Overheating assessment competence and dynamic modelling

Approved Document O gives two ways to show a new home will not overheat, and they need different people. The simplified method is a set of geometric limits on glazing area and openable free area that a designer can apply from the drawings. The dynamic route is a thermal simulation of the building over a summer, run against defined weather data and occupancy assumptions, and overheating assessment competence in that second route means simulation experience rather than a general sustainability background.

Confusing the two costs money at the wrong moment. A scheme designed to the simplified limits and then tested dynamically at technical design stage, because the planning condition asked for modelling, often fails on the bedrooms.

The two routes and who can run each

The simplified method caps the proportion of glazing by orientation and sets a minimum free area for ventilation, with different values depending on whether the location is treated as higher risk. It is a compliance check, and an architect or building services engineer can apply it with care. Its weakness is that it says nothing about a single aspect flat with restricted window opening, which is exactly the type that overheats.

The dynamic route runs a thermal model, and for homes the recognised methodology is the CIBSE approach for predominantly naturally ventilated dwellings. It tests each modelled room against criteria for daytime exceedance and for bedroom temperature at night. The person running it needs to know the software, the methodology and the assumptions the methodology fixes, and needs to be able to explain each of those to a reviewer.

There is no statutory register of overheating modellers. CIBSE maintains registers of low carbon consultants and energy assessors, membership of a relevant institution is the usual proxy, and a modeller's project list in comparable building types tells you more than either.

What dynamic thermal modelling involves

A thermal model has geometry, construction, gains, ventilation and weather. Each of those is a place where a result can be produced that has nothing to do with the building as it will be built.

Geometry has to include the things that shade and the things that block airflow. Balconies above windows, deep reveals, opposing blocks across a courtyard, and the corridor in a single aspect flat with mechanical ventilation. The methodology expects communal corridors in flatted schemes to be modelled, because a warm corridor supplying a ventilation unit changes the result inside the flat.

Gains and occupancy are largely fixed by the methodology rather than chosen, which is a good thing, since it stops a modeller producing a pass by assuming an empty flat. Where a modeller has departed from the standard profiles, the report has to say so and justify it.

Weather files and the assumptions inside them

The weather file is the assumption most often glossed over and the one that changes the answer most. Design summer year files exist for different locations, different years and different percentiles, and Approved Document O specifies which file applies for compliance. A model run against a different file is not comparable, and a model run against a mild year will pass a building that fails against the specified one.

State in the report which file was used, for which location, and why. Where a local policy or a client brief asks for a future climate scenario in addition to the compliance file, run both and present them separately rather than merging them.

Urban heat island effects are embedded in the file choice for some locations. Do not add an allowance on top without saying what you have done.

The inputs that actually decide the result

Six inputs decide most residential outcomes.

Glazing area and the g-value of the specified glass. Window opening: not the size of the window, but the actual free area once frames, restrictors and any acoustic ventilator are accounted for. Whether the flat is single or dual aspect. External shading, since internal blinds are of limited help and the methodology constrains what can be claimed for them. The ventilation strategy and whether windows can be opened at night. And thermal mass, which is the one most often assumed rather than specified.

Noise is the constraint that produces the hardest cases. Where an acoustic assessment concludes that windows must remain closed at night to meet internal noise levels, the overheating model has to be run in that condition, and the scheme then usually needs mechanical cooling or a redesign of the facade. Model the windows-closed case wherever the site is noisy, and read the Noise Impact Assessment and the overheating work together rather than in sequence.

Where the model diverges from the drawings

Three divergences recur, and all of them appear after planning permission.

Openable free area shrinks. A window drawn as fully openable becomes a restricted opening for safety at height, and the free area drops well below the modelled figure. Check the restrictor detail against the model input.

Glazing specification changes during procurement. A lower cost unit with a higher g-value passes the energy target and fails the overheating one.

Shading disappears. Brise soleil, projecting balconies and external blinds are the first items to be value engineered out, and each of them was carrying part of the result.

Guard against this by writing the modelled parameters into the specification, not just into the report. An Overheating Risk Assessment that lists the glazing g-value, the free area per room and the shading depth as performance requirements gives the contract something to hold onto.

Checking the output tables

You do not need to run the model to review it. Five checks catch most problems.

1. Confirm the weather file, location and percentile against what Approved Document O or the local policy requires.
2. Check which rooms were modelled, and confirm the worst cases are among them: top floor, single aspect, west or south facing, smallest openable area.
3. Look at the bedroom night-time criterion separately from the daytime result, because a scheme commonly passes one and fails the other.
4. Check whether the windows-open or windows-closed case was run, and whether that matches the acoustic advice.
5. Read the failures. A report showing every room passing comfortably on a constrained urban site deserves a second look at the inputs.

Commissioning it alongside the energy statement

Run both from the same geometry, by the same consultant, at the same time. The fabric that improves the Energy Statement result worsens the overheating one, and separating the two commissions produces a design that satisfies each in isolation and neither together.

Order matters as well. Get an early view on overheating before the elevations are fixed, because the least costly fix is a change to window size, orientation and shading at concept stage. After planning permission the fixes available are mechanical, and they cost.

Our note on who can prepare an overheating assessment goes further into evidencing the modeller's experience, and the professional directory is the route to a reviewer who can check a draft before it is submitted.

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. For specialist technical reports, qualified professional input may be required.