Modelling assumptions in an overheating risk assessment
Occupancy, gains, weather files and criteria are prescribed in a dynamic overheating assessment. What the modeller controls is the building, and that is where reports fail.
An overheating risk assessment is only as credible as its input assumptions, and most of the assumptions in a dynamic model following the building services overheating methodology are fixed rather than chosen. Occupancy profiles, internal gains, weather files and the temperature criteria are prescribed. What the modeller controls is the building: glazing area and specification, shading, thermal mass, ventilation openings and their control. When a report fails at review, it is usually because a prescribed input was quietly altered or a real-world constraint on opening a window was ignored.
The distinction that causes the most confusion is between this modelling route and the simplified route in Approved Document O. They are different tests with different pass criteria, and a scheme can pass one and fail the other.
Occupancy and internal gain profiles
The methodology sets standard profiles for bedrooms, living rooms and kitchens, covering when each room is occupied, by how many people, and what equipment and lighting gains apply at each hour. Those profiles exist so that two assessors modelling the same flat get comparable answers.
Do not adjust them to suit the scheme. A bedroom modelled as unoccupied during the day, or a living room with equipment gains reduced because the developer expects efficient appliances, produces a result that a reviewer will reject as soon as the input tables are read. If an assumption has to depart from the standard profile, say so explicitly, give the reason, and present the result both ways.
Kitchens carry the heaviest gains and are frequently the room that fails. In an open plan flat the kitchen and living area are a single zone, which concentrates cooking gains into the space that also has the largest glazed area. That combination is the single most common cause of a failing result in small flats.
Weather file selection and future climate files
Weather file choice changes the answer more than almost any other input. The methodology uses the design summer year files for the location nearest the site, at the specified emissions scenario and percentile, and the compliance run uses the baseline file rather than a future one.
Three points to state explicitly in the report. Which weather station the file relates to, and how far it is from the site. Which design summer year and scenario were used. And whether any urban heat island adjustment was applied, which matters in London where more than one design summer year is published and they represent different kinds of hot summer.
Future climate files belong in the report as a resilience test, not as the compliance run. A scheme that passes on the baseline file and fails badly on a mid-century file is worth flagging to the client, because retrofitting shading into a completed block is expensive and sometimes impossible where the elevation has been approved. Present the two clearly labelled and do not blur them together.
Window opening assumptions and their limits
Natural ventilation results depend entirely on what the model assumes about opening windows, and this is where optimistic modelling shows up.
| Assumption | What the model often says | What the building will do |
|---|---|---|
| Openable area | Full clear area of the sash | Restricted to a safety limit above ground floor |
| Night opening | Windows open through the night | Closed on a ground floor flat for security |
| Opening trigger | Opens whenever internal temperature rises | Occupant may be out, or asleep |
| Free area | Manufacturer's stated area | Reduced by frame, trickle vent position and opening angle |
Model the restrictor. Where guarding or child safety requires opening to be limited, the reduced free area is the one to use, and the report should say which windows are restricted and why. The same applies to any window that cannot be left open at night for security reasons on the lower floors.
Cross ventilation deserves its own sentence in the report. A single aspect flat with openings on one elevation only cannot cross ventilate, and a model that assumes it can is simply wrong.
Blinds, shading and their modelled operation
Shading can be credited only where it is part of the permitted scheme. External shading devices, fixed brises soleil, deep reveals and balconies above are all legitimate inputs because they appear on the approved elevations.
Internal blinds are more contentious. They can be modelled where they are specified and fitted as part of the development, with a stated control regime, but a model that assumes occupants deploy blinds at exactly the right moment every day is assuming a level of behaviour that does not happen. Curtains supplied by the occupier are not a design measure and should not appear in the model at all.
Where a scheme relies on shading to pass, name the products and areas on the drawings and secure them by condition. Shading described only in a report tends to disappear at value engineering stage, and the Energy Statement and the overheating work should describe the same specification rather than diverging.
Noise and security constraints on ventilation
This is where the overheating model meets the acoustic assessment, and the two have to be reconciled in the same submission.
If the acoustic evidence concludes that windows must remain closed to achieve acceptable internal noise levels, then the overheating model cannot assume open windows. Approved Document O addresses the point directly by limiting reliance on openings where noise at night in bedrooms would be excessive, and it sets out the levels at which that applies. Where the site is on a busy road or near a railway, run the overheating model in the windows-closed condition with mechanical ventilation and, if needed, cooling.
Get the acoustician's internal noise levels before the thermal model is run rather than afterwards. A Noise Impact Assessment that lands after the overheating report has been issued usually means both documents get reissued.
Reporting failed rooms honestly
Report every assessed room, not a representative sample. Give the room reference, orientation, floor level, the criterion applied and the result, with the margin of pass or failure.
Where rooms fail, say which and by how much, then set out what would fix them. That might be reduced glazing, added external shading, a different opening type, or mechanical cooling. A report that presents only compliant rooms invites the reviewer to assume the omitted ones failed, and on a large scheme the reviewer will ask for the full schedule anyway.
Include the sensitivity of the result to the two or three inputs it actually turns on. If a flat passes by a tenth of a percentage point on the assumption of a fully open window at night, the report should say so plainly rather than presenting a pass.
Where Part O and the modelling route diverge
Approved Document O offers a simplified method based on limits to glazing area and minimum free areas for ventilation openings by orientation and location, and a dynamic modelling route based on the building services methodology with modifications set out in the Approved Document itself.
The modifications matter. The Approved Document specifies which weather files to use, applies criteria across occupied rooms, and constrains what can be assumed about opening windows at night where noise or security is an issue. So a model run purely to the base methodology is not automatically a Part O compliance model.
At planning stage the question is usually different again. Councils and the London Plan's cooling hierarchy want to see that passive measures have been exhausted before active cooling is proposed, which is a design narrative rather than a pass or fail. Set the modelling out so it answers both: the compliance result for the Approved Document, and the hierarchy narrative for the planning authority, cross-referred to the Sustainability Statement rather than repeated in it.
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Frequently asked questions
Can I change the occupancy profiles in an overheating model?
No, not without saying so. The methodology fixes occupancy, internal gains and equipment profiles for bedrooms, living rooms and kitchens so that two assessors modelling the same flat produce comparable results. If an assumption has to depart from the standard, state the departure, give the reason and present the result both with and without it.
Which weather file should a compliance run use?
The design summer year file for the location nearest the site, at the emissions scenario and percentile the methodology specifies, using the baseline rather than a future climate file. State the weather station, its distance from the site, the design summer year used and whether any urban heat island adjustment was applied.
Should future climate files be included?
Include them as a clearly labelled resilience test alongside the compliance run, never in place of it. A scheme that passes on the baseline file but fails badly on a mid-century file is worth flagging to the client, because adding external shading to a completed block after the elevations are approved is difficult and often impossible.
How should window restrictors be modelled?
Model the restricted free area, not the full clear area of the sash. Where guarding or child safety limits opening above ground floor, or where a ground floor flat cannot be left open at night for security, use the reduced area and identify in the report which windows are restricted and why.
What happens when the acoustic report requires windows closed?
The overheating model must then run in the windows-closed condition with mechanical ventilation, and cooling if needed. Approved Document O limits reliance on open windows where night-time noise in bedrooms would be excessive. Obtain the acoustician's internal noise levels before running the thermal model rather than afterwards.
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.