In July 2026, CIBSE published the latest edition of Technical Memorandum 59 (TM59): Overheating Risk in Dwellings – A Design Stage Methodology (2026).

TM59 sets out the industry’s recommended approach for assessing overheating risk in new residential developments and major refurbishments using dynamic thermal modelling. The guidance explains how thermal models should be applied, outlines the required assumptions, and defines a three-stage methodology for overheating assessments. It applies to a wide range of residential and accommodation types, including houses, apartments, sheltered housing, care homes, student accommodation, prisons, and bedrooms in hotels and hospitals.

Building on more than a decade of real-world application and the latest academic research, the 2026 edition strengthens the guidance to help deliver homes that are resilient to future climate conditions while supporting lower operational carbon emissions and reduced energy costs. Notable updates include a new bedroom overheating criterion, informed by recent research from Loughborough University, and revised ceiling fan modelling, highlighting ceiling fans as an effective passive comfort solution.

A significant change in the updated guidance is the requirement for all dwellings to be assessed under unconstrained conditions using projected 2050s weather data. This places greater emphasis on passive design strategies from the earliest stages of the design process, encouraging the creation of homes that remain comfortable during warmer summers without relying on active cooling.

To download and read the full TM59 report, please visit the CIBSE website here.