How hot is too hot inside a house?

People search this question for two different reasons: because they want to know whether their house is unusually bad, or because they are worried about someone. Both are answerable, but not with a single number — the honest answer is a small ladder of thresholds, plus the point at which the question stops being about comfort at all.

Safety

Step by step

1. Know that there is no legal maximum.UK law sets no maximum temperature for a home or for a workplace. Workplace regulations require a reasonable temperature and employers have general health and safety duties, and guidance discusses thermal comfort, but there is no number above which work must stop. Anyone telling you there is a legal limit is repeating a myth.
2. Use the sleep threshold as the practical one.Sleep is where indoor heat bites first. Most people sleep best in a bedroom in the high teens Celsius. Above the mid-twenties, falling asleep takes longer and sleep is more broken, for almost everyone, regardless of how used to it they think they are. If your bedroom is not dropping below the mid-twenties overnight, that is the number to act on.
3. Treat the high twenties indoors as a health threshold, not a comfort one.For an infant, an older person, someone pregnant, or anyone with a heart, lung or kidney condition, sustained indoor temperatures in the high twenties and above carry real risk, and the person affected is often the last to notice. At that point the job changes from making the house pleasant to keeping one room genuinely cool and checking on people.
4. Measure in the right place.Put the thermometer at roughly the height people occupy, away from direct sun, away from a window, and away from anything warm like a television or a router. A thermometer in a sunbeam reads the sunbeam. Check the bedroom at bedtime and again in the small hours, because the evening figure is not the one you sleep in.
5. Compare inside with outside before blaming the house.If the indoors is cooler than outdoors during the day and warmer at night, the building is behaving normally — that lag is thermal mass. If the indoors is consistently hotter than outside even at night, something is wrong: solar gain that is never shaded, a roof with no insulation, or ventilation that is never opened when it should be.
6. Watch humidity as well as temperature.How hot a given temperature feels depends heavily on humidity, because sweat evaporating is how you lose heat. A humid twenty-six feels considerably worse than a dry twenty-eight, and it is the reason fans stop being much help at the top of the range.
7. Know when the number stops mattering.If someone is confused, has stopped sweating, has hot dry skin or is unresponsive, the thermometer is irrelevant. That is heatstroke and it is a 999 call.

Tips

• A cheap digital thermometer with a memory that records the overnight minimum and maximum tells you far more than glancing at a dial, because the number that matters is the one at three in the morning.
• If the bedroom will not come down overnight, sleeping downstairs is a legitimate and effective answer rather than an admission of defeat.
• Fans stop being useful above roughly body temperature, because moving very hot air across skin adds heat rather than removing it. Below that they help by speeding evaporation.
• Rooms feel different at the same temperature depending on the surfaces — a room with a hot ceiling radiating downwards feels hotter than the air reading suggests, which is exactly the loft-conversion problem.

Common mistakes

• Believing there is a legal maximum indoor or workplace temperature — There is not, in UK law. The duty on employers is to provide a reasonable temperature and to manage risk, which is a judgement rather than a threshold, and no number obliges anyone to send staff home.
• Judging a bedroom by the evening reading — The temperature that affects sleep is the one in the early hours, after the fabric has released the day's heat. A room at a comfortable figure at ten can be several degrees warmer at two.
• Assuming an older relative will say if they are too hot — Thirst and heat perception both become less reliable with age and with several common medications. Checking the room temperature yourself is more dependable than asking.

Questions people ask