You may already have had the email saying it is impossible to work in the south-facing rooms, while senior management asks whether that is really true. If you look after building operations and indoor climate, it is a familiar position to be in.
Overheating is when the room temperature during occupied hours sits higher than the purpose of the room can reasonably tolerate — warmer, in practice, than sedentary work can be done in comfortably. The summer problem is rarely that one day was hot. It is that nobody can account for how many hours it lasted, in which rooms, and whether it happened on ordinary days or only during a heatwave. Without that answer the discussion becomes one claim against another, and the complaints about the indoor climate pile up in a way that is hard to answer.
Why the question usually stalls
Four things get in the way every time.
The complaints arrive when things are at their worst and fade when the weather turns. By then it is too late to measure. The thermometer on the windowsill stands in the sun and shows something other than the place where people actually sit. Everyone talks about different rooms as though they were one building. And finally, two questions get mixed together that each have their own answer: was the building built correctly, and is it too warm to work in right now?
The last one is worth keeping separate. The Building Regulations set requirements at construction and major refurbishment — Part O covers overheating in new residential buildings — and compliance is demonstrated by calculation before anyone moves in. For buildings without cooling, CIBSE TM52 and TM59 are the recognised overheating criteria. A perfectly compliant building can still have one room that is unpleasant once the sun has been on the façade since lunchtime. The two statements do not contradict each other — they simply answer different questions.
What actually applies in the workplace
The Workplace (Health, Safety and Welfare) Regulations 1992 require, in regulation 7, a ‘reasonable’ temperature indoors. The numbers sit in the accompanying Approved Code of Practice: normally at least 16 °C, or at least 13 °C where the work involves significant physical effort. Those are minimums. There is no legal maximum workplace temperature in the UK, and that is deliberate: the HSE’s reasoning is that no meaningful upper limit can be set, because high temperatures in a bakery or a foundry follow from the work itself and not only from the weather.
The duty comes from somewhere else instead. Under the Health and Safety at Work etc. Act 1974 and your risk assessment, the HSE expects a thermal comfort risk assessment once employees complain — and that is precisely where measured data changes the conversation. With no number in law to point at, what was actually recorded, in which rooms and for how many hours, becomes the whole basis of the case. For guidance rather than law, CIBSE Guide A recommends an operative temperature of 21–23 °C in winter and 22–24 °C in summer for offices. That is an industry standard, not legislation, and it is worth saying so out loud before it turns into a quoted limit in an email thread.
The difference between ‘too warm on an entirely ordinary day’ and ‘too warm during a heatwave’ is not academic. It decides whether the problem is the building or the weather, and therefore whether the answer is solar shading or a jug of water.
Turning the hot hours into documentation
With our solution Indoor climate in RoomAlyzer Air you get data that lets you judge indoor climate complaints on a factual basis. The sensors measure around the clock, so the hot weeks are on record no matter when the question is raised. From there the accompanying software gives you three levels, depending on what you need.
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The chart: Shows how a single room behaved, with the green, amber and red zones of the scenario drawn in. That is enough to settle whether the room peaked at 3 pm or sat high all day.
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Deviations: Works through the whole building: how much of the time each room spent above the threshold, which rooms are worst, and — through the 24-hour distribution — when in the day it happens. This is where the pattern turns into a priority, because you can see it is the same five rooms every time.
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The Workplace heat report: Collects it in a PDF built for exactly this discussion. You choose the rule package, the period, and which hours count as occupied hours — the default is Monday to Friday 08–16, because those are the hours the case is about. Two packages are available: AT 25 °C, which uses 25 °C as its threshold, and ASR A3.5, which is the stricter of the two. Both are there, and the choice is yours — pick the one that matches your own policy on thermal comfort. The report contains a room table, the worst days, a calendar covering the whole period where each cell is a day coloured by that day’s worst level, and a bar chart of hours per week split into OK, above threshold and serious.
The decisive trick is the outdoor source. The report takes the outdoor temperature from an Outdoor sensor or from weather data at the location’s map position, and uses it to separate the two kinds of exceedance. In the AT package the columns are called ‘Normal >25’ (indoors above 25 °C while it was not a heatwave outside) and ‘Heatwave’. Without outdoor figures the hours are still counted, but they appear as ‘No outdoor’, because nobody can determine which kind of exceedance it was.
What you can do once the pattern is clear
Once you know which rooms and which hours you are dealing with, the remedies become concrete and usually cheaper than feared: solar shading on the façade that is causing it, night ventilation that cools the building down before the day starts, moving the most exposed workstations, or shifting meeting times in the room that is warmest after lunch.
The measurement series carries on, and that is the second half of the benefit. Once the blinds are up, you can compare this summer with last summer for the same rooms and the same hours — and see whether it worked, instead of asking people whether they think it feels better.
One last honest note: the sensors are operational monitoring, not accredited measurement. A formal thermal comfort assessment sets requirements for measurement height and radiant shielding among other things, and where the sensors are placed is your responsibility. The report documents what was measured during occupied hours — it does not replace a professional health and safety assessment.
If you need the rules and the report in detail, the workplace temperature rules are set out here. And if it is the day-to-day threshold breaches you want a grip on, deviations are explained here.
