Cooling hierarchy evidence for a south facing flat

A single aspect flat facing south is the hardest overheating case in a residential scheme, and the cooling hierarchy is the order in which an officer expects you to have attacked it. The London Plan sets out six steps: reduce heat entering the building through orientation, shading, albedo, glazing design, insulation and green infrastructure, then minimise internal heat generation, then manage heat within the building through thermal mass and ceiling height, then provide passive ventilation, then mechanical ventilation, and only then active cooling.

The evidence requirement is not that you reach step four and stop. It is that you can show what each earlier step achieved and why the next one was needed. A report that jumps from "the flat is south facing" to "comfort cooling is proposed" will be sent back, and rightly.

The six steps and why the order matters

The hierarchy is a design sequence, so each step changes the inputs to the one after it. Cut solar gain by a third at step one and the ventilation rate needed at step four falls with it. Reverse the order and you size a cooling system for a load you never tried to reduce.

The policy consequence is that active cooling is acceptable where the earlier steps have been exhausted and a residual risk remains, and unacceptable where it has been used as a substitute for design. Officers test this by reading the dynamic model inputs rather than the conclusion, which is why an iteration table matters more than a results page.

Showing each step was tested

Build the report around a table of iterations. One row per design variant, columns for the measure applied, the resulting solar gain, the modelled exceedance against the criteria, and the decision taken.

That is an illustrative structure rather than a set of results, and the numbers belong to the scheme. The point is that each row shows a measure and its effect, so the reader can see where the design ran out of passive options.

Two things to avoid. Do not present a single compliant run with no history, because it tells the officer nothing about whether cooling was designed in from the start. And do not list measures you did not model, since a shading device described in the text and absent from the model is the most common inconsistency found on review.

Solar control glazing against external shading

Solar control coatings are the easy answer and the weaker one. They reduce the g-value year round, which also reduces useful winter solar gain and daylight, and they do nothing about the angle of incidence. On a south elevation, where summer sun is high and winter sun is low, external horizontal shading does the work that glass coatings cannot.

So the defensible south-facing strategy is usually geometric: a projecting balcony, a brise soleil, a deep reveal, or a recessed window within the structural zone. Each of those can be drawn, measured and verified on site, and each survives value engineering better than a specification line.

Where the elevation also faces west, horizontal devices stop working because the sun is low in the afternoon, and vertical fins or an operable shutter become the only options. Say so explicitly if any part of the flat has a west-facing window, because an officer who sees a single horizontal shading solution applied to both orientations will question the modelling.

Green infrastructure sits in step one too. A planted roof or planted terrace lowers surface temperatures and roof heat gain, and it can be coordinated with other roof uses, which is the subject of our note on biosolar roofs.

Ventilation where noise closes the windows

This is the real constraint on urban single aspect flats, and the one that pushes schemes to mechanical solutions. If the flat sits on a busy road, the internal noise levels needed with windows open are often unachievable, so the overheating model cannot assume a free area the occupier would never use.

The honest route is to run the model on a closed-window basis where the acoustic evidence shows openable ventilation is not viable at night, and to say that is what you have done. That usually means a measured noise survey rather than a desk estimate, and it means coordinating the Overheating Risk Assessment with the Noise Impact Assessment so that both documents use the same facade attenuation and the same assumed window position.

Acoustic ventilators and attenuated background ventilation can bridge part of the gap. Secure night purge through a restricted opening or a louvred panel is often better, because it delivers air at the time the bedroom criterion is tested. Where neither works, mechanical ventilation with a summer bypass is the step the policy expects before cooling.

When active cooling becomes acceptable

Three situations, broadly. Where the acoustic environment rules out openable ventilation and mechanical ventilation alone cannot meet the bedroom criterion. Where the dwelling type requires it, such as accommodation for vulnerable occupiers with a clinical reason for tighter temperature control. And where the external microclimate, including a heat island effect reflected in the weather file, leaves a residual risk after every passive measure.

In each case the report has to state the residual risk, show the measures that were exhausted, and describe how the cooling is provided and controlled. A heat pump providing reversible cooling is easier to defend than a separate condenser array, because the plant already exists for heating and the carbon consequence is smaller. External plant location, noise from condensers and discharge of warm air into a courtyard then become planning issues in their own right.

Modelling outputs an officer looks for

Name the methodology and the version. State the weather file and the year set used, and say whether a future climate year was tested, since a scheme that passes on a current file and fails on a 2050s file is worth flagging rather than concealing. Give results room by room, not as a building average, and identify the worst room.

Include the occupancy profile and internal gains assumptions, because the easiest way to pass an overheating model is to assume the flat is empty. Include the assumed openable free area as a measured figure from the window schedule, and confirm that restrictors, guarding and security requirements have been applied to it. That last point catches a lot of schemes: a window modelled as fully openable but fitted with a restrictor to meet falling safety requirements is not the window that was modelled.

The compliance route also needs stating. Approved Document O offers a simplified method with limits on glazing area and a requirement for removing excess heat, and a dynamic modelling route. A planning submission usually uses the modelling route, and saying which one applies avoids an officer asking.

Writing the sequence as a narrative

The structure that works runs to five short sections: the constraint, the measures tested in hierarchy order, the point at which passive options were exhausted, the residual risk, and the cooling provided with its controls. Two pages, with the iteration table and the model outputs as appendices.

Cross-refer to the Energy Statement rather than repeating its figures, and make sure the glazing areas, g-values and ventilation rates match between the two documents. Contradictions between the energy and overheating submissions are found on almost every review, usually because the two were produced from different drawing revisions.

Overheating assessment is a report type where councils expect a suitably qualified author and professional review is strongly advised. PlanScope drafts the planning narrative and structure around a modeller's outputs, and inserts flags where figures are still to be confirmed rather than assuming them. 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. Enter your postcode on PlanScope to see which reports your scheme is likely to need.