Overheating criteria inside a TM59 assessment

The TM59 overheating criteria are two tests, not one, and a dwelling has to pass both. The living space test is an adaptive comfort test applied across all occupied rooms over the summer period. The bedroom test is a fixed temperature test applied to the sleeping hours. Most failures are bedroom failures, and most of those come from a modelling assumption rather than from the fabric, which is why the assumptions section of the report deserves more scrutiny than the results table.

The methodology is CIBSE TM59, and Part O of the Building Regulations adopts it as the dynamic modelling route with its own additions on weather data and night noise. So a TM59 assessment submitted for planning purposes has to be run to satisfy both, and the report should say which requirements each assumption answers.

What the two criteria measure

For a predominantly naturally ventilated home the assessment runs both of the following over the period from 1 May to 30 September.

The first criterion tests all occupied rooms, including bedrooms, against an adaptive comfort threshold derived from the running mean external temperature. The number of hours in which the operative temperature exceeds the acceptable maximum by one degree or more must not exceed three per cent of occupied hours.

The second criterion applies to bedrooms only, between 10pm and 7am, and holds that the operative temperature must not exceed 26 degrees for more than about one per cent of annual hours, which works out at roughly 32 hours across the summer period.

Where a dwelling is mechanically ventilated without openable windows the approach changes, and a single fixed temperature test is applied to occupied rooms for a stated proportion of annual occupied hours. Say which route you have used, because the two are not comparable.

Both criteria are about hours of exceedance rather than peak temperature. A dwelling can reach an uncomfortable peak on two afternoons and still pass, and a dwelling that sits at 26.5 degrees every night for a month will fail on a handful of degrees.

The bedroom criterion and night time hours

Thirty-two hours is not many. Spread across five months it allows an average of about a quarter of an hour per night, which means a bedroom that runs marginally warm for a fortnight of settled weather has used its whole allowance.

That is why bedrooms drive the result, and why the night ventilation strategy is the most important thing in the assessment. If windows cannot be opened at night, for security, for fall protection or because of road noise, the heat accumulated during the day has nowhere to go.

Three design responses appear in reports that pass. Reducing the solar gain reaching the room during the day, through orientation, glazing area and external shading. Increasing the effective opening area available at night, through secure night vents, acoustically attenuated vents or openings protected by guarding. And exposing thermal mass so that the daytime peak is absorbed rather than passed to the air.

Ceiling fans are increasingly proposed. They shift the comfort threshold rather than the temperature, and the current methodology and Part O treat their contribution in a specific way, so check the treatment rather than assuming a credit.

The living space criterion and adaptive comfort

The first criterion is more forgiving because the threshold moves with the weather. When the running mean external temperature rises, the acceptable internal temperature rises with it, on the basis that occupants adapt their clothing and expectations.

That has a practical consequence worth understanding. A living room in a hot spell is judged against a higher threshold than the same room in a cool spell, so an assessment can show a room passing during the hottest week and failing in June. Reviewers who expect the worst results to coincide with the hottest weather sometimes read that as an error.

Kitchens deserve separate attention. Cooking gains are substantial and the methodology prescribes them, so an open plan kitchen and living space carries the gains of both functions in one volume with one set of openings.

Weather files and choosing the location

The weather file choice can move a result from pass to fail, and it is the first thing a reviewer should check.

The methodology uses design summer year files for a set of UK locations, with a primary file representing a moderately warm summer and further files representing hotter and more extreme patterns. Part O prescribes which files and which future period to use, and also distinguishes locations by overheating risk.

Four points to state explicitly in the report:

1. The weather station or location file used, and why it is the appropriate one for the site.
2. The design summer year variant and the period modelled.
3. Whether an urban heat island effect has been accounted for, and how.
4. Whether the additional hotter files were run as a sensitivity test, and what they showed.

Choosing a coastal file for an inland urban site, or a rural file for a dense city centre, understates the risk. Where the nearest file is not representative, say so and run the next nearest as a sensitivity.

Occupancy and window opening assumptions

The methodology prescribes occupancy profiles so that assessments are comparable: bedrooms occupied overnight and partly during the day, living spaces occupied through the day and evening, with defined gains from people, lighting, equipment and cooking. Departing from those profiles without explanation is the most common way a favourable result is manufactured.

Window opening is where the judgement sits. The modelling has to reflect what an occupant can actually do:

• Windows above ground floor level without guarding are restricted to a limited opening, which sharply reduces the effective free area.
• Ground floor and otherwise accessible windows should not be assumed open overnight where security is a real concern.
• Where external noise at night exceeds the relevant internal criteria with windows open, the windows have to be modelled closed and an alternative ventilation route provided.

That last point ties the overheating assessment to the acoustic one. A Noise Impact Assessment that identifies the facades where windows cannot be opened at night defines the inputs to the thermal model, and the two reports have to agree. Where they do not, one of them is wrong, and the discipline of identifying the affected receptors is the same as in our note on receptor identification in a noise impact assessment.

Single aspect units and purge ventilation

Cross ventilation does most of the work in a naturally ventilated dwelling, and a single aspect unit has none. Air comes in and goes out through openings on the same facade, driven by buoyancy and by wind pressure differences that may be small.

So single aspect units concentrate the failures, particularly on upper floors, on west facing elevations and behind recessed balconies. Purge ventilation provision under the ventilation requirements is a minimum rather than a cooling strategy, and an assessment that relies on purge openings alone in a deep single aspect plan will usually fail.

Illustrative example: a single aspect west facing one bedroom flat on the fifth floor, with a recessed balcony over the living room glazing and restricted window openings above ground floor. The bedroom exceeded the night criterion by roughly three times the permitted hours. Reducing glazing on the bedroom elevation, adding an acoustically attenuated night vent and changing the balcony to a projecting type brought the unit within both criteria. The figures are illustrative and used to show the sequence.

Reporting failures room by room

Report every room, not just the failures, and report both criteria for every bedroom.

A usable results table gives the unit reference, orientation, floor level, room, the criterion, the permitted hours, the modelled hours and the pass or fail. Then a separate schedule of the units that fail, what causes each failure, and the mitigation applied.

Three things a reviewer will look for beyond the numbers. The full assumptions schedule, covering weather file, occupancy, gains, opening areas, opening schedules and any restrictions. A statement of what was modelled as built rather than as aspired to, particularly for shading that is drawn but not yet specified. And confirmation that the mitigation is secured, which usually means it appears on the approved drawings.

Councils normally expect an Overheating Risk Assessment from a suitably qualified author, and professional review is strongly advised because the assumptions rather than the arithmetic decide the outcome. And be honest about the units that only just pass. A unit at two point eight per cent against a three per cent limit is a unit that will fail when the shading is value engineered out. Browse all 66 PlanScope report types and start a draft.

This is general guidance, not legal advice, and the applicable criteria and weather data requirements depend on the building type and the current regulations, so check the position with your local planning authority and a qualified building physics adviser before relying on it.

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