultants to assess overheating risk in residential buildings. The updated edition strengthens the methodology used during design and places greater emphasis on testing homes against future climate conditions.
A key change is the requirement for all homes to be assessed under unconstrained conditions using future weather files representing the 2050s climate. That approach is intended to bring overheating risk into the earliest design decisions and encourage passive measures before schemes become dependent on energy-intensive cooling.
The updated publication also incorporates revised approaches to evaluating bedroom overheating, informed by recent research undertaken by Loughborough University. Ceiling fans are given revised modelling guidance, recognising their potential role as a low-energy comfort measure in residential buildings.
Overheating has become a more urgent design issue as new and existing homes face higher summer temperatures, more intense heat events, and changing occupancy patterns. Many homes are vulnerable because of extensive glazing, limited ventilation strategies, poor orientation, constrained façades, or layouts that restrict night-time cooling. In dense urban locations, heat island effects can intensify the problem.
The update sits within a wider tightening of building performance expectations, with energy, comfort, and climate resilience becoming harder to separate. Recent movement around tougher commercial EPC standards has shown how operational performance is moving higher up the built environment agenda. In housing, overheating is particularly sensitive because design teams must reduce heat risk without simply shifting the burden onto mechanical cooling and higher electricity demand.
Passive design measures such as solar control, shading, ventilation strategy, thermal mass, window design, layout, and low-energy air movement need to be considered before cooling systems become the default answer. The use of future weather files also changes the design conversation. A home that appears acceptable under present-day assumptions may perform poorly under the conditions expected during its service life.
The guidance also affects major refurbishment. Existing homes often have limited capacity for façade changes, ventilation upgrades, or layout alterations, yet they may still be expected to deliver higher comfort and energy performance. Retrofit teams must manage overheating alongside insulation, airtightness, moisture, fire safety, heritage constraints, resident disruption, and cost limits.
Developers and design teams are likely to face greater scrutiny of overheating analysis at an earlier stage. Modelling assumptions, internal gains, ventilation opening areas, shading, fan use, and bedroom conditions will all need to be handled consistently. Poor assumptions at planning or technical design stage can later become costly redesign issues, particularly where façades, window sizes, or mechanical strategies have already been fixed.
The revised guidance also strengthens the role of building services engineers in housing design. Overheating is not solely an architectural issue, nor is it simply a matter of adding cooling. It sits between fabric, services, planning, resident behaviour, climate data, and compliance. Stronger outcomes will depend on early coordination between architects, engineers, energy specialists, and developers.
As hotter summers become more common, overheating will become a mainstream quality and safety issue in residential construction. The updated TM59 gives the sector a clearer assessment route, but its value will depend on whether design teams use it early enough to influence buildings rather than late enough merely to document risk.
