Setting up a TM59 overheating assessment

Most TM59 assessments that fail do so because of two input assumptions rather than because the building is badly designed. The first is the weather file. The second is whether the windows can be opened at night. Get the TM59 overheating assessment inputs agreed early and the modelling becomes a design tool. Leave them to the end and the report becomes an argument about method.

CIBSE TM59 is a design methodology for assessing overheating risk in homes. It fixes the occupancy, the gains, the weather and the criteria so that two assessments of the same building are comparable, which is exactly why departing from its assumptions attracts questions.

What the methodology tests

TM59 applies two criteria to naturally ventilated homes and a single criterion to mechanically ventilated ones.

For predominantly naturally ventilated dwellings, Criterion A applies the adaptive comfort test from CIBSE TM52 to living rooms, kitchens and bedrooms. The number of occupied hours in May to September during which the operative temperature exceeds the comfort threshold by one degree or more must stay below three per cent of occupied hours.

Criterion B is the bedroom test and it is the one that decides most schemes. Between 10pm and 7am, the operative temperature in a bedroom must not exceed 26 degrees for more than one per cent of annual hours, which works out at around 32 hours across the summer period.

For homes with mechanical ventilation, the test changes to a fixed threshold across all occupied rooms rather than an adaptive one, on the reasoning that occupants of a sealed building have fewer adaptive options.

Weather file and location

The dynamic model runs against a CIBSE design summer year file. TM59 calls for DSY1 for the nearest CIBSE weather location, 2020s, high emissions, fiftieth percentile.

DSY1 is a moderately warm summer. DSY2 is a more intense short heatwave and DSY3 a longer, less intense one. Testing against all three is worth doing where the occupants may be vulnerable, such as in older persons housing or a care setting, and some London boroughs ask for it.

Location choice carries more weight than it looks. The CIBSE set covers a limited number of UK locations, and the London Weather Centre file carries the urban heat island effect that a nearby suburban file does not. Picking a cooler coastal file for an inland site is the oldest trick in this field and reviewers look for it. State the file you used, the scenario and the percentile in the report, on the first page.

Occupancy and internal gains are fixed

TM59 specifies occupancy profiles and internal gains rather than leaving them to the modeller. Bedrooms are occupied overnight and partly through the day, living spaces through the evening, with stated numbers of people per room type and stated equipment and lighting gains.

The point of this is comparability. A modeller who reduces occupancy hours, lowers equipment gains or assumes an empty flat during the working day will produce a pass that means nothing, and a reviewer who knows the method will spot it in the appendix.

Two areas do need judgement. Communal corridors in blocks of flats are assessed separately and are frequently the worst performing spaces, particularly where risers and heating distribution run through them. And where a dwelling has a non-standard layout, such as a studio or a room used for both sleeping and living, say how it was treated rather than quietly picking the more favourable profile.

Window opening, noise and air quality

This is where most of the argument sits. TM59 allows windows to be opened when the internal temperature rises above a threshold, but only where they can actually be opened in practice.

Three constraints reduce the openable area, and all three need evidence.

• Safety restrictors, which limit openings above a certain height above floor level and are routine in flats.
• Security, which usually prevents ground floor and accessible windows being left open overnight.
• External noise and air quality, which may mean windows cannot be relied on at night at all.

The noise constraint is the one that interacts with the rest of the application. If the acoustic evidence shows that bedroom windows must stay closed at night to meet internal noise levels, then the overheating model must run with those windows closed, and the ventilation strategy has to change accordingly. Running the acoustic and overheating work in isolation produces two reports that contradict each other, which is a validation problem as much as a design one. Commission the Noise Impact Assessment early enough to inform the model rather than after it.

Air quality does the same job near a busy corridor or an Air Quality Management Area, and the two often point at the same facades.

Reporting a failure and fixing it

A failing room is not a failing scheme. What matters is the order in which remedies are applied, because the cooling hierarchy in policy expects passive measures first.

1. Reduce solar gain through glazing area, orientation and a lower g-value, which costs nothing at design stage.
2. Add external shading: balconies, reveals, brise soleil, shutters or fixed fins. Internal blinds and curtains are not accepted as a solution in the building regulations route.
3. Improve ventilation through dual aspect layouts, larger openable areas, or ventilation paths that do not depend on a single facade.
4. Add mechanical ventilation, and then active cooling only where the preceding steps have been shown not to work.

Report the failing rooms individually with their criterion results rather than giving a scheme-wide pass rate. A planning officer reading an Overheating Risk Assessment needs to see which units fail, by how much, and what changed as a result. This is a report type where councils expect a suitably qualified author with dynamic thermal modelling experience, and professional review before submission is strongly advised.

How it relates to the building regulations route

Approved Document O offers two compliance routes for overheating in new residential buildings: a simplified method based on glazing areas and cross ventilation, and dynamic thermal modelling carried out in accordance with TM59 with the modifications set out in the Approved Document.

The modifications matter. The building regulations route specifies its own weather data and treatment of shading, and it does not allow reliance on internal blinds and curtains. So a TM59 assessment prepared for a planning application is not automatically a Part O compliance demonstration, and a Part O simplified method result is not a TM59 assessment at all.

In London, the London Plan expects the cooling hierarchy to be followed and dynamic modelling on major residential schemes, which is usually presented alongside the Energy Statement rather than as a standalone document. Elsewhere the requirement comes from local policy or a validation list, so check before assuming one is needed.

Run the assessment at the stage where the answer can still change the drawings. An overheating report produced after planning permission tends to produce either an expensive retrofit of shading or an awkward conversation about mechanical cooling.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice, so confirm requirements with your local planning authority. Browse the PlanScope report types to see what a residential scheme usually submits alongside it.