The overheating section of a sustainability statement

The overheating section of a sustainability statement has to do two things that pull against each other. It must show the design has worked down a cooling hierarchy, reducing heat gain before managing it and managing it before cooling mechanically. And it must report dynamic modelling results, including the rooms that fail, in a way an officer can check. Statements that show only the second, a summary table of passing rooms with no account of the design decisions behind them, get treated as compliance paperwork rather than as evidence.

Why overheating sits in a sustainability document

Two regimes are in play, and they are not the same thing.

Approved Document O of the Building Regulations sets the construction standard for new residential buildings in England, with a simplified method and a dynamic modelling route. It is checked by building control, after planning.

Planning policy asks a different question at an earlier stage, because overheating is governed by orientation, massing, glazing ratios, depth of plan and the relationship to neighbouring buildings, and all of those are fixed by the planning drawings. By the time building control sees the scheme, the inexpensive fixes have gone. That is why London policy and a growing number of local plans ask for overheating evidence with the application.

A dedicated Overheating Risk Assessment may be required in its own right on larger schemes, in which case the sustainability statement summarises it rather than repeating it.

The cooling hierarchy and its order

The hierarchy used in London, and widely adopted elsewhere, runs in six steps.

1. Reduce the amount of heat entering the building in summer, through orientation, shading, albedo, fenestration proportions, insulation and green roofs or walls.
2. Reduce internal heat generation, through efficient lighting, appliances and, above all, well insulated pipework in communal heating systems.
3. Manage the heat within the building, using exposed thermal mass and higher ceilings.
4. Provide passive ventilation.
5. Provide mechanical ventilation.
6. Provide active cooling, using the lowest carbon option available.

The order is the argument. A statement that reaches step six needs to have shown its work at steps one to five, and reviewers read the section backwards for exactly this reason.

Step two is the one most often skipped in flats. Uninsulated communal heating risers running through corridors and apartments are a major internal gain, and in many schemes they are a larger contributor than solar gain in circulation spaces. Address it explicitly.

Passive measures and what counts as evidence

Evidence means drawings and numbers, not adjectives.

For shading, give the depth of a brise soleil or balcony overhang in millimetres, the orientation of the facade it serves, and the modelled effect. External shading works, internal blinds do very little, and stating which you modelled matters.

For glazing, give the glass to wall ratio per orientation and the g-value of the specified unit. A statement that describes a reduced glazing area without giving the ratio is asking to be queried.

For ventilation, give openable areas as a proportion of floor area, the free area after accounting for restrictors and insect mesh, and whether cross ventilation is achievable in each dwelling type. Single aspect flats facing south or west are the ones that fail, and identifying them early is more useful than discovering them in the results table.

For thermal mass, say what is exposed. A concrete soffit behind a plasterboard ceiling is not thermal mass in any useful sense.

Reporting modelling results honestly

Dynamic modelling for homes is normally carried out to CIBSE TM59, which applies two criteria. For occupied rooms, hours of exceedance during the summer months should stay within a small percentage of occupied hours, and for bedrooms, temperatures should not exceed 26°C for more than 1 per cent of annual night hours between 10pm and 7am.

Report the following and the section becomes checkable.

• The software used and its version.
• The weather file, including which design summer year was used and whether future weather scenarios were tested.
• The dwelling types modelled, how many of each, and why they were selected as representative.
• The occupancy and internal gain profiles applied.
• The results for every modelled room against both criteria, with failures shown.
• The measures added in response to failures, and the re-run results.

Selecting only the best performing dwelling types is the most common way these assessments mislead. Model the worst case, which usually means the top floor, single aspect, south or west facing unit with the highest glazing ratio, and say that is what you did.

Where future weather scenarios are tested and the scheme fails under them, say so and describe the adaptation route. That is a stronger position than silence, because the question will be asked.

Where active cooling is accepted

Active cooling is accepted where the passive route has been exhausted and the residual risk is real. Deep plan buildings, noisy or polluted locations that prevent window opening, and some non-residential uses are the usual cases.

What the statement has to show is that cooling is the last step rather than the first design assumption. Where cooling is provided, describe the system, its carbon consequence, and how it is counted in the Energy Statement, since cooling load feeds the operational carbon figures and the two documents must agree. The energy hierarchy sections of an energy statement deal with the same tension from the other direction.

Comfort cooling designed in from the outset, with the overheating assessment written afterwards to justify it, reads exactly as it is.

The collision with noise and air quality

Passive ventilation assumes open windows, and open windows assume an acceptable acoustic and air quality environment. On urban sites that assumption often fails.

Where the overheating strategy relies on window opening, the acoustic assessment has to show internal noise levels with windows open, against the guidance criteria rather than against closed window values. If the Noise Impact Assessment concludes that windows must stay shut at night on a facade, the overheating model cannot open them.

The same applies near a busy road or an air quality management area, where mechanical ventilation with filtration may be required and the overheating strategy has to work with it.

Run the two assessments together. The version where the acoustician and the thermal modeller each assume the other has solved the problem produces a scheme that cannot be built as drawn, and it is usually discovered at technical design when changing the facade is expensive.

Conditions and verification

Expect a condition requiring the approved overheating mitigation to be implemented, and on larger schemes a condition or obligation requiring post-completion verification.

Write the strategy so it survives value engineering. The measures that tend to be removed are external shading, higher specification glazing and mechanical ventilation with heat recovery, and each of those removals changes the modelling. Where a measure is load bearing for the results, say so in the statement, because that sentence is what an officer will point to when a discharge submission arrives with the shading deleted.

The Sustainability Statement is where all of this is drawn together for the application, and the overheating section should be short, numerical and honest about the rooms that came close.

Browse all 66 PlanScope report types to see which sustainability documents a scheme is likely to need.

This is general guidance, not legal advice. Check current Building Regulations requirements and local policy 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.