Cooling hierarchy: proving the case before mechanical cooling
Active cooling sits at the bottom of the hierarchy for a reason. What has to be modelled and evidenced at each earlier step before it becomes defensible.
The cooling hierarchy planning argument is lost at the point where a scheme arrives with mechanical cooling already in the design and a paragraph explaining why the earlier steps were not possible. Reviewers read that paragraph as a decision that was taken first and justified afterwards, because it usually was.
The hierarchy is sequential. Each step has to be worked through and evidenced before the next one is reached, and active cooling sits at the bottom as the measure of last resort. In London the sequence is set out in the London Plan policy on managing heat risk. Elsewhere the same logic arrives through Approved Document O and local plan sustainability policies.
The order the hierarchy sets
The steps, in the order they have to be taken:
- Minimise internal heat generation through energy efficient design.
- Reduce the heat entering the building in summer, through orientation, shading, surface reflectance, glazing design, insulation and green roofs and walls.
- Manage the heat within the building, using exposed thermal mass and generous floor to ceiling heights.
- Provide passive ventilation.
- Provide mechanical ventilation.
- Provide active cooling.
The order is not arbitrary. The first three reduce the load, the next two remove heat without refrigeration, and the last adds energy demand and plant. A scheme that has worked the first three properly usually finds that the remaining load is small enough for ventilation alone.
What reviewers look for is evidence at each level, not a statement that it was considered. Considered is not a test anyone can check.
Minimising internal heat gains
This step gets skipped most often, because it sits with the services engineer rather than the architect and rarely appears in the drawings.
The gains that matter in a dwelling are hot water distribution pipework in risers and corridors, the heat interface unit where there is a communal system, lighting, and appliances. In a communal heat network the distribution losses can be the single largest summer heat gain in a corridor, and they are largely a design decision about insulation thickness, flow temperatures and riser layout.
What counts as evidence: stated pipework insulation standards, the design flow and return temperatures, the calculated corridor gain, and the lighting power density. Where the scheme is all electric with individual heat pumps rather than a network, say so and show the difference, which is the point our note on the all electric decision develops.
Corridors deserve a specific mention. An overheated communal corridor undermines the ventilation strategy for every flat that opens onto it, and a purge path that relies on drawing air from a corridor at 30 degrees does not work.
Shading, glazing and thermal mass
This is where the architecture does its work, and where the trade with daylight and views is real.
Glazing ratio is the first lever. A south or west facing elevation with a high glazed proportion and no shading will overheat whatever is done later. Reducing the glazed area is the cheapest single measure and the one clients resist most.
External shading outperforms internal blinds by a wide margin because it stops the heat before it enters. The usable options are fixed horizontal shading on south elevations where the sun is high, vertical fins or deep reveals on east and west where it is low, balconies used deliberately as shading for the storey below, and solar control glazing where nothing else fits.
Solar control glass carries a penalty. A low g-value also cuts daylight transmission, so the internal daylight assessment and the overheating assessment have to be run against the same specification rather than against two convenient sets of assumptions.
Thermal mass only helps where it is exposed and where night ventilation can discharge it. An exposed soffit above a plasterboard ceiling does nothing. Where the scheme uses lightweight construction, say so and do not claim mass benefits the building does not have.
Green roofs reduce surface temperature and help the wider urban heat effect, and they can be combined with photovoltaic arrays, which is covered in our note on biosolar roofs.
Ventilation before cooling
Passive ventilation means openable windows sized and positioned to give both purge and, where possible, cross flow. Single aspect flats are the difficulty, because there is no cross flow and the purge depends entirely on the depth of the plan and the free area of the opening.
Approved Document O offers two routes for new residential buildings. A simplified method that sets maximum glazing areas and minimum free areas for openings by orientation and cross ventilation type, and a dynamic thermal modelling route following the CIBSE methodology for domestic overheating. The simplified method is quicker and more restrictive. The modelling route is more permissive and requires the assumptions to be stated.
Mechanical ventilation comes next, and it is not the same as cooling. Supplying tempered outside air at a higher rate removes heat without refrigeration, and for many schemes it closes the gap.
