Overheating criteria under CIBSE TM59

CIBSE TM59 sets two criteria for naturally ventilated homes. Criterion A allows the operative temperature to exceed the adaptive comfort threshold by one degree or more for no more than 3 per cent of occupied hours between 1 May and 30 September, in living rooms, kitchens and bedrooms. Criterion B allows bedroom temperatures to exceed 26 °C for no more than 32 hours across that same period, between 22:00 and 07:00. Both have to pass. A flat that sails through criterion A and fails criterion B by four hours fails the assessment.

For predominantly mechanically ventilated homes the method switches to a single fixed threshold, with occupied rooms not exceeding 26 °C for more than 3 per cent of annual occupied hours.

What the methodology assesses

TM59 is a design methodology for assessing overheating risk in homes using dynamic thermal modelling. It is not a standard, it sets no legal requirement, and it does not predict what any particular household will experience. What it does is fix the inputs, so that two assessments of the same building by different consultants should produce broadly the same answer.

That is the point of it. Before TM59 published in 2017, residential overheating assessments varied so widely in occupancy assumptions, window opening behaviour and weather data that results were not comparable. The methodology constrains those choices.

It covers the home as built and occupied, which means it captures solar gain through glazing, internal gains from people, lighting and equipment, the thermal mass of the construction, and the ventilation available by opening windows or by mechanical means. It does not assess the building's energy use, which is a separate calculation entirely.

The two criteria and how they apply

Criterion A is adaptive, so the acceptable indoor temperature rises with the running mean of outdoor temperature. People tolerate more indoors during a warm spell than during a cool one, and the criterion reflects that.

Criterion B is not adaptive, because sleep is not. Core body temperature regulation during sleep does not adjust to the weather, which is why the bedroom test is a flat 26 °C and why it is the criterion that usually fails.

The 32 hour allowance is small. Across a five month period it amounts to roughly three and a half nights of mild exceedance, or one hot night in a heatwave. On a south or west facing single aspect flat with no cross ventilation, that budget is spent in a single July week.

Weather files and the choice of year

A dynamic model is only as representative as the weather file driving it. CIBSE publishes Design Summer Year files for a set of UK locations, each one representing a different kind of warm summer: a moderately warm summer, a short intense heatwave, and a longer less intense warm period.

TM59 compliance assessments are normally run on the moderately warm summer file, with the other two used as a sensitivity test to show how the building behaves in a heatwave. The emissions scenario, percentile and time period all have to be stated, because a file representing current conditions and one representing the 2050s give materially different results for the same building.

State the file name, the location, the scenario and the percentile in the report. An assessment that reports a pass without naming the weather file cannot be checked, and a reviewer will ask.

Location choice matters more than people expect. The nearest published location is not always the right one for a site in an urban heat island, and London sites in particular should use a London file rather than a regional one.

Bedroom and living space treatment

TM59 prescribes occupancy profiles rather than leaving them to the modeller. Bedrooms are treated as occupied overnight and partially occupied during the day. Living rooms and kitchens are occupied through the day and evening. Internal gains from people, lighting, appliances and cooking are set out by room type, as are gains from hot water distribution pipework, which on a scheme with a communal heat network is a substantial load and a common source of failure.

Window opening follows a rule rather than a behaviour. Windows open when the room is occupied and the internal temperature passes a set point, and they close again below it. Where opening has to be restricted, for fall protection on a first floor window or for security at ground level, the restricted opening area is what the model uses, and that single input often decides the result.

Which rooms to model: the methodology expects the worst affected units rather than every flat. On a block that means the top floor south or west facing single aspect units, corner units with two exposed elevations, units above a plant room, and a sample of typical mid-block units for comparison. The reasoning behind the selection belongs in the report, and the detail of those inputs is covered in TM59 overheating inputs for a single aspect flat.

Relationship with Building Regulations Part O

Approved Document O deals with overheating in new residential buildings and offers two routes to compliance.

The simplified method limits glazing area by orientation and requires a minimum free area for ventilation, with different values for locations identified as higher risk. It is quick, it needs no modelling, and it constrains the design heavily. On an urban apartment scheme it is rarely achievable.

The dynamic route uses TM59, with Part O specifying the weather data, the scenarios and some of the conditions to be used, which are not identical to a TM59 assessment run for planning purposes. Running one model and presenting it for both purposes without checking the differences is a common error.

Part O also carries a noise provision that matters on any site near a road or railway. Where the overheating strategy relies on opening windows, the guidance expects night-time noise in bedrooms to stay within stated limits, around 40 dB LAeq,8hour and 55 dB LAFmax. Where road noise exceeds that with windows open, the windows cannot be assumed open, and the design has to deliver cooling another way.

That interaction is the hardest technical problem on urban infill sites, and it has to be solved jointly by the acoustician and the modeller rather than sequentially.

An illustrative result for a single aspect flat

The figures below are illustrative, to show how the two criteria behave.

Take a top floor one bedroom flat facing south west, 50 m², with a glazing to floor area ratio of about 30 per cent on the single exposed elevation, lightweight internal construction, mechanical extract ventilation and windows restricted to 100 mm opening for fall protection.

A first run might give criterion A exceedance of around 2 per cent of occupied hours in the living room, which passes, and criterion B bedroom exceedance of around 90 hours, which fails by a wide margin. The building passes the adaptive daytime test and fails the sleeping test, which is the characteristic result for this unit type.

The design responses that move criterion B are, in rough order of effect, external shading on the exposed elevation, reducing the glazing ratio, adding a second opening to create cross ventilation where the plan allows it, increasing exposed thermal mass, and only then mechanical cooling. The order matters because the cooling hierarchy in policy expects passive measures to be exhausted first, and the sequence is set out in overheating in single aspect flats and the design responses.

What a planning submission should include

A reviewer needs enough to reproduce the result.

The software and version, the weather file with location, scenario, percentile and period, the units modelled with the reason for selecting them, the construction build-ups with U-values and thermal mass, the glazing specification including g-value, the ventilation strategy with openable areas and any restrictors, the occupancy and gains profiles with any departure from TM59 justified, and the results against both criteria room by room rather than as a single pass statement.

Where cooling is proposed, show the passive measures tested first and why they were insufficient. Where a scheme passes only with windows open, state the façade noise levels so the consistency with the acoustic strategy is visible.

An Overheating Risk Assessment is usually expected from an author competent in dynamic thermal modelling, and professional review before submission is strongly advised. The question of competence is set out in who can prepare an overheating assessment to TM59.

The honest uncertainty here is the gap between modelled and measured performance. Post-occupancy data on completed schemes remains thin, and the window opening rule in particular is a modelling convention rather than an observed behaviour.

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.