Overheating Risk Assessment outline to the dynamic method

An overheating risk assessment outline for the dynamic route has a fixed skeleton, and reviewers check it in the same order every time: the compliance route, the weather file, the rooms modelled, the ventilation assumptions, the results against each criterion, and the mitigation actually built into the drawings. Miss the weather file or the window opening assumption and the results mean nothing, however many rooms were modelled.

Approved Document O offers two routes for new residential development. The simplified method sets limits on glazing area and on the free area of openings by risk category and orientation. The dynamic method uses thermal modelling to the CIBSE methodology for homes. Most schemes of any complexity end up on the dynamic route, because the simplified method's glazing limits are hard to meet on a modern block of flats.

Which compliance route applies

Say which route you have taken in the first paragraph, and why. Where a scheme mixes forms, for example houses and a block of flats, the route may differ between them and the report should treat them separately.

Three situations force the dynamic route. Glazing areas above the simplified method's limits. Cross-ventilation that cannot be achieved because units are single aspect. And any case where windows cannot be assumed openable at night, which on a busy road is most urban sites.

Note also that the dynamic route is not a softer option. It gives design freedom in exchange for having to demonstrate performance, and a failed model is a clearer refusal than a marginal simplified calculation.

Model inputs and weather files

This section is where a reviewer decides whether to trust the rest. Set out every input.

• Software name and version, and the calculation engine.
• Weather file: the CIBSE design summer year for the nearest appropriate location, with the location choice justified, and the urban heat island position addressed for city centre sites.
• Sensitivity testing against the hotter design summer year files, and against a future climate scenario where the local authority asks for one.
• Construction: U-values, thermal mass and the g-value and light transmittance of each glazing type, by orientation.
• Shading: overhangs, reveals, brise soleil, balconies above, and any external obstruction from neighbouring buildings, modelled geometrically rather than as a blanket factor.
• Internal gains: the standard occupancy, lighting, equipment and hot water profiles from the methodology, and any additional gains such as heat loss from communal heating distribution pipework, which is the usual cause of corridor overheating in blocks with a district system.
• Ventilation: openable free area per window, restrictor positions, and the control assumption for when windows open and close.

State the design summer year used by name. A report that says a typical weather file was used has told the reviewer nothing.

Room selection and worst case units

The methodology expects all occupied rooms to be assessed, and in practice the modelling covers a representative selection with the worst cases identified and justified. Explain the selection logic rather than presenting a list.

The units that usually fail:

1. Top floor units, with roof gains and often the largest glazing.
2. Single aspect units facing south or west, with no cross-ventilation path.
3. Corner units with two exposed elevations.
4. Units with an inset balcony that blocks the airflow while adding a reflective surface.
5. Small studios, where the ratio of glazing and gains to volume is highest.
6. Units adjoining a communal corridor carrying heating pipework.

Communal corridors need assessing in their own right, against the corridor temperature criterion in the methodology, and they fail more often than the flats do.

Ventilation assumptions and the noise constraint

This is the assumption that decides the result, and it is where assessments are most often optimistic.

Approved Document O requires the modelling to reflect what occupants can actually do. Where noise means windows cannot be opened at night, the model must assume them closed. Where fall protection restricts opening above ground floor level, the free area must reflect the restrictor. Where security or air quality means windows stay shut, the same applies.

So the acoustic assessment and the overheating assessment have to share one set of assumptions. A scheme whose noise report specifies closed windows with mechanical ventilation, and whose overheating model opens the same windows at 02:00, will be sent back by whichever officer reads both. Resolve it once, in writing, and cross-reference each document to the other. The same conflict arises with air quality mitigation on exposed frontages.

Purge ventilation, night cooling and mechanical ventilation with a summer bypass all need their control strategy stating, including what happens when the occupant does nothing.

Results presentation

Present results room by room, keyed to a plan, with the criterion each result is tested against and a plain pass or fail. The criteria for predominantly naturally ventilated homes are the hours of exceedance test across the May to September period and the separate bedroom night-time test, with the fixed temperature criterion applying to mechanically ventilated dwellings.

Do not bury failures in an appendix. Where a room fails, say so, say by how much, and say what has changed in the design as a result. A report showing three failures and three design responses is more credible than one showing universal compliance on a south-facing single aspect block, and the detail of how to lay the numbers out is covered in reporting TM59 results.

Illustrative example

A block of 54 flats on a main road models 14 rooms across six representative units plus two corridor cases. Bedrooms on the road frontage are modelled with windows closed at night because the acoustic report requires it, ventilated mechanically with a summer bypass. Two top floor west-facing living rooms fail the hours of exceedance test on the first run. External shading fins and a lower g-value glazing on that elevation bring both within the criterion, and the corridor case passes once the heating distribution is relocated and insulated. The figures are illustrative and each scheme depends on its own model.

Mitigation hierarchy applied

Set the mitigation out in the order the guidance expects, and show what each step achieved in the model rather than listing measures.

Reduce the gains first: glazing area and orientation, then external shading, then solar control glazing. Remove excess heat next: cross-ventilation where the plan allows it, openable area, night purge, and thermal mass to absorb the daytime peak. Only then consider mechanical ventilation, and only after that mechanical cooling, which the guidance treats as the last resort and which brings its own energy and carbon consequences into the Energy Statement.

Two practical notes. External shading is far more effective than internal blinds and needs to be on the planning drawings, because adding it later changes the elevations. And glazing area reductions are the cheapest fix available and the hardest to get agreed, so run the model early enough that the elevation can still change.

Reporting for the planning submission

Planning and building regulations want different things from the same model. The building control submission is about compliance. The planning submission is about whether the design has responded, which means the report has to be legible to a planning officer who is not a thermal modeller.

For planning, include a short summary of the route, criteria and outcome in plain terms, a table of results by unit type, the mitigation register cross-referenced to the drawing numbers where each measure appears, the shared ventilation assumption with the acoustic and air quality reports, and a statement of the cooling hierarchy where local policy requires one. In London the cooling hierarchy in the London Plan is the relevant policy and the energy assessment guidance sets out what a referable scheme must submit.

An Overheating Risk Assessment draft gives you the structure, the criteria framing and the policy narrative. The modelling itself needs a competent modeller with the software and the judgement to set the assumptions, and the question of who that should be is covered in who can prepare an overheating assessment to TM59.

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, and councils normally expect dynamic thermal modelling from a suitably qualified author, so professional review is strongly advised.