Occupancy profiles in an Overheating Risk Assessment

Two dynamic models of the same flat can reach opposite conclusions because of what was assumed about the people inside it. An overheating assessment occupancy profile sets when rooms are occupied, how many people are present, what equipment is running and whether windows can be opened, and those inputs move the result further than most fabric changes do.

Which is why reviewers go to the assumptions table first. A report that buries its profiles in an appendix, or omits them, invites a rerun with the reviewer's own assumptions.

Why profiles change the answer

The recognised design methodology for assessing overheating risk in homes tests two criteria. For living rooms, kitchens and bedrooms, the number of hours the operative temperature exceeds a comfort threshold across the summer period must stay within a defined percentage of occupied hours. For bedrooms, the temperature between 22:00 and 07:00 must not exceed 26 degrees for more than one per cent of annual hours.

Both criteria are expressed against occupied hours. Change the occupancy pattern and you change the denominator, the internal gains and the hours being tested, all at once.

The bedroom criterion is the one most schemes fail, and it is the one most sensitive to profiles, because a bedroom is occupied by two people through the hottest part of a summer night with the windows the only means of cooling.

The standard profiles and where they break down

The methodology sets out default profiles: living areas occupied through the day and evening with associated gains, double bedrooms occupied overnight by two people, single bedrooms by one, and defined equipment and lighting gains for each space.

These defaults are a reasonable representation of a general-needs dwelling. They are a poor representation of several building types that are now common.

Student accommodation is occupied differently, with study desks generating gains through the day in rooms that also serve as bedrooms. Co-living and small studio flats compress living and sleeping into one space, so the daytime living criterion and the night-time bedroom criterion apply to the same room. Homes designed for occupants working from home carry daytime gains in rooms the defaults assume are empty.

Where the defaults do not fit, depart from them, say so and justify it. Departing without saying so is the failure mode that costs a scheme a condition discharge later.

Vulnerable occupancy

Where a development is intended for occupants less able to regulate their own environment, including care homes, extra care housing and some sheltered schemes, the methodology applies a stricter treatment: continuous occupancy, a fixed rather than adaptive comfort threshold, and no assumption that occupants will adapt their clothing or behaviour.

This matters for two reasons. The stricter criteria are harder to meet, so the design has to respond earlier with shading, thermal mass or mechanical cooling. And the classification decision is a planning matter as much as a technical one, because a scheme marketed as age-restricted housing will be held to it.

State the occupancy classification applied and the basis for it on the front page of the report.

Window opening, security and noise

The single most consequential assumption is whether windows open at night, and how far.

Ground floor and accessible upper floor windows are often modelled with restricted opening for security, commonly to around 100 millimetres. Where restrictors are assumed, the model has to use the restricted free area rather than the full openable area, and the restrictor must actually be specified in the building.

External noise is the other constraint. Building regulations guidance on overheating limits the extent to which night-time window opening can be relied on where external noise levels are high, and where a site is affected by road, rail or entertainment noise, the acoustic evidence and the overheating model must agree. Two reports assuming opposite things about the same window is a familiar way to lose several weeks. The acoustic side of that argument is covered in our note on rating penalties in a BS 4142 assessment.

Air quality does the same work near busy roads, and a dwelling within an air quality management area may need the windows treated as closed.

Where windows cannot be relied on, the design has to respond with mechanical ventilation, secure night vents, acoustic attenuated vents or cooling, and the report should show the option testing rather than jumping to the answer.

Internal gains from equipment and lighting

Equipment and lighting gains are defined by the methodology, and the temptation is to reduce them on the basis that appliances have improved.

Resist it unless you can evidence the alternative. Reviewers treat reduced gains as a thumb on the scale, and the reduction is rarely what saves a marginal scheme.

Two gains are worth more attention than they usually get. Hot water distribution losses within the dwelling, particularly where a heat network riser passes through, can dominate a small flat, and corridor and riser temperatures on a communal heating system are a known cause of failure. Where a heat network serves the building, the secondary distribution losses should be modelled explicitly rather than taken from a default.

Communal corridors are not subject to the dwelling criteria, but authorities increasingly ask for corridor temperatures to be reported, because a corridor at 30 degrees drives heat into every flat door.

Where the planning stage and the building control stage diverge

An overheating assessment at planning stage answers a different question from the one building control asks. Planning is concerned with whether the scheme as designed can meet the standard without changes that would alter its appearance or its layout, such as external shading, deeper reveals or a different window pattern. Building control is concerned with compliance at completion.

That gap is where developers lose money. A scheme permitted with clear glazing and shallow reveals, then found at technical design to need brise soleil across a principal elevation, needs an amendment application and a conversation about design that should have happened a year earlier.

Run the model early, at concept stage, on the profiles you expect to defend. The output at that point is not a compliance document. It is a design tool that tells you which elevations are in trouble.

What to put on the face of the report

1. The occupancy classification, standard or vulnerable, and the basis for it.
2. The profiles used for each room type, with any departure from the defaults and its justification.
3. Equipment, lighting and hot water distribution gains, with sources.
4. Window opening assumptions by facade and floor, including restrictor dimensions and free areas.
5. The weather file used, including the location, the year and whether a future climate scenario was run.
6. Shading modelled, including balconies, reveals, overhangs and any external obstructions.
7. Results by room against each criterion, with the failing rooms named rather than summarised.

Item five deserves emphasis. A model run on a historic weather file for a benign location will pass schemes that fail on a warmer file, and authorities in southern England increasingly specify which file to use. The Overheating Risk Assessment should name it.

Illustrative example, for method only. A single-aspect south-facing one-bedroom flat passes the living area criterion comfortably and fails the bedroom criterion by a small margin. Testing shows the failure is driven by the restricted night opening assumed for security on the second floor and by distribution losses from the heat network riser. Insulating the riser to a higher standard and specifying a secure night vent resolves it without changing the glazing. Reducing the assumed equipment gains would also have produced a pass, on paper, and would have been rejected on review.

Where the strategy relies on ventilation or cooling, the outcome feeds the Energy Statement and should be consistent with it. Setting the model up correctly in the first place is covered in setting up a TM59 overheating assessment.

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.