Overheating Risk Assessment or Energy Statement

An Energy Statement is about carbon. An Overheating Risk Assessment is about whether people can sleep in the building in August. They use overlapping inputs, they are often produced by the same consultant from the same thermal model, and they pull the design in opposite directions often enough that running them in sequence rather than together is a false economy.

The distinction that catches architects out is jurisdictional. Overheating is controlled through Approved Document O of the Building Regulations, which is not a planning matter at all. Plenty of local plans, and the London Plan in particular, require an overheating assessment at planning stage anyway, and the two regimes use different tests.

What each assessment tests

An Energy Statement demonstrates how the development reduces regulated carbon emissions against a defined baseline, and how it meets any local plan target beyond that baseline. It works through an energy hierarchy: reduce demand through fabric and form, supply energy efficiently, then use renewable technologies, and in London report the outcome through the monitoring stage that policy adds at the end.

An Overheating Risk Assessment demonstrates that habitable rooms will not experience excessive temperatures in summer. It works through a cooling hierarchy: reduce internal heat generation, reduce the heat entering the building in summer, manage the heat within the fabric, provide passive ventilation, then mechanical ventilation, and only as a last resort active cooling.

Both are forward-looking predictions made from a model rather than measurements. That is worth stating in each document, because a reader who treats a modelled figure as a promise will be disappointed by the post-occupancy position.

Carbon reduction against thermal comfort

The conflict is real and it is structural. Everything that reduces heating demand in winter tends to increase overheating risk in summer.

More insulation keeps heat in. Greater airtightness removes the uncontrolled infiltration that used to flush warm air out overnight. Larger glazed areas bring in daylight and useful winter solar gain, along with a great deal of unwanted summer gain. Communal heating networks with poorly insulated risers push heat into circulation cores and, through them, into flats.

A single-aspect south or west facing flat with a large window, a heat network riser in the hall and a facade on a busy road is the case that fails, and it fails in both directions: the overheating model cannot be solved with openable windows because of noise, and the carbon calculation gets worse if you add cooling.

Catching that at concept stage costs nothing. Catching it at technical design costs a facade redesign.

Methods and standards applied

For carbon, dwellings are assessed using the national calculation methodology for domestic buildings and non-domestic buildings are assessed using the equivalent non-domestic procedure, both operating against the targets in Approved Document L. Local plan targets usually sit on top, expressed as a percentage improvement, and London adds a whole life carbon requirement on larger schemes.

For overheating, Approved Document O offers two routes. The simplified method limits glazing areas and sets minimum free areas for ventilation openings by orientation and by whether the location is classed as high risk. The dynamic thermal modelling method follows the CIBSE methodology for residential overheating assessment, using hourly simulation across a summer weather year.

Choose the route deliberately. The simplified method is quicker and suits straightforward layouts with generous opening areas. Dynamic modelling is necessary for constrained sites, for single aspect flats, and wherever the simplified glazing limits would compromise the design. Councils asking for an overheating assessment at planning stage almost always want the modelled route, because that is the one that produces evidence about specific rooms.

Weather files matter more than most people expect. Modelling against a current design summer year and against a future weather year gives a very different picture, and where a plan asks for future climate resilience the later file is the one that governs.

Where the two conflict in design

Four decisions do most of the work, and each has to be taken with both assessments in view.

Glazing ratio and orientation. The daylight case, the winter gain case and the summer gain case point in different directions, and the resolution is usually a lower glazing ratio with better glass rather than more glass with shading bolted on.

Shading strategy. External shading is far more effective than internal blinds, and it is an elevational decision that has to be made before the planning drawings are fixed. Adding brise soleil after a permission has been granted means a non-material amendment at best.

Ventilation strategy. Openable area, opening restrictors, cross ventilation and secure night purging all sit at the boundary between the overheating model and the facade design. Fall protection and window opening limits reduce the free area the model relies on, and models that assume unrestricted openings at first floor and above will not survive review.

Heat network design. Riser insulation and heat interface unit location affect both the carbon figure and the internal gains the overheating model has to dissipate.

Ventilation, glazing and acoustic constraints

On noise-affected sites the overheating assessment and the noise assessment have to be run together, because each makes assumptions about the other.

Approved Document O requires overheating to be assessed with windows closed where external noise means occupants would keep them shut, which forces the design towards mechanical ventilation or acoustically attenuated openings. The acoustic assessment, meanwhile, sets internal noise criteria that determine whether windows can be treated as openable at all.

So the acoustician and the energy consultant need to agree the facade assumptions before either report is written. That includes the assumed open area, the attenuator performance, and the hours during which openings are assumed to be used. The choice of noise method matters here too, and the difference between BS 4142 and the residential planning and noise approach changes which criteria the facade is designed to.

Where the two cannot be reconciled without cooling, say so in both documents and explain why the cooling hierarchy has been exhausted. A planning officer will accept mechanical cooling with a reasoned justification far more readily than a model that quietly assumes windows nobody will open.

Conditions securing each

Energy commitments are normally secured by a condition requiring the development to be carried out in accordance with the approved energy strategy, sometimes with an as-built verification stage. In London, the monitoring stage of the energy hierarchy adds a reporting obligation after completion, and on major schemes a carbon offset contribution is secured through a planning obligation.

Overheating commitments are secured in two ways at once, and that catches people out. A planning condition may require the approved mitigation to be implemented. Approved Document O applies regardless, at building control stage, and compliance there is assessed against the final design rather than the planning drawings.

If the design changes between permission and construction, both have to be revisited. A revised glazing specification approved by building control can breach a planning condition tied to the approved elevations.

Running them in parallel

Do three things at the start of the project.

Build one thermal model and use it for both assessments, with the same geometry, the same construction build-ups, the same occupancy profiles and the same internal gains. Two models with different assumptions produce two documents that contradict each other, and officers notice.

Fix the facade assumptions with the acoustician before modelling begins. Opening areas, restrictors, attenuation and the noise environment are shared inputs.

Report both in the planning submission with a short section explaining the interaction, rather than as two unconnected appendices. Where a sustainability statement is also required, the overheating section of a sustainability statement should draw on the same model rather than restating conclusions from memory.

Browse the full list of report types to see how energy, overheating and sustainability documents sit together, or enter your postcode on PlanScope to see which reports your scheme is likely to need.

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