Frequently asked questions
What is the difference between a TM59 and a TM52 overheating assessment?
TM59 applies to homes and TM52 applies to non-domestic buildings. Under TM59 (2017), Criterion A limits living rooms, kitchens and bedrooms to no more than 3% of occupied hours at 1 K or more above the adaptive threshold, and Criterion B limits bedrooms to 26 degrees C for no more than 1% of night hours (about 32 hours). TM52 has three criteria: hours of exceedance (3% of occupied hours), daily weighted exceedance (no more than 6) and an upper limit of 4 K above the threshold. A space passes TM52 by meeting two of the three. CIBSE published TM59:2026 in July 2026 with a revised bedroom criterion based on night-time temperatures, but Approved Document O still refers to the 2017 method, so confirm with Building Control which version applies.
How do I show TM59 overheating compliance for a planning condition or Part O?
Run a dynamic thermal simulation of every habitable room to CIBSE TM59 and report room-by-room results against the criteria. Approved Document O accepts this dynamic route as an alternative to its simplified method. Submit a report covering the model inputs, weather file, ventilation assumptions, results and any mitigation. Models normally use CIBSE Design Summer Year weather data, with DSY1 for the 2020s as the standard test, and London schemes are often also tested against DSY2 and DSY3 and a future climate. If a room fails, show that the cooling hierarchy has been followed: reduce solar gain and heat gains first, then ventilate, then use passive cooling, with mechanical cooling as a last resort.
Technical information last reviewed October 2026.
