CIBSE TM59 overheating assessment for residential

A TM59 assessment passes only if a dwelling meets both of its criteria, and the second one catches more schemes than the first. Criterion A allows the operative temperature to exceed the adaptive comfort threshold by 1 K 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 applies to bedrooms alone and allows the night-time operative temperature between 22:00 and 07:00 to exceed 26 degrees Celsius for no more than 1 per cent of those hours. A single-aspect bedroom facing south-west in a flat with communal heating risers will usually fail Criterion B long before Criterion A becomes interesting.

The methodology is set out in CIBSE TM59, and Approved Document O of the Building Regulations adopts a dynamic method based on it with modifications. Planning and Building Regulations therefore ask closely related questions with different assumptions, and the two assessments are not interchangeable.

What the method tests

TM59 is a dynamic thermal modelling exercise, not a spreadsheet. Each room is modelled hourly across the summer period with defined occupancy, internal gains, ventilation openings and shading, and the output is a pass or fail against each criterion for every assessed room.

For dwellings that rely on mechanical ventilation or cooling rather than opening windows, the standard applies a fixed criterion instead of the adaptive one, based on hours above 26 degrees Celsius in all occupied rooms. State which route you are using and why in the opening page of the report, because a reviewer reading a fixed-criterion result against adaptive criteria will reach the wrong conclusion.

Sample the worst cases rather than every unit. A defensible sample covers each orientation, the top floor, single-aspect units, the smallest units, units with the highest glazing ratio and any unit adjoining a heating riser or plant room.

Weather file and climate scenario

Use the CIBSE Design Summer Year file for the nearest appropriate location, and state which one. The base assessment uses DSY1 for the 2020s, high emissions, 50th percentile. In London the weather station choice matters because the urban heat island is embedded in the file, and using a rural file for a central London site is a straightforward error.

Where the London Plan applies, expect the additional sensitivity runs with the more extreme summer files, and where the client wants comfort over the life of the building, a 2050s run is worth doing even when nobody has asked for it. Report each run separately rather than averaging them.

A future weather run that fails is not a reason to refuse. It is a reason to design in the capacity to add cooling later, and saying that plainly in the report is more credible than presenting a marginal pass and hoping.

Occupancy profiles and internal gains

TM59 fixes the occupancy and gains so that results are comparable between consultants. Bedrooms are occupied overnight, living areas through the day and evening, with defined equipment and lighting gains and a set metabolic contribution per person.

Two inputs decide most outcomes.

The first is communal heating distribution. Where a scheme has a communal boiler or heat network, the standing losses from risers and laterals inside the dwelling are a continuous summer gain, and TM59 requires them to be included. Assessments that omit them, or that assume a pipework insulation standard the mechanical drawings do not show, are the most common reason a result cannot be relied on.

The second is glazing. Modellers sometimes reduce g-values or add shading that is not on the planning drawings. Every input has to be traceable to a submitted drawing or specification, and a reviewer should be able to map each assumed shading device to an elevation.

Internal blinds cannot be relied on in the base case, because occupant behaviour is not a design feature.

Window opening, noise and security

Free area is where modelling meets reality. The model needs the openable area of each window, not the whole window, and restrictors, child safety limits above ground floor and the depth of a reveal all cut it down.

Where external noise means windows cannot be opened at night, the assessment must model them closed and rely on mechanical means, which usually turns a comfortable pass into a fail unless the ventilation strategy was designed for it. That is the hinge point between the acoustic and the thermal assessments, and both consultants need the same assumption about whether a bedroom window opens at 02:00 beside a distributor road.

Put the assumption in a table like this in the report, and cross-reference the acoustic report by document number.

Reporting room by room

Report results room by room, with orientation, floor level, glazing ratio, free area and the result against each criterion. A summary that says the scheme passes, with an appendix nobody can read, gets sent back.

Include a short methodology statement naming the software, the version, the weather file, the criteria applied and the rooms sampled, with the reasons for the sample. Where an officer or a reviewing consultant asks for the model, releasing it is usually quicker than three rounds of questions about inputs. Keep the input schedule as a table rather than as prose, because that is the part a reviewer checks line by line against the drawings.

Where a room fails, say which criterion, by how much, and what changes it. Percentages of hours, not adjectives. Our note on cooling hierarchy: proving the case before mechanical cooling sets out the order in which authorities expect mitigation to be tested, and the Overheating Risk Assessment page explains where that sits in the submission.

Mitigation in order of effectiveness

Reduce the gains first. Insulate and relocate heating distribution, reduce glazing area on the exposed elevations, specify lower solar transmittance where daylight allows, and use a summer bypass on heat recovery ventilation.

Then work on the fabric and form: external shading such as brises-soleil, reveals and balconies that shade the glazing below, exposed thermal mass where the construction allows, and dual aspect to permit cross ventilation. Dual aspect is the single most effective change available on most residential layouts, and it is a planning decision rather than a services one.

Mechanical ventilation and then active cooling come last, and a scheme that needs cooling should say so in the Energy Statement rather than leave it to be discovered at technical design. For student and co-living schemes the occupancy assumptions differ, as energy assessment for student accommodation schemes explains, and for schemes running into the next regulatory change future homes standard: advising on schemes in progress covers the timing question.

One honest uncertainty. TM59 fixes occupancy so results can be compared, which means it does not predict what any real household will experience. Treat a marginal pass as what it is: a modelling result within the uncertainty of the method, not a guarantee of comfort.

Draft the structure of your overheating and energy documents on PlanScope, then have the dynamic modelling carried out and signed by a qualified building services engineer.

For specialist technical reports, qualified professional input may be required. 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.