TM59 overheating modelling and modeller competence

There is no statutory register of overheating modellers, which means the competence question is answered by what the modeller can show rather than by a certificate. The practical test has three parts: do they use dynamic simulation software validated against a recognised test suite, can they justify every assumption they made about window opening, and have they read the acoustic report before setting the ventilation strategy. A TM59 assessment that fails any of those will be picked apart by a reviewer.

CIBSE Technical Memorandum 59 sets the methodology for assessing overheating risk in homes, and Approved Document O of the Building Regulations adopts a modified version of it as one of two compliance routes. The two documents are related but not identical, and knowing where they diverge is part of the competence.

What the methodology asks of the modeller

TM59 applies two criteria to predominantly naturally ventilated homes.

The first, taken from TM52, tests living rooms, kitchens and bedrooms: the number of hours during which the operative temperature exceeds the comfort threshold by one degree or more must not exceed three per cent of occupied hours between May and September.

The second applies to bedrooms only: the operative temperature must not exceed 26 degrees for more than one per cent of annual hours between 10pm and 7am.

For homes with mechanical ventilation and restricted openings, TM59 uses fixed temperature thresholds instead, applied for no more than one per cent of occupied hours.

The methodology also fixes the internal gains. Occupancy profiles, lighting loads, equipment loads and hot water distribution gains are specified so that two modellers assessing the same building should produce comparable answers. Departing from those profiles is permitted where the building type demands it, but the departure has to be stated and justified. An assessment that quietly reduces the equipment gains to pass is the commonest form of a bad report.

Software capability and validation

The model has to be a dynamic thermal simulation running at hourly or sub-hourly time steps, with a natural ventilation airflow network, not a steady-state calculation with a solar gain correction.

Ask for four things before appointing:

• The software name and version, and evidence that it has been validated against CIBSE TM33 or the ASHRAE Standard 140 test suite.
• Confirmation that the airflow network models buoyancy and wind-driven flow, since single-sided ventilation in a deep flat behaves quite differently from cross-ventilation.
• The shading approach, including whether surrounding buildings and balconies above are modelled as geometry rather than as a blanket reduction factor.
• A sample output pack from a previous scheme, redacted if necessary.

The last of those tells you more than the first three. A modeller who produces a results table without an assumptions register is a modeller whose work cannot be checked.

Weather files and which year is used

The weather file is the single input that changes results most, and it is where inexperienced work goes wrong.

CIBSE publishes Design Summer Year files for a set of UK locations, in three variants representing different kinds of hot summer, and for different future time periods and emissions scenarios. TM59 specifies which file to use for a current assessment, and Approved Document O specifies its own requirements for the compliance route, including which locations must be used for certain parts of London.

Three points to check on any submitted assessment:

1. The weather location chosen, and whether it is the nearest available or the one the guidance requires.
2. The design summer year variant and the time period, stated explicitly rather than implied.
3. Whether any future-period file has also been run as a sensitivity test, which is increasingly expected on larger schemes even where it is not a compliance requirement.

A model run against a test reference year rather than a design summer year is not a TM59 assessment at all.

Window opening assumptions and who sets them

This is where the modeller's judgement is exposed, and where a reviewer will look first.

TM59 sets a standard opening profile: windows open when the room is occupied and the internal temperature rises above a stated threshold, and close otherwise. Real buildings depart from that in ways the model has to reflect:

• Restrictors on windows above ground floor level, commonly limiting the opening to 100 mm for fall protection, which reduces the free area substantially.
• Windows that cannot be opened at night for security on ground floor or accessible upper floor units.
• Windows that cannot be opened because of external noise.
• Openings shielded by a balcony above or by a deep reveal.
• Tilt-before-turn or top-hung units, whose effective free area is a fraction of the clear opening.

Every one of those should appear in the assumptions register with a reason. The architect sets most of them, not the modeller, which is why the assumptions have to be agreed rather than assumed. A model that opens a window the fire strategy or the fall protection detail will not allow is a model that describes a different building.

Coordination with the acoustic strategy

The contradiction is familiar. The acoustic report says windows must stay shut at night to meet internal noise criteria. The overheating model opens them to lose heat. Both cannot be true.

Approved Document O addresses this directly by setting noise levels above which opening windows cannot be treated as a reasonable means of removing excess heat, which forces the design towards acoustically attenuated background ventilation, mechanical ventilation with a cooling strategy, or a layout that puts bedrooms on the quiet elevation.

The practical sequence is to run the acoustic survey first, establish the free field levels at each facade, then set the ventilation strategy, then model. Doing it the other way round produces two reports that contradict each other in the same application. Where the ground floor use or nearby plant is the noise source, the Noise Impact Assessment has to be shared with the modeller rather than filed separately.

BS 8233 internal noise guideline values for living rooms and bedrooms are the reference most acousticians use, and the overheating model should state which of those the ventilation strategy is designed to meet with windows closed.

Choosing between the compliance routes

Approved Document O offers a simplified method and a dynamic modelling method, and the choice is a project decision rather than a technical one.

The simplified route costs less and moves faster but is unforgiving on single-aspect flats and on any elevation with substantial glazing. Modelling costs more and usually permits a better building. Decide at stage two, not at stage four, because switching route late means redrawing.

Planning and Building Regulations are separate processes here. A local plan policy may require an overheating assessment at application stage, and some authorities set their own criteria above the Approved Document. An Overheating Risk Assessment prepared for planning should state which standard it addresses, since a document written to the planning policy will not automatically discharge the Building Regulations requirement.

What the submission must record

A complete assessment contains, at minimum:

• The compliance route and the criteria applied, with the source document and edition.
• The weather file, location, variant and time period.
• The rooms modelled and the basis for selecting them as worst case, which normally means the most exposed orientation, the top floor, single-aspect units and the smallest bedrooms.
• Geometry inputs, including surrounding massing and any overshadowing.
• Fabric, glazing g-value and light transmittance, and any shading devices with their control regime.
• The ventilation strategy, with free areas and the opening profile.
• Internal gains, stated against the TM59 profiles with any departures justified.
• Results per room against each criterion, with pass or fail stated plainly.
• Remedial measures where a room fails, and the re-run results.
• An assumptions register with a named author.

The assumptions register is what makes the document reviewable, and it is what a reviewer with a different view can argue with. Our note on overheating assessment competence and dynamic modelling goes further into how reviewers interrogate these inputs, and the professional directory sets out the review route for a draft.

This is general guidance, not legal advice. Overheating requirements depend on the current edition of the Approved Document and on local policy, so check both and take advice from a qualified building services engineer.

Browse all 66 PlanScope report types to see where an overheating assessment sits alongside the energy and acoustic evidence. 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.