The overheating compliance route in a TM59 assessment

A TM59 overheating assessment is one of two ways to demonstrate that a new home will not overheat, and choosing between them is a design decision rather than a reporting decision. Approved Document O offers a simplified route, which limits glazing area and prescribes minimum openable free areas by risk category, and a dynamic thermal modelling route built on the CIBSE methodology for assessing overheating risk in homes. The simplified route is quick and inflexible. The modelling route is slower, costs more, and is the only option once the design wants large glazing, a single aspect, or windows that cannot be opened at night.

Decide which route you are on before the elevations are fixed, because the simplified route's glazing limits are not something you can retrofit into a design that has already been sold to a client.

The two compliance routes compared

There is a third case worth naming. Where a planning authority's own policy requires an overheating assessment, for instance under a cooling hierarchy policy, the modelling route may be required for the planning application even where the simplified route would satisfy the building regulations. Check the local policy before assuming that building control is the only audience.

Weather files, scenarios and occupancy profiles

Three inputs determine the answer more than the building fabric does, and all three should be stated plainly on the first page of the report.

The weather file. The methodology uses design summer year files for the nearest available location, and the approved document specifies which file and which emissions and percentile scenario forms the compliance case. The alternative files, covering a short intense warm spell and a longer less intense one, are used as sensitivity tests rather than as the compliance case. A report that does not name the file it used cannot be checked.

The location. Using a weather file for a city 80 kilometres away because it was to hand will change the result. So will ignoring the urban heat island effect on a dense inner-city site, where the published file may understate night-time temperatures.

The occupancy profile. The methodology sets standard profiles, with bedrooms occupied overnight, living spaces occupied through the day and evening, and stated internal gains from people, lighting and equipment. Those profiles are deliberately conservative, and substituting a lighter occupancy assumption to pass the criteria is the single most common manipulation in this field. If a non-standard profile is used, it has to be justified and flagged.

The criteria for bedrooms and living spaces

For predominantly naturally ventilated homes there are two criteria and both must be met.

The first applies to occupied rooms across the May to September period, and limits the hours during which the operative temperature exceeds the comfort threshold by one degree or more to a small percentage of occupied hours.

The second applies to bedrooms at night, between 22:00 and 07:00, and limits the hours during which the operative temperature exceeds 26 degrees to one per cent of the annual hours in that period. That works out at around 32 hours across the year, which is a tight allowance and the one most schemes fail.

For homes that are predominantly mechanically ventilated, a single criterion applies, limiting hours above 26 degrees across all occupied rooms to a small percentage of annual occupied hours.

Report every room against every applicable criterion, not just the worst one. An officer or a building control body checking the work needs to see which rooms pass by a wide margin and which pass by an hour, because the second group is where a change during construction will cause a failure. An Overheating Risk Assessment that reports only a headline pass is not checkable.

Window opening assumptions and the noise constraint

This is where most modelling goes wrong, and it goes wrong in the applicant's favour.

The model assumes windows open when the room is warm. Whether they can be opened in reality depends on the openable free area after restrictors, guarding and the opening geometry, on security at ground floor level, on whether the flat has a single aspect so no cross-ventilation is possible, and on the external noise environment at night.

The approved document deals with the noise point directly. Where windows have to be closed at night to keep internal noise within acceptable limits, the model has to reflect that, which usually means the scheme needs mechanical ventilation and sometimes cooling. You cannot assume an open window for thermal purposes and a closed window for acoustic purposes in the same room on the same night.

So check three things in any model. What openable free area was assumed, room by room, and does it match the window schedule. What noise level was assumed at the facade, and does it match the Noise Impact Assessment. And whether restrictors, which are commonly fitted above ground floor level, were applied in the model.

Mitigation in the order the hierarchy requires

The hierarchy is not a preference. It is the order in which measures have to be considered, and a scheme that jumps to cooling without exhausting the earlier steps will be asked to go back.

1. Reduce unwanted solar gains through orientation, glazing area and proportion, glazing specification and g-value.
2. Provide external shading, whether fixed brise soleil, deep reveals, balconies acting as shading, or external blinds, which outperform internal blinds substantially.
3. Reduce internal gains, mainly by insulating and locating heating and hot water distribution properly, since uninsulated pipework in a riser is a large and avoidable gain in flats.
4. Provide passive means of removing heat, including openable area, cross-ventilation where the plan allows, and night purge.
5. Provide mechanical ventilation where passive means are constrained.
6. Provide active cooling only where the preceding steps cannot deliver compliance, and state the energy consequence.

Two measures worth more than their reputation. Deeper reveals and recessed windows cost little and shade a surprising amount on south and west elevations. And moving a bedroom from a west-facing single aspect position to an east-facing one at plan stage solves a problem that no amount of glazing specification will fix afterwards.

Where overheating meets acoustics and air quality

The three assessments constrain each other, and on an urban site they have to be run together rather than in sequence.

Noise sets whether windows can open at night, which sets whether the thermal model can rely on them. Air quality does the same on a site within or beside a management area, where opening a window onto a busy road at a low level is not an acceptable ventilation strategy, and an Air Quality Assessment may point towards filtered mechanical supply. Daylight pulls the other way, since reducing glazing to control solar gain reduces daylight, and a Daylight and Sunlight Assessment may set a floor below which the glazing cannot go.

Run one coordination exercise where the acoustician, the air quality assessor and the thermal modeller agree the facade strategy per elevation. Doing it in series, with each consultant inheriting the previous one's assumptions, produces a design that fails one test whichever way it is adjusted.

Reporting the results for a planning submission

A submission that survives review states the software used and its version, the weather file and scenario by name, the location and any urban heat island adjustment, the occupancy and gains profiles with any departure from the standard set flagged, a room schedule identifying every occupied room modelled and every one excluded with a reason, the openable free area assumed for each room cross-referenced to the window schedule, the facade noise levels assumed and their source, results for every room against every applicable criterion in a table, the sensitivity results using the alternative weather files, the mitigation described in hierarchy order, and a statement of residual risk.

Where the scheme also needs an Energy Statement, keep the two consistent. A fabric specification in the energy work that differs from the one in the thermal model is the most frequently spotted inconsistency in a planning submission, and it undermines both documents.

This is general guidance, not legal advice; check the position with your local planning authority, building control body 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. Dynamic thermal modelling is work councils expect a suitably qualified author to carry out, so professional review is strongly advised.

Browse the report types on PlanScope to see how the overheating, energy and acoustic documents fit together.