CIBSE TM59 - Modelling and Analysis Guide

CIBSE TM59 is a standardised approach to predicting overheating risk for residential building designs (new-build or major refurbishment) using dynamic thermal analysis. The testing of the methodology has focused on flats, as they tend to represent a higher overheating risk than houses. However, the methodology should also be applicable to houses.

There are two versions of CIBSE TM59, and the details of how to use these two versions are described in separate sections:

TM59:2017 - the original TM59 methodology introduced in 2017.
TM59:2026 - the 2026 revision first implemented in DesignBuilder v2025.1.

Differences between TM59 2017 and 2026

The 2026 revision introduces several important changes shaped by industry feedback, practical experience and recent research. While it retains the overall dynamic thermal modelling approach and the naturally ventilated / mechanically ventilated split, it changes several of the pass criteria and adds an elevated air speed provision.

New bedroom assessment criteria

One of the most notable changes in TM59:2026 relates to how overheating is assessed in bedrooms. Under the previous methodology, bedroom overheating was assessed based on the number of hours the room exceeded a specified temperature during sleeping hours. TM59:2026 introduces a new approach based on mean night-time temperatures, reflecting the latest research into sleep quality and the impact of heat on occupants overnight. This change is intended to provide a more realistic assessment of sleeping comfort and how people experience overheating during the night.

New 3-stage assessment process

TM59:2026 introduces a three-stage process for assessing overheating risk:

Stage 1 – Considers passive design measures only
Stage 2 – Takes account of local constraints, such as noise, security and air quality restrictions
Stage 3 – Allows for mechanical cooling where required

This structure reinforces the principle that passive measures should be considered first, with mechanical cooling used only where it is necessary.

Updated weather files

TM59:2026 adopts a dedicated weather file framework based on the latest climate projections. This provides a more robust basis for overheating assessments by considering future climate conditions as well as current weather patterns.

More detailed real-world modelling

TM59:2026 provides expanded guidance on practical issues that can affect overheating risk in real homes, including:

Noise restrictions
Security concerns
Air quality limitations
Window opening constraints

These factors can have a significant influence on ventilation strategies and how occupants are able to use their homes during periods of hot weather. By recognising these real-world constraints, the updated methodology supports a more practical and realistic assessment of overheating risk.

Recognition of passive cooling measures

TM59:2026 provides clearer recognition to measures that can improve thermal comfort without significantly increasing energy use. This includes updated guidance on the use of ceiling fans, alongside continued emphasis on passive cooling measures such as solar shading and natural ventilation.

The table below summarises the changes that affect how a model is set up and assessed.

Topic	TM59 (2017)	TM59 (2026)
Criteria set	Same four tests present but not lettered: a nat-vent adaptive test, a bedroom night test, a fixed-temperature test for mechanically ventilated homes, and an advisory corridor check.	The same tests are now given explicit letters A to D: A living rooms/kitchens/bedrooms adaptive exceedance; B bedroom night mean; C mechanically ventilated or cooled fixed temperature; D communal / circulation. C and D existed in 2017 but were not lettered.
Bedroom night criterion (B)	Operative temperature 22:00 to 07:00 must not exceed 26 °C for more than 1% of annual hours (32 h; 33 h or more is a fail).	Mean operative temperature during sleeping hours must not exceed the night threshold Tn on more than 4 nights. Tn = 27 °C (Cat II) or 26 °C (Cat I, vulnerable).
Sleeping window	22:00 to 07:00.	23:00 to 08:00.
Assessment period	Criterion A over May to September; the night and mechanically ventilated tests read on an annual-hours basis.	All four criteria assessed over May to September.
Home office activity	No home-working profile. Living spaces modelled with evening-weighted occupancy.	New home office activity for daytime home-working occupancy. Assessed under Criterion A only; Criterion B (the night test) is not applied to it.
Mechanically ventilated / cooled homes	Assessed against the fixed-temperature test (Criterion C) only.	Must pass both Criterion B (bedroom night mean) and Criterion C (fixed temperature). Bedrooms in these homes are no longer exempt from the night test.
Elevated air speed / ceiling fans	Credited by adjusting the operative temperature: the assessed temperature was reduced according to fan speed, then compared against the fixed comfort limit.	Credited by lifting the criterion instead: operative temperature is left unchanged and the comfort limit is raised by the air-speed uplift. Naturally ventilated adaptive (A only): +0.2 °C per 0.1 m/s, capped at +1.2 °C, applied above 25 °C. Mechanically ventilated / cooled (C): +0.35 °C per 0.1 m/s, capped at +2.1 °C.
Vulnerable occupants	Type I occupancy assumed for predominantly naturally ventilated care homes and similar.	Category I applied (a 1 K reduction on the adaptive threshold) for criteria A and B.
Communal / circulation (D)	Corridors flagged as a significant risk if operative temperature exceeds 28 °C for more than 3% of annual hours. Advisory only.	Criterion D on a 28 °C basis, assessed over May to September. Common circulation (TM59_CommonCirculationAreas) is a pass / fail; in-dwelling circulation (TM59_CirculationAreas) reports a significant-risk flag.