The criteria section of an overheating risk assessment
Criteria selection decides an overheating assessment before any simulation runs. How the CIBSE method and Approved Document O differ, and what to record.
The overheating risk assessment criteria you choose decide the answer before a single simulation runs. Two frameworks are in play for homes in England: the CIBSE design methodology for assessing overheating risk in homes, and Approved Document O of the Building Regulations. They use related but different criteria, different weather assumptions and different rules about what mitigation you may claim, and an assessment that reports against the wrong one gets sent back.
State in the first page of the report which framework applies, why, and which criteria are being tested. Officers and building control bodies both look for that.
Which criteria apply to which building type
Approved Document O applies to new residential buildings, covering dwellings, institutions where people sleep and student accommodation. It is a Building Regulations requirement, so compliance is mandatory for those buildings, and many local planning authorities also ask for an assessment at application stage where a policy requires it.
The CIBSE methodology for homes is a design methodology rather than a regulation. It remains the reference point for existing buildings, for major refurbishment, for schemes assessed against a local plan policy that names it, and for the dynamic modelling route within the Approved Document itself.
For non-domestic buildings, neither applies directly. The adaptive comfort criteria in the earlier CIBSE technical memorandum on overheating in European buildings are the usual reference, with three criteria covering hours of exceedance, daily weighted exceedance and an absolute upper limit on the temperature difference.
Say which of these you are using and name the document and edition. A report that refers to overheating criteria without a source is asking the reader to guess.
The residential criteria in the CIBSE methodology
For homes that rely mainly on opening windows, the methodology sets two criteria and both have to be met.
The first applies the adaptive comfort test to living rooms, kitchens and bedrooms during occupied hours across the summer period. The number of hours during which the operative temperature exceeds the adaptive comfort threshold by one degree or more must stay within a small percentage of occupied hours.
The second applies to bedrooms at night. Operative temperature must not exceed 26 degrees for more than one per cent of the annual night time hours between 22:00 and 07:00, which works out at a low tens of hours across the year.
For homes with mechanical ventilation, the methodology uses a fixed threshold test instead, with all occupied rooms assessed against a temperature limit for a small percentage of occupied hours.
Two practical points. The bedroom criterion is the one most schemes fail, because it is absolute rather than adaptive and the night time window is when a building releases the heat it absorbed during the day. And communal heating is a frequent cause of failure in blocks of flats, because heat losses from risers and distribution pipework raise corridor and hallway temperatures, which the methodology requires to be included.
How Approved Document O differs
The Approved Document offers two routes to compliance.
The simplified method sets maximum glazing areas by orientation and minimum free areas for ventilation, with different values for higher risk locations. It is quick and it constrains the design tightly.
The dynamic thermal modelling method uses the CIBSE methodology with modifications, and the modifications are what catch people out.
| Point | CIBSE methodology | Approved Document route |
|---|---|---|
| Status | Design guidance | Regulatory requirement |
| Weather data | Design summer year, with sensitivity testing encouraged | Specified weather year and location set by the document |
| Internal blinds and curtains | May be modelled with justification | Restricted as a means of limiting solar gain |
| Openable windows | Modelled according to design intent | Cannot be assumed open where noise or safety prevents it |
| Rooms assessed | Living rooms, kitchens, bedrooms | As specified in the document |
The noise restriction is the significant one for urban sites. Where external noise means windows cannot be opened at night, the assessment has to assume them closed, and a scheme that relied on cross ventilation then fails and needs mechanical cooling. Our note on overheating assessments on a constrained urban site works through that interaction, and the acoustic evidence comes from a Noise Impact Assessment rather than from an assumption.
Check the current edition of the Approved Document before setting up the model. It has been amended since publication and the detail of the restrictions matters.
Choosing the weather file and the year
Weather selection changes results more than most design decisions.
CIBSE publishes design summer year files for locations across the United Kingdom, in three variants: one representing a moderately warm summer, one a short intense warm spell, and one a longer less intense warm period. A building can pass against one and fail against another, because a heavyweight structure copes badly with a long warm period while a lightweight one struggles with a short intense spell.
Files also come by emissions scenario, percentile and future time period. A building assessed against current weather may fail comfortably against a 2050s file, which is a relevant question for a scheme with a long design life even where no policy requires it.
