Who can prepare an overheating assessment to TM59

There is no licence to carry out a CIBSE TM59 overheating assessment, and no register a council can check. What there is instead is a methodology detailed enough that an inexperienced modeller produces a result nobody can rely on, and a set of inputs where small, defensible-looking choices move a scheme from fail to pass. That is why councils and reviewers look at the assumptions rather than the conclusion.

For an architect commissioning the work, the competence question comes down to four things: does the assessor build dynamic thermal models routinely rather than occasionally, do they use the correct weather files, will they model the units that are actually worst rather than a convenient sample, and will they tell you the answer before it is too late to change the design.

What the methodology asks of the modeller

TM59 is a design methodology for assessing overheating risk in homes, built on dynamic thermal modelling rather than a spreadsheet. The model runs the building hour by hour across a summer period, using defined occupancy, equipment and lighting profiles, and reports the hours during which internal conditions exceed the criteria.

For predominantly naturally ventilated homes the assessment applies the adaptive criteria from CIBSE TM52 to living areas, and a separate fixed threshold to bedrooms at night, because sleep is where overheating does the most harm. For homes relying on mechanical ventilation the criteria become fixed temperature thresholds applied across occupied hours. Blocks of flats also need the communal corridors assessed, and those often fail when the flats pass, because of heat loss from risers and long dwell times.

A competent modeller can explain which criterion each space is being tested against and why. One who cannot is reading numbers out of a software report.

Weather files and why the choice matters

Dynamic modelling is only as meaningful as the weather data behind it. CIBSE publishes Design Summer Year files for locations across the United Kingdom, in several variants representing different kinds of hot summer, and for different future time periods and emissions scenarios.

Two choices need to be stated explicitly in any assessment: which location file was used, and which year and scenario. A scheme modelled against a current weather file may pass and fail against a future one, which is the point of asking. In London the urban heat island effect makes the location choice more consequential still, and the London Plan requires proposals to follow a cooling hierarchy that puts passive measures ahead of mechanical cooling.

If the assessment does not name the weather file on its front page, send it back.

The inputs that decide pass or fail

Four assumptions do most of the work, and all four are places where an assessment can be quietly optimistic.

• Window opening. TM59 assumes windows can be opened, but a window that cannot be opened because of noise, security, pollution or a restrictor is not available in reality. The opening assumption must match the noise strategy, so if the Noise Impact Assessment concludes that windows must stay closed to meet internal criteria, the overheating model has to be run on that basis.
• Which units are modelled. Top floor, single aspect, south or west facing, small floor area with large glazing: that is the unit that fails. A sample that omits it is not a sample.
• External shading. Surrounding buildings, balconies above, reveals and fins all reduce gains, and omitting them is conservative. Including shading from a building that does not exist yet, or from trees, is not.
• Internal gains. TM59 sets profiles for occupancy, lighting and equipment. Altering them to suit the scheme is the clearest sign of a result that has been engineered.

The tension between noise and overheating is the one that catches schemes near roads and railways. Two consultants, two reports, two sets of assumptions, and nobody reconciling them until an officer notices. Ask for one page in each report that states the window position assumed, in the same words.

How this sits with the Building Regulations route

Approved Document O provides two routes to compliance for new dwellings: a simplified method with prescribed limits on glazing area and free area for ventilation, and a dynamic thermal modelling route based on the TM59 methodology with specified modifications.

Those routes are not identical to a planning overheating assessment, and confusing them causes real problems. A scheme can satisfy the simplified method and still show unacceptable results in a planning assessment, and a planning assessment run to TM59 without the Part O modifications does not demonstrate Building Regulations compliance.

State which you are doing. An Overheating Risk Assessment submitted with a planning application should say whether it also demonstrates compliance with the Building Regulations route, and if it does not, say what will be done at the next stage. Councils usually expect a suitably qualified author for this work, and professional review is strongly advised.

Reviewing a submitted assessment

A reviewer should be able to check the following in half an hour.

1. Weather file location, year and scenario, stated on the face of the report.
2. Which criteria were applied to which spaces, and whether the ventilation strategy justifies that choice.
3. The unit schedule, with the reasoning for the units selected and confirmation that the worst cases are among them.
4. Window opening assumptions, cross-checked against the noise and air quality evidence.
5. Shading inputs, cross-checked against the site plan and the surrounding context.
6. Whether the modelled geometry, glazing areas and construction build-ups match the submitted drawings.
7. What fails, by how much, and what is proposed about it.

Point seven is where weak reports give themselves away. An assessment reporting that every space passes every criterion on the first run, in a dense urban scheme with large south-facing glazing, deserves a second look rather than a signature. The wider question of what a reviewer takes responsibility for is covered in peer review and sign-off.

Mitigation that survives value engineering

Overheating mitigation falls into two groups, and the difference becomes visible about 18 months later.

Measures built into the form of the building survive. Reduced glazing ratios, a deeper reveal, fixed external shading, a brise soleil, orientation, dual aspect layouts giving cross ventilation, exposed thermal mass, and a shading coefficient fixed in the glazing specification are all hard to remove without redesigning.

Measures that depend on behaviour or on a later package do not survive. User-operated blinds, a promise of purge ventilation through a window that is later restricted for safety, mechanical cooling that gets omitted at tender, and secure night ventilation through a vent that is deleted from the door schedule: these are the items that disappear.

Design the passive measures in first and treat mechanical cooling as the last resort it is described as in policy. It is also the measure that carries an energy and carbon penalty, which the Energy Statement then has to account for.

Discharging the condition

Where a permission carries an overheating condition, the discharge submission normally needs the model rerun against the as-built specification, with the glazing, shading, ventilation and construction details confirmed from the contractor's information rather than the planning drawings.

That is the point at which value-engineered changes surface. Keep the original model and the input schedule so the rerun is a day's work rather than a rebuild, and keep the assessor engaged through the technical design stage. For finding a reviewer who can check the work independently, the professional directory is the route.

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.