20mph at midnight

A speed limit is a number. Safe driving isn't. You don't hold 70 on the motorway in fog or nose-to-tail traffic — and 20mph past a school at 3pm is sensible, while 20mph on the same empty road at 3am is just absurd.

The number is a guide. The conditions decide.

Building heating ignores that completely. One setpoint holds every room at a single temperature, around the clock — through the night, through the empty working day, through the half-term week when nobody comes in. It treats a guide as a target, and the conditions as noise.

It assumes there is one 'correct' temperature a home should always sit at