Operative temperature in overheating assessment
Overheating criteria for homes are written in operative temperature, not air temperature. What the metric combines, how models report it and what a report must state.
Operative temperature is the comfort metric used in dynamic overheating assessment, and it combines the air temperature of a room with the mean radiant temperature of the surfaces around the occupant. At the low air speeds found in a dwelling it is close to the average of the two. Report air temperature instead and you will understate the result, which is why an officer or a reviewer checking an overheating assessment looks first at which metric the model output actually reports.
The distinction is not academic. A sunlit room with large glazing can be a couple of degrees warmer in operative terms than its air temperature suggests.
What operative temperature combines
A person in a room exchanges heat two ways that matter here. Convection with the surrounding air, driven by air temperature and air movement, and radiation with the surfaces around them, driven by those surface temperatures.
Mean radiant temperature is the area-weighted average of the surface temperatures the occupant can see, including glazing, walls, floor, ceiling and any radiant source. Operative temperature combines it with air temperature, weighted according to air speed. In still conditions the weighting is roughly equal, so the operative temperature sits midway between the two. As air speed rises, the convective term takes more weight.
The older term dry resultant temperature describes essentially the same quantity at low air speed, and it still appears in legacy specifications and older model outputs.
| Quantity | What it describes |
|---|---|
| Air temperature | Dry bulb temperature of the air in the space |
| Mean radiant temperature | Area-weighted average of surrounding surface temperatures as seen by the occupant |
| Operative temperature | Weighted combination of the two, used as the comfort metric |
| Dry resultant temperature | Older term, equivalent to operative temperature at low air speed |
Why air temperature alone is inadequate
Consider a south-facing bedroom under a flat roof on a July afternoon. The air may be at 25 degrees, having been tempered by ventilation. The glazing surface, the exposed plasterboard ceiling below a warm roof deck, and a sunlit wall are all considerably warmer. The occupant radiates to those surfaces and feels the difference.
An assessment reporting the air temperature records a room within the threshold. The same room assessed in operative terms exceeds it. The physics has not changed; only the reported quantity.
This is also why fabric choices that do nothing to air temperature still improve the result. Shading reduces the surface temperature of the glazing and prevents direct solar radiation falling on the occupant. Exposed thermal mass lowers the peak surface temperature of the ceiling. Neither shows in an air temperature trace to the same degree.
Radiant effects from glazing and surfaces
Glazing is the dominant radiant surface in most overheating cases, for two reasons. It reaches higher temperatures than opaque elements under solar load, and it admits direct beam radiation that falls on the occupant without heating the air at all.
The specification variables that move the radiant term are the glazed area as a proportion of the facade, the total solar energy transmittance of the glass, the presence and type of shading, and whether the shading is external or internal. Internal blinds are much less effective, because the solar energy has already entered the room and the blind itself becomes a warm radiant surface.
Lightweight construction with insulation on the inside face produces warmer internal surfaces than a heavier build-up. And where an exposed concrete soffit is available, with acoustic and fire requirements satisfied, exposing it does real work.
How models report the value
Dynamic simulation tools calculate air temperature, mean radiant temperature and operative temperature as separate outputs. The default output variable in a template is not always the one the assessment criteria require, and that is a common and easily missed error.
Check three things in any report you receive.
- Which variable the exceedance hours were counted against, stated explicitly rather than inferred from a graph legend.
- The air speed assumption used in the weighting, since a model assuming raised air speed from a mechanical system will weight air temperature more heavily.
- Whether the same variable is used consistently across every room and every criterion in the assessment.
An Overheating Risk Assessment that does not state its metric cannot be reviewed, and the correct response from a reviewer is to send it back rather than to assume.
Effect on criteria thresholds
The industry criteria for homes are written in operative temperature. For predominantly naturally ventilated dwellings, the adaptive comfort approach counts hours where the operative temperature exceeds the comfort temperature by one degree or more across the summer period, with a limit expressed as a percentage of occupied hours. A separate night-time criterion for bedrooms limits the hours above 26 degrees operative temperature between 10pm and 7am.
For mechanically ventilated or cooled dwellings, a fixed threshold approach applies, again in operative temperature.
Because the criteria are percentages of occupied hours, a small shift in the reported temperature produces a disproportionate change in the result. A room sitting just below a threshold for most of the summer crosses it for many hours once the radiant term is included properly. That sensitivity is the reason the metric matters so much in review.
An illustrative comparison
Take a single-aspect bedroom with a large south-west window, assessed twice on the same weather file. In the first run the exceedance hours are counted against air temperature. In the second they are counted against operative temperature, with everything else identical.
The second run returns more exceedance hours, because every hour in which the glazing and internal surfaces are warmer than the air moves closer to or beyond the threshold. The scheme that passed on the first run can fail on the second. This example is illustrative; the size of the difference depends on orientation, glazed proportion, solar transmittance, shading and construction weight, and it has to be calculated rather than estimated.
The useful consequence is that the measures which reduce the gap between operative and air temperature are the same measures that improve the overall result. External shading, lower solar transmittance glazing, reduced glazed area on the exposed elevation, and exposed mass.
What to state in a report
A reviewer should be able to reproduce the assessment from the report. That means stating the software and its version, the weather file including the location and the design summer year variant used, the future climate scenario if one is applied, the metric reported, the air speed assumption, internal gains and occupancy profiles, the window opening logic including any restriction imposed for fall protection, and the blind or shading operation assumed.
State the criteria in full and report the result room by room against each one. Where a room fails, say so and set out the measure proposed rather than adjusting an assumption until it passes. The assumption set is where these assessments are won and lost, and an officer who has seen a hundred of them will go to the window opening logic first.
Where the glazing specification is also carrying an energy target, the two assessments have to be run on the same specification. A change made for the Energy Statement can move the overheating result in the opposite direction, and both documents end up attached to the permission.
Browse the report types PlanScope can draft to see what a residential scheme needs.
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 operative temperature?
A comfort metric combining the air temperature of a space with the mean radiant temperature of the surrounding surfaces, weighted by air speed. At the low air speeds typical in dwellings the weighting is roughly equal, so the value sits approximately midway between the air temperature and the area-weighted average surface temperature.
How does it differ from air temperature?
Air temperature describes only the dry bulb temperature of the air. Operative temperature also accounts for radiant exchange with surfaces the occupant can see, including warm glazing and sunlit walls. In a room under solar load the operative value is typically higher, so reporting air temperature understates the overheating result.
Is dry resultant temperature the same thing?
Effectively yes, at the low air speeds found in dwellings. Dry resultant temperature is the older term and still appears in legacy specifications and in some model outputs. Where a report uses it, confirm the air speed assumption so the comparison with current criteria holds.
Which metric do the overheating criteria use?
Operative temperature. The adaptive approach for naturally ventilated homes counts hours where operative temperature exceeds the comfort temperature by one degree or more across the summer, and the separate bedroom criterion limits hours above 26 degrees operative temperature at night. Fixed threshold criteria for mechanically ventilated homes also use it.
Why do internal blinds perform poorly?
Because the solar energy has already passed through the glazing before the blind intercepts it. The blind then warms up and becomes a radiant surface facing the occupant. External shading stops the energy before it enters, which reduces both the air temperature rise and the mean radiant temperature.
What should I check in a modelling report?
Which variable the exceedance hours were counted against, stated explicitly rather than read off a graph legend. Then the air speed assumption used in the weighting, and whether the same variable is applied consistently across every room and criterion. A report that does not state its metric cannot be reviewed.
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.