Overheating Risk Assessment: reporting CIBSE TM59 results

A TM59 overheating assessment report turns on two criteria. One covers occupied hours across the whole dwelling, the other covers bedrooms at night. Everything else in the document exists to show that those two results came from inputs a case officer can check.

Officers rarely argue with the pass rate itself. They argue with the weather file, the occupancy profile, the assumed window opening, and whether the rooms that failed were reported at all.

CIBSE TM59 is a design methodology for assessing overheating risk in homes using dynamic thermal modelling. It is the method named in Approved Document O of the Building Regulations, and where a local plan carries a cooling or overheating policy, an Overheating Risk Assessment built on TM59 is usually what the validation list has in mind. Check the council's published validation requirements before you commission anything, because this is one of the documents that varies most between authorities.

Model inputs that decide the answer

Geometry comes first. Every habitable room is modelled as a zone with its real glazing area, orientation, shading from balconies and reveals, and shading from neighbouring buildings. A model that ignores the block opposite will overstate the risk on a tight urban site and understate it on an open one.

Then the weather file. TM59 assessments are run on CIBSE Design Summer Year data for the nearest available location, and the edition and emissions scenario have to be stated in the report. Two assessments of the same building on different weather years are not comparable, so a reviewer should be able to read the file name off the page.

Internal gains are where reports diverge most. TM59 sets out occupancy, lighting and equipment profiles so that different consultants produce something close to the same answer for the same building. Bedrooms are treated as occupied overnight and partly during the day. Living rooms and kitchens carry their own profiles. Heat from hot water distribution matters in flats served by a communal system, and leaving out the losses from risers and heat interface units is the single most common way a scheme passes on paper and cooks in practice.

Fabric and ventilation assumptions round it out. Reflectances, thermal mass exposed or hidden behind plasterboard, purge ventilation openings, trickle vents and any mechanical ventilation rates all need to be listed in a table the reader can audit against the drawings.

The two criteria and how they are reported

For predominantly naturally ventilated homes, TM59 applies two tests at once.

The first is an adaptive comfort test across living rooms, kitchens and bedrooms during the summer period. It limits the number of occupied hours in which the operative temperature runs above the adaptive comfort threshold to a small percentage of those hours. The second applies to bedrooms only and looks at night-time sleeping hours, limiting the hours above a fixed temperature.

Homes that rely on mechanical ventilation rather than opening windows are assessed against a single fixed-temperature test instead. The report has to say which category the dwelling falls into and why, because a building assessed as mechanically ventilated when the occupants will in fact open windows is being flattered.

Results belong in a table, one row per room, showing both criteria, the result and a plain pass or fail. Percentages rounded to one decimal place. No colour coding that hides a marginal failure.

Where Approved Document O differs

Approved Document O is the Building Regulations route, and it offers two ways to demonstrate compliance. The simplified method caps glazing areas and sets minimum free areas for openings according to location and cross-ventilation. The dynamic thermal modelling route uses TM59 with modifications set out in the Approved Document itself.

Those modifications matter for planning. Where external noise would prevent windows being opened at night, the Approved Document expects the assessment to reflect that, which usually means modelling with windows closed during sleeping hours. Guarding and restrictor assumptions are also constrained.

A planning-stage report that quietly assumes wide-open windows on a road frontage will be undone at building control. Better to run the constrained case at planning and design for it, which is the argument for pairing the overheating work with the Energy Statement rather than treating them as separate exercises.

Openable area, security and noise

Three real-world constraints cut the ventilation the model is allowed to claim.

Restrictors on windows above ground floor level limit the opening, and the model has to use the restricted free area rather than the full sash. Ground floor windows and those accessible from balconies or flat roofs raise security questions, and a scheme that depends on ground floor windows being open all night needs another answer. Noise is the third. If the acoustic assessment concludes that windows must stay shut to meet internal noise guideline values, the overheating model cannot then open them.

Where those three bite at once, on a busy urban frontage with restrictors and a noise constraint, the honest conclusion is often that passive measures alone will not get every bedroom through. Say so in the report and set out what follows.

Reducing risk before adding cooling

The order of measures is the argument, and it should be written as one.

1. Reduce heat gains through orientation, glazing ratio, external shading, solar control glazing and lighter external finishes.
2. Reduce internal gains, chiefly by insulating hot water distribution and specifying efficient lighting and appliances.
3. Manage heat inside the dwelling using exposed thermal mass where the construction allows.
4. Use passive ventilation, including cross-ventilation and secure night purge.
5. Use mechanical ventilation where passive means cannot deliver the air change rates.
6. Use active cooling only for the residual risk that the earlier steps cannot remove.

Most refusals on overheating grounds come from schemes that skip to step six, or from schemes that never wrote the sequence down. An officer reading a report wants to see that the glazing ratio was tested before a heat pump was specified.

Reporting the rooms that fail

A report that shows every room passing on the first run invites scrutiny. Real buildings have a south-west corner bedroom under a flat roof that struggles, and the credible document is the one that names it.

Set out the failing rooms, the margin by which they fail, the measures tested and the residual position. Where a design change fixes it, show the before and after. Where it does not, explain what the occupant is left with and whether an alternative such as a ceiling fan or a fixed external shade is being carried into the detailed design.

This is also where the limits of an AI-assisted draft are worth stating plainly. PlanScope can structure the narrative, cite the method and pull the policy references together, and the modelling itself has to come from a suitably qualified assessor running the software. Councils usually expect a named author with dynamic thermal modelling experience, and professional review is strongly advised before submission. The same logic applies to the sustainability documents that travel with it, which is why a BREEAM pre-assessment credit tracker is only as good as the assessor behind it.

Conditions and what happens after permission

Expect a condition tying the development to the submitted assessment, or requiring the measures listed in it to be built and retained. On larger residential schemes some authorities go further and seek verification at completion or a post-occupancy monitoring commitment.

Read the condition wording before the design is frozen. A condition that names external shading fins will not be discharged by a solar control glazing substitution, and the variation application to fix it costs more than getting the specification right at submission. If you want the detail of how the inputs and outputs fit together before you brief an assessor, the walk through the TM59 inputs and outputs covers the model side in more depth.

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. Enter your postcode on PlanScope to see which reports your site is likely to need.