Only when those are exhausted does active cooling become defensible, and even then the assessment should show what proportion of the accommodation needs it rather than applying it across the scheme.
Evidencing that passive measures were exhausted
The evidence that works is comparative. Show the modelled result at each stage rather than only the final one.
| Stage modelled | What it demonstrates |
|---|---|
| Base case with no mitigation | The scale of the problem |
| With reduced glazing and external shading | The effect of the fabric measures |
| With thermal mass and increased ceiling height where applicable | The effect of managing heat internally |
| With passive ventilation at the designed free areas | Whether openable windows alone succeed |
| With mechanical ventilation | Whether cooling is avoidable |
| With active cooling | The residual case, by unit type |
Report the worst case units separately. A scheme wide pass conceals the top floor, south west corner, single aspect flat that fails every criterion, and that unit is what an officer will ask about.
State the assumptions that drive the result: occupancy profiles, internal gains, window opening behaviour, blind use, external shading from surrounding buildings, and the weather file used. Changing the weather file alone can move the answer, so name it.
Where noise and air quality close the windows
The honest exception to the hierarchy is a site where the windows cannot be opened. That case has to be made with evidence rather than asserted.
For noise, that means measured levels at the facade, an assessment against the internal criteria for daytime and night time, and a demonstration that the criteria are exceeded with windows open. A Noise Impact Assessment prepared by a suitably qualified acoustician is the supporting document, and professional review is strongly advised.
For air quality the equivalent is modelled or monitored pollutant concentrations at the facade against the relevant objectives, with the intake position for any mechanical system shown.
Two things are worth saying plainly. First, a noise constrained facade does not automatically justify cooling across the whole building, only for the affected elevations. Second, the better answer is often acoustic ventilation that allows purge without a fully open window, and a reviewer will ask whether that was tested.
And if the noise or air quality problem is severe enough to close every window, that is also a comment on whether the layout puts habitable rooms on the right elevation.
Presenting the conclusion
The Energy Statement carries the cooling hierarchy narrative, and it should read as a sequence of decisions with numbers attached rather than a description of the finished design.
A structure that works: the hierarchy step by step, the measure adopted at each step, the modelled outcome after each step, the residual overheating risk, and the case for any active cooling by unit type and elevation. Then a short section on the assumptions and their sensitivity.
Keep the supporting Overheating Risk Assessment consistent with it, using the same model, the same weather file and the same unit references. Councils usually expect a suitably qualified author for that assessment and professional review is strongly advised.
Check the current version of the policy and the Approved Document before you start. Both have been revised, and a statement written to a superseded version invites a comment you did not need to attract.
Browse all 66 PlanScope report types to see which documents your scheme is likely to need.
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.
Frequently asked questions
What is the cooling hierarchy?
A sequence that starts with minimising internal heat generation, then reducing heat entering the building through orientation, shading and glazing design, then managing heat internally with thermal mass and ceiling height, then passive ventilation, then mechanical ventilation, and only then active cooling. Each step has to be evidenced before the next is reached.
Does Approved Document O apply outside London?
Yes. Approved Document O applies to new residential buildings across England and offers two routes: a simplified method setting maximum glazing areas and minimum opening free areas, or dynamic thermal modelling following the CIBSE domestic overheating methodology. The London Plan cooling hierarchy applies in addition within London.
Can I justify cooling because the site is noisy?
Only with evidence. That means measured facade levels, an assessment against internal criteria for day and night, and a demonstration that the criteria are exceeded with windows open. It justifies cooling for the affected elevations rather than the whole building, and a reviewer will ask whether acoustic ventilation was tested first.
Does thermal mass count if the building is lightweight?
No. Thermal mass only helps where it is exposed to the room and where night ventilation can discharge the stored heat. An exposed soffit above a plasterboard ceiling contributes nothing. Where the construction is lightweight, say so and model it that way rather than claiming a benefit the building does not have.
How should the assessment be reported?
Show the modelled result at each stage of the hierarchy rather than only the final design, and report worst case units separately by orientation, floor and aspect. State the occupancy profiles, internal gains, window opening assumptions, external shading and the weather file used, because changing the weather file alone moves the answer.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.