The TM59 modelling section of an overheating assessment

The modelling section of an overheating assessment is where the result is decided, and almost all of it is fixed by the methodology rather than chosen by the modeller. A cibse tm59 overheating assessment modelling section that reads like a description of the software has missed the point. What a reviewer wants to see is the weather file, the occupancy and gains profiles, the window opening rules, and any place where the modeller departed from the standard assumptions, with the reason.

TM59 exists to stop the same building producing different answers in different hands. Write the section so that someone else could rebuild the model from it.

The two criteria for homes

For predominantly naturally ventilated dwellings, TM59 applies two tests, and a room has to pass both.

The first is the adaptive comfort criterion taken from TM52, applied to living rooms, kitchens and bedrooms: the number of occupied hours between May and September during which the operative temperature exceeds the adaptive threshold by one degree or more must not exceed three per cent of occupied hours.

The second applies to bedrooms at night. Between 22:00 and 07:00 the operative temperature must not exceed 26 degrees for more than one per cent of annual night hours, which works out at 32 hours a year.

For dwellings that are mechanically ventilated, the adaptive criterion is replaced by a fixed threshold applied across occupied rooms. Where occupants are vulnerable, for example in supported or care accommodation, TM59 directs a stricter approach again.

State in the report which criteria set was applied and why, because reviewers regularly find an adaptive criterion applied to a flat that has no openable windows worth the name.

Weather file choice and future scenarios

Design summer year files come in three forms for each location: one representing a moderately warm summer, one a short intense warm spell and one a longer, less intense warm period. Each exists for different future time periods, emissions scenarios and percentiles.

Approved Document O specifies which file is to be used for compliance, and that is the file the pass or fail rests on. TM59 encourages testing additional files to understand how resilient the design is, which is useful information for a client and is not the compliance test.

Report them separately. A table showing the compliance result alongside the resilience runs, clearly labelled, is far more useful than a merged figure, and it protects you when a consultee asks why a different file appears in the appendix.

Say which location file was used. A scheme in a large city modelled against a rural weather file understates the urban heat island effect that is built into the city file.

Occupancy profiles and internal gains

TM59 fixes these, and that is the point of it. Bedrooms are treated as occupied overnight and partly during the day, living areas during the evening, with defined gains from occupants, lighting, equipment and cooking. A modeller cannot assume an empty flat during a heatwave.

Where a departure is unavoidable, for example for a building type the methodology does not cover, the report has to state the profile used, the source and the reason. Anything else reads as a result engineered backwards from the answer.

Two profile details are worth checking in any report. Whether kitchen gains were included at the defined level, and whether the modelled occupancy matches the bedroom count rather than an assumed household size.

Window opening assumptions and their limits

This is the assumption that moves the result most, and the one most often overstated.

TM59 defines when windows are opened in the model, based on internal temperature during occupied hours. What the modeller has to get right is how much they open in reality. Free area is reduced by frames, by trickle vents being irrelevant to summer purge, and above all by restrictors fitted for safety at height, which on a flat above ground floor commonly limits an opening to around 100 millimetres.

Two constraints override the opening rule altogether. Where an acoustic assessment concludes that windows must stay shut at night to meet internal noise levels, the model has to be run closed. Where air quality requires the same, likewise. In both cases a single aspect flat usually fails without mechanical intervention.

Security and pollution constraints should be stated in the modelling section rather than left to the acoustic report, because a reader checking the overheating result should not have to open three documents to discover the windows cannot be opened.

Shading, blinds and cross ventilation

External shading counts: balconies above, deep reveals, brise soleil, and shading from opposing blocks. Model them at their actual dimensions.

Internal blinds are handled cautiously by the methodology, because their use depends on occupants and they do little to stop solar gain once it is through the glass. Do not claim a result that rests on internal blinds being closed all day.

Cross ventilation is the other big lever. A dual aspect flat with openings on opposite facades behaves entirely differently from a single aspect one of the same area, and where a corridor supplies ventilation air to a flat, TM59 expects communal corridors to be modelled so that the supply temperature is realistic rather than assumed.

Mechanical ventilation and cooling as a last resort

The hierarchy runs: reduce gains, then shade, then ventilate, then cool. A report that arrives at comfort cooling without showing the earlier steps invites a policy objection, since local plans generally require the passive measures to be exhausted first.

Where mechanical cooling is proposed, state the extent, the plant location, the effect on the energy assessment and the acoustic consequences of external condensers. Those three items move between documents and get lost.

Reporting results room by room

Give a table with every modelled room, its orientation, its aspect, the criterion applied, the result and the pass or fail. Identify the worst case rooms explicitly: top floor, west or south facing, single aspect, smallest openable area.

A report in which every room passes comfortably on a constrained urban site deserves a second look at the inputs, and a reviewer will give it one. Failures, where they exist, should be stated plainly with the mitigation that follows. An Overheating Risk Assessment that hides two failing bedrooms in an appendix will have them read out by somebody else.

Reconciling TM59 with Approved Document O

They are related but not identical. Approved Document O offers a simplified route and a dynamic route, and the dynamic route draws on the TM59 methodology while setting its own compliance requirements, including the weather file to be used.

So a TM59 pass is not automatically a building regulations pass, and a planning condition requiring a TM59 assessment is not discharged by a Part O calculation. Where both are needed, model once and report twice, with a short section explaining how the same model satisfies each.

Our note on who can prepare an overheating assessment covers the competence question, and reporting TM59 results deals with the presentation of the output tables. The full report index shows where the overheating work sits against the energy and acoustic documents it depends on.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. 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.