Worked example: TM59 inputs for a single aspect flat

Get the TM59 overheating inputs wrong and a single aspect flat will fail on paper when it would be comfortable in practice, or pass on paper when it would not. The method fixes most of the assumptions for you. What the modeller still chooses is the weather file, the openable free area, the shading and whether windows can realistically be left open at night, and those four choices decide the result far more than the fabric does. What follows sets out an illustrative input set for a south facing single aspect one bedroom flat and shows where each assumption bites.

What a TM59 assessment tests

CIBSE TM59 is a design methodology for assessing overheating risk in homes using dynamic thermal modelling. It does not invent comfort criteria of its own. For predominantly naturally ventilated homes it applies the adaptive criteria from TM52 to living rooms, kitchens and bedrooms, then adds a separate fixed temperature test for bedrooms at night.

Two results come out of it. The first is whether occupied hours exceed the adaptive comfort threshold too often across the summer period. The second is whether bedrooms stay above 26 degrees overnight for more than a small fraction of the hours between 10pm and 7am.

Mechanically ventilated homes are treated differently, with a single fixed criterion applied across all occupied rooms. The distinction matters at the outset, because a flat served by mechanical ventilation with heat recovery and openable windows has to be modelled as what it actually is rather than as whichever gives the kinder answer.

Choosing the weather file and location

TM59 asks for a Design Summer Year file, and the baseline run uses DSY1 for the nearest suitable location under a moderate future scenario. Assessments that quietly use a Test Reference Year instead will overstate performance, because a TRY describes an average year rather than a warm one.

Location choice is not a formality. An inner urban site takes a file that reflects the urban heat island, and a site on the edge of a city served by a coastal or airport weather station will read differently. Say in the report which file was used, which scenario, and why that station was chosen.

Running DSY2 and DSY3 as a sensitivity is worth the modelling time. They describe different kinds of hot summer, one with a short intense heatwave and one with a long warm spell, and a design that passes DSY1 comfortably but collapses under DSY2 is worth knowing about before it is built.

The fixed occupancy and gains profiles

The method sets the occupancy, lighting and equipment profiles so the assessment cannot be tuned by the modeller. Bedrooms are treated as occupied overnight and partly during the day. Living rooms and kitchens are occupied through the day and evening. Cooking, lighting and appliance gains follow prescribed profiles, and heating and hot water distribution losses have to be included where pipework and cylinders sit inside the dwelling.

That last item catches a lot of flats. An unlagged heat interface unit and its associated pipework in a hallway cupboard puts a steady load into the dwelling all summer, and on a district heating scheme it can be the difference between a pass and a fail. Model it honestly, and if the distribution losses are the problem, the answer is insulation and controls rather than a different weather file.

Corridors in apartment buildings deserve the same treatment. TM59 includes a separate check on communal corridors for exactly this reason.

Openable area and the assumptions behind it

This is where most disputes sit. The model needs the free area of the opening, not the area of the window, and a top hung casement restricted to a 100 millimetre opening gives a fraction of what the elevation suggests.

Three constraints reduce openable area in practice:

1. Fall protection. Where there is a drop below the opening and no guarding, restrictors are fitted and the free area falls sharply.
2. Security. Ground floor and accessible upper floor windows are commonly assumed to be closed when the flat is unoccupied and at night.
3. External noise. Where the noise climate means a resident would not sleep with the window open, the model has to run with it closed.

Single aspect layouts then run into the physics. Single sided ventilation works over a limited depth, conventionally taken as around two and a half times the ceiling height, so the back of a deep single aspect flat is barely ventilated regardless of how large the window is. Cross ventilation, where it exists, works over roughly five times the ceiling height. A deep plan single aspect unit with one openable window is the hardest case in UK housing, which is why boroughs resist the typology.

The noise constraint links the overheating work to the acoustic work, and the two reports have to agree with each other. If the noise assessment concludes that windows must stay shut to meet internal criteria, the overheating model cannot assume they are open, and the sections an officer checks in a BS 4142 noise report are the ones that determine what the thermal model is allowed to do.

An illustrative input set for a single aspect flat

The figures below are illustrative only. They describe a notional 50 square metre one bedroom flat on the third floor of an urban block, single aspect facing south, and they exist to show the shape of the input schedule rather than to be copied.

Change the last line to an open window and this flat will usually pass. Leave it closed and it usually will not. That single assumption carries more weight than the glazing specification, and it has to be justified by the acoustic evidence rather than asserted.

Reading the pass and fail criteria

For the naturally ventilated case, the living room, kitchen and bedroom are tested against the adaptive criterion across the summer period, and the bedroom is separately tested for hours above 26 degrees between 10pm and 7am. Both have to be satisfied. Reporting a pass on one while staying quiet about the other is the most common reporting failure officers see.

Present the results room by room, with the number of exceedance hours and the threshold alongside. A table of percentages without the underlying hours is hard to check. Where a room fails, say by how much, because a room failing by a handful of hours points to a different design response than one failing by hundreds.

Design responses when a flat fails

Work down the order of effectiveness rather than reaching straight for cooling. External shading first: brise soleil, deep reveals, balconies above windows, or fixed fins on the western elevation where late afternoon gain dominates. Then glazing, reducing the ratio or improving the g-value, accepting that a low g-value coating trades daylight for heat control.

Then ventilation. Increasing the free area, changing the opening type from top hung to side hung, providing secure night ventilation through an acoustically attenuated vent, or dual aspect where the plan can be reworked. Then internal gains, which usually means the heating distribution.

Mechanical cooling is the last resort, and a scheme that arrives at cooling without evidence of the earlier steps will be challenged on energy policy grounds, particularly where the energy statement follows the standard hierarchy. The overheating work and the carbon reporting are read together, and the structure of that reporting is covered in the note on whole life carbon assessment reporting.

Keeping the model consistent with Part O

Approved Document O offers a simplified method based on glazing and free area limits, and a dynamic method that uses TM59 with modifications. The modifications matter. User operated internal blinds cannot be relied on, and where night-time noise would prevent windows being opened, the model has to reflect that.

Run one model and report it consistently to both audiences. Where the planning submission claims open windows and the building control submission assumes them closed, somebody will notice, and the discrepancy will follow the scheme. The design narrative in the design and access statement should describe the shading and ventilation strategy in the same terms the model used.

PlanScope drafts the overheating narrative and the supporting statements from your scheme details, flagging every input a modeller has to supply, and a suitably qualified assessor then completes and reviews the dynamic modelling. See the full set of report types that a residential scheme usually needs.

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.