Record four things in the report: the location file used, the variant, the scenario and percentile, and the reason for the choice. Where a policy or the Approved Document specifies a file, use that one and say so. Where it does not, test more than one variant and report all the results rather than the best.
Occupancy and internal gain assumptions
Assumptions here are prescribed rather than free, and departures need justification.
The methodology sets standard occupancy profiles by room and dwelling type, with bedrooms occupied overnight and living areas through the day, together with standard gains for people, lighting, cooking and equipment. It also requires heat gains from hot water distribution and from any communal heating pipework within the dwelling to be included, which is the item most often omitted.
Where a scheme has unusual gains, model them and say so. A dwelling with a plant room on the party wall, a flat above a commercial kitchen, or a unit adjoining a communal riser cupboard will behave differently from the standard case.
Do not adjust occupancy to secure a pass. A reviewer comparing your profiles against the published tables will find the change, and once one assumption is shown to be non standard the rest of the model is read with suspicion.
Reporting pass and fail room by room
Report every assessed room individually, with the criterion applied and the result. A summary saying the scheme complies, with no room level data, is not an assessment.
The structure that works:
- A schedule of assessed rooms, with dwelling reference, orientation, floor level and room use.
- The criterion applied to each and the numerical result against the limit.
- Failing rooms identified plainly, with the margin of failure.
- The mitigation applied and the post mitigation result for those rooms.
- Communal corridors and circulation assessed separately where communal heating is present.
- A statement of the worst case dwellings selected and why they represent the scheme.
Sampling is acceptable, and the justification for the sample is part of the assessment. A top floor south west corner flat with single aspect is the case to model, not the ground floor dual aspect unit. Our note on reporting the results of a residential overheating assessment sets out the presentation in more detail, and the outputs feed the Overheating Risk Assessment submitted with the application.
Mitigation that survives value engineering
The mitigation that appears in an assessment and the mitigation that gets built are often different things, and the difference is where overheating failures originate.
Measures that tend to survive are those embedded in the fabric and the form: reduced glazing ratios, glazing with a lower solar factor, external shading integral to the elevation such as reveals, balconies and brise soleil, window opening geometry and free area, and dual aspect layouts. They survive because removing them means redesigning.
Measures that tend to disappear are internal blinds, external shutters added late, mechanical ventilation with a summer bypass that gets substituted for a simpler unit, and window restrictors specified for safety that then limit free area below the modelled figure.
So write the assessment to name the parameters the result depends on, with their values, and recommend a condition or a specification note that fixes them. Where the Energy Statement and the overheating assessment pull in different directions, which happens with airtightness and glazing, say so rather than presenting both as compatible.
One honest uncertainty. These criteria assess a modelled building with standard occupants, and real occupants close windows, block trickle vents and place furniture against radiators. Passing the criteria reduces risk rather than removing it, and a report that acknowledges that is more useful to a design team than one that claims otherwise.
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Frequently asked questions
Does Approved Document O replace the CIBSE methodology?
No. The Approved Document is the regulatory requirement for new residential buildings, and its dynamic modelling route uses the CIBSE methodology with modifications. The CIBSE method remains the reference for existing buildings, major refurbishment and schemes assessed against a local plan policy that names it.
Which criterion do schemes usually fail?
The bedroom night time criterion, which limits hours above 26 degrees between 22:00 and 07:00 to around one per cent of annual night hours. It is an absolute threshold rather than an adaptive one, and night is when a building releases the heat absorbed during the day. Communal heating pipework makes it harder still.
Which weather file should be used?
The one the Approved Document or local policy specifies, and where neither does, test more than one design summer year variant and report all the results. Record the location file, the variant, the emissions scenario and percentile, and the reason for the choice, because a building can pass against one variant and fail another.
Can internal blinds be modelled as mitigation?
Under the CIBSE method they can be modelled with justification. The Approved Document route restricts reliance on internal blinds and curtains as a means of limiting solar gain, and it also prevents windows being assumed open where noise or safety prevents it. Check the current edition before setting up the model.
How many dwellings need to be modelled?
A justified sample rather than every unit, representing the worst cases. That means top floor, single aspect, south and west facing units, and any dwelling adjoining plant, risers or commercial uses. State which dwellings were selected and why they represent the scheme, and assess communal corridors separately where communal heating is present.
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.