TM52 vs TM59: Which One Do You Need?
They sound interchangeable and they are not. One is for homes, one is for everything else, and only one has a night-time bedroom test. A short answer and the reasoning behind it.
Two CIBSE documents, two numbers, a lot of confusion. The short answer first.
The short answer
- TM59 is for homes. It is the methodology Approved Document O points to for dynamic overheating assessment of new residential buildings.
- TM52 is for non-domestic buildings. Offices, schools, and other free-running spaces.
If you are assessing dwellings for Part O compliance, you want TM59.
Why they are related
TM59 did not invent its comfort test. TM59's daytime test is TM52's adaptive comfort criterion, applied to homes: hours where operative temperature exceeds the adaptive threshold must stay within 3% of occupied hours from May to September.
That shared ancestry is why the two get muddled. The adaptive model underneath is the same.
The difference that matters
TM59 adds a second test that TM52 has no equivalent for.
The night-time test looks only at bedrooms, only between 10pm and 7am, and uses a fixed 26°C threshold rather than an adaptive one. Hours above it must not exceed 1% of annual night-time hours.
That exists because of something specific to homes: people sleep in them. Sleep quality degrades sharply above roughly 26°C, and unlike an office worker, someone in bed cannot move to a cooler room, change what they are wearing, or go home at five.
This is why you cannot substitute a TM52 assessment for a TM59 one on a residential development. It is not a stricter version of the same test. It is missing the test that residential buildings most often fail.
What TM59 also fixes
Beyond the extra test, TM59 prescribes the inputs: occupancy profiles, internal gains, and window-opening behaviour for domestic use. TM52 leaves more of that to the assessor, which is appropriate for the variety of non-domestic buildings and unhelpful if you want two assessors to agree about a flat.
So, which do you need?
| You are assessing | You need | | --- | --- | | New dwellings for Part O | TM59 | | Student accommodation, residential institutions | TM59 | | Offices, schools, non-domestic | TM52 | | A mixed-use building | Both, for their respective parts |
If somebody has sent you a TM52 report for a residential Part O submission, it is the wrong document, and the thing it is missing is the bedroom test.
AutoTHERMAL is built specifically for TM59 and Part O. [Join the waitlist](/waitlist) for early access.
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