Choosing between TM52 and TM59 overheating criteria

Pick the wrong overheating methodology and the assessment answers a question nobody asked. The short version: TM59 is the domestic methodology and applies to homes, TM52 is the adaptive comfort methodology for free-running non-domestic spaces, and TM59 borrows one of TM52's criteria rather than replacing it. A mixed use building usually needs both, applied to different parts.

The confusion arises because TM59 is not a separate test regime. It is a design methodology that specifies modelling assumptions for homes, then applies TM52 criterion one to the habitable rooms and adds a bedroom criterion of its own.

What each methodology covers

TM52 sets out the adaptive comfort approach for buildings without mechanical cooling, where occupants adjust clothing, open windows and tolerate higher internal temperatures when it is warm outside. The comfort temperature moves with the running mean external temperature, so the same internal figure can be acceptable in August and uncomfortable in May. It is written for offices, schools, and other non-domestic spaces that run free.

TM59 takes that framework and fixes the inputs for dwellings: occupancy profiles for living spaces and bedrooms, internal gains from appliances and lighting, assumptions about curtains and blinds, window opening behaviour, and the treatment of communal corridors and risers. It then applies two criteria to naturally ventilated homes and a different criterion set to predominantly mechanically ventilated homes.

So the choice is not really a choice for a residential scheme. It is a choice for the non-residential parts of a mixed building, and for the question of which criteria apply to a space that is neither clearly domestic nor clearly an office.

The criteria side by side

The three TM52 criteria work together and a space fails if it breaches more than one of them, which gives some tolerance to a brief extreme. The second and third criteria are the ones that catch a space with a short severe peak rather than a long mild excess, and they are why a space can pass on total hours and still fail.

For homes, the adaptive criterion and the night-time criterion test different things. A living room with large south glazing tends to fail the daytime criterion. A single aspect bedroom with a west window and restricted opening tends to fail the night-time one. They respond to different design moves, so report them separately rather than giving a single pass or fail per dwelling.

Bedrooms and the overnight test

The night-time bedroom criterion is the one that drives design on urban sites, and it is the hardest to satisfy. A bedroom retains heat gained during the day, the external temperature falls slowly in a built-up area, and if noise or security restricts the openable area, the room cannot purge.

Three consequences for the design team. Shading on east and west elevations matters more than on south elevations for bedrooms, because low-angle sun in the late afternoon heats the room shortly before the test period begins. Exposed thermal mass helps only if there is a night ventilation path to discharge it, so a sealed room with a concrete soffit performs worse than an insulated one with a purge route. And restrictors fitted for falling safety reduce the free area actually available, so model the restricted opening rather than the nominal one.

Where the acoustic environment rules out night opening altogether, mechanical ventilation with a summer bypass is normally the next step. Planted roof surfaces also reduce the heat reaching top-floor bedrooms, which brings the overheating design into contact with the other demands on a roof, as our note on biosolar roofs describes.

Mixed use buildings and which applies

Apply the domestic methodology to the dwellings and the adaptive non-domestic methodology to the commercial floors, and say in the report which applies where. Three space types sit awkwardly between them.

Communal corridors and lobbies in a residential block are addressed within the domestic methodology, and they frequently fail where risers carry heating distribution pipework. The usual fixes are insulation of the pipework, separation of the riser from the corridor, and mechanical ventilation of the corridor, all of which are design decisions rather than reporting decisions.

Student rooms, co-living rooms and hotel bedrooms are rooms used for sleeping that are not self-contained dwellings. The pragmatic approach, which councils generally accept, is to apply the domestic bedroom criterion to the sleeping space and the non-domestic approach to communal areas, with the choice explained.

Accommodation for occupiers who cannot easily adapt, such as a care home or supported housing, needs tighter thresholds than either default. The domestic methodology includes guidance for vulnerable occupants, and a scheme that ignores it will be queried by an environmental health or housing consultee rather than by the planning officer.

Weather files and future climate years

The weather file choice changes the answer more than most design decisions do. Design summer year files come in three forms for each location: a moderately warm summer, a short intense warm spell, and a longer less intense warm period. A design that passes against one can fail against another, because the three test different failure modes.

Report the file you used by name, including the location, the percentile and the time period. Run at least one sensitivity case against a different file, and say what happened. Where a future climate year is tested, say which decade and which emissions scenario, since a 2050s file with a high emissions scenario is a different test from a 2030s median file.

A scheme that passes on a current file and fails on a future one is worth disclosing rather than hiding. Officers in authorities with a climate resilience policy increasingly ask for it, and volunteering the result with an explanation of the adaptation capacity built into the design reads much better than being asked.

Where Approved Document O sits

Part O of the Building Regulations is a separate compliance regime from planning, and it offers two routes: a simplified method with limits on glazing area and requirements for removing excess heat, and a dynamic thermal modelling route. The modelling route uses the domestic methodology with its own specified criteria, weather data and modelling assumptions, so a model built for Part O compliance is not automatically the model a planning authority wants.

In practice the sensible approach is one model, run twice, reported once. State which criteria set each result relates to, and keep the geometry, glazing and ventilation inputs identical between runs so that the two reported outcomes can be reconciled. Where they differ, say why.

Coordination with the Energy Statement matters here too, because the overheating model and the energy model share glazing specifications, g-values and ventilation rates. Discrepancies between the two are found on almost every review.

What to state in the report

Six items, each in one line near the front. The methodology and its edition. The criteria applied and to which spaces. The weather file by full name. The software and version. The rooms modelled and the worst performing room. And the compliance route being demonstrated.

Then the results, room by room, with the occupancy profile and internal gains in an appendix so a reviewer can check them. An Overheating Risk Assessment structured that way takes a reviewer twenty minutes rather than two hours, and that alone shortens the determination period.

Overheating modelling is a report type where councils expect a suitably qualified author and professional review is strongly advised. PlanScope drafts the planning narrative and structure around a modeller's outputs, with flags where figures are still to be confirmed. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which reports your scheme is likely to need.