Overheating Risk Assessment: TM59 inputs and outputs

An overheating risk assessment to CIBSE TM59 takes a thermal model of the building, applies fixed occupancy and internal gain profiles, runs it against a summer weather file and reports every habitable room against two temperature criteria. Rooms pass or they do not. The reason this matters at planning stage rather than only at building control stage is that the fixes for a failing room are usually architectural: orientation, glazing area, shading, dual aspect and floor-to-ceiling height. Change those after permission and you are back for an amendment.

Most of the argument on a TM59 report is about assumptions rather than about physics. Two competent engineers modelling the same block can reach opposite answers by choosing different weather files and different openable areas, and a case officer with no thermal modelling background has to be able to see which choices were made.

What TM59 tests

TM59 splits homes into two families. For predominantly naturally ventilated dwellings there are two criteria and a room has to satisfy both.

Criterion A is the adaptive comfort test taken from CIBSE TM52. It counts the occupied hours from May to September in which the operative temperature exceeds the adaptive comfort threshold by one degree or more, and limits those hours to three per cent of occupied hours. It applies to living rooms, kitchens and bedrooms.

Criterion B is a night-time test for bedrooms. Operative temperature between 10pm and 7am must not exceed 26 degrees for more than one per cent of the annual hours in that window across the same summer period, which works out at a low tens of hours rather than a comfortable margin.

Mechanically ventilated dwellings are assessed differently, against a fixed operative temperature threshold across occupied hours rather than the adaptive criterion, because occupants of a sealed home cannot respond by opening a window.

Homes for vulnerable occupants sit in a stricter category again. Sheltered housing, extra care and care accommodation are tested against tighter thresholds, and a scheme of that type should never be reported on the standard residential basis without saying so.

Where Part O and the planning assessment diverge

Part O of the Building Regulations introduced a duty to limit unwanted solar gain and to provide a means of removing excess heat from new residential buildings. It offers a simplified method, which caps glazing areas and sets minimum free ventilation areas by orientation and by whether the site is cross ventilated, and a dynamic thermal modelling method which uses the TM59 approach.

A planning Overheating Risk Assessment is not the same document. It usually has to demonstrate the cooling hierarchy set out in local policy, test future climate scenarios rather than only present-day ones, and show that the layout and the fabric were driven by the overheating result rather than checked against it afterwards. Where the scheme is in London, the assessment normally sits alongside the Energy Statement and is judged against the same policy chapter.

The practical consequence is that a Part O simplified-method compliance note will not discharge a planning requirement for a TM59 assessment. Councils that ask for one want the room-by-room output.

Weather files and the scenario you model

TM59 is run on a CIBSE Design Summer Year for the nearest weather location. The Design Summer Year set gives three different summer types, each available for different emissions scenarios, percentiles and time slices, and the choice is the single largest lever in the whole exercise.

State four things in the report: the weather station, the summer year variant, the emissions scenario and percentile, and the time slice. If the authority asks for a future time slice as a sensitivity test, present both sets of results in the same table so the difference is visible.

Urban heat island effects matter for large city sites. Using a rural weather file for a site in a dense inner urban block understates night-time temperatures, which is exactly where criterion B bites.

Assumptions that decide the answer

Occupancy and equipment gains are fixed by the methodology, which is the point of it. What remains open, and what a reviewer should check first, is a short list.

• Openable area actually available, after restrictors, opening geometry and reveal depths are taken off the nominal window area.
• Whether windows can be open at night at all, which is an acoustic question before it is a thermal one.
• Shading from balconies, reveals, external fins, neighbouring buildings and trees, and whether internal blinds have been credited when they should not be.
• Corridor and riser temperatures in a block with communal heating, since uninsulated distribution pipework can make a lobby the warmest space in the building.
• Ventilation strategy in single aspect flats, where cross ventilation is unavailable by definition.

Single aspect flats facing south or west are where schemes fail. That is not a reason to hide them. It is a reason to fix the plan while it is still a drawing, and the design evolution section of the Design and Access Statement is the right place to show which layout options were tested and discarded.

The cooling hierarchy and why openable area comes first

London Plan policy sets a cooling hierarchy which is now used well beyond London: reduce the heat entering the building through orientation, shading, albedo and insulation, then minimise internal heat generation from services and appliances, then manage the heat within the structure through thermal mass and exposed surfaces, then provide passive ventilation, then mechanical ventilation, and only then active cooling.

The hierarchy is assessed in order. A report which jumps to comfort cooling in a two-bedroom flat because the model failed will be sent back, because it has not demonstrated that the earlier steps were exhausted.

The tension with acoustics is real and worth stating openly in the report. Where a Noise Impact Assessment concludes that internal noise levels can only be met with windows closed, the thermal model must be run closed, and the scheme then needs a ventilation route that is not the window. Mechanical ventilation with a summer bypass, acoustically attenuated background ventilators or a change to the layout are the usual answers. Silence about the conflict is the one option that fails at determination.

Reporting rooms that fail

Give the case officer a table: unit reference, room, orientation, aspect, criterion A result, criterion B result. Then a short schedule of the rooms that do not pass, the reason, and what has changed in response.

Averaging across a block hides the problem and invites a consultee objection. And reporting only the worst-case unit is no better, because the officer cannot tell whether it is representative.

Where a residual failure remains after the design has been through the hierarchy, say so and explain the mitigation. An honest report with three failing bedrooms and a stated fix is more likely to be accepted than a report claiming every room passes on assumptions no one can check.

Conditions and post-completion checks

Authorities commonly condition the overheating strategy, requiring the modelling to be resubmitted once glazing specification, shading and ventilation products are fixed, and sometimes requiring in-use temperature monitoring after occupation. Read the condition wording carefully before agreeing it, because a condition that requires all rooms to pass with no tolerance is a condition you may not be able to discharge.

An overheating assessment is a modelling exercise, and councils usually expect a suitably qualified author, so treat an AI-assisted draft as the structure and the policy framing rather than as the calculation. Enter your postcode on PlanScope to see which environmental reports a residential scheme on your site is likely to need.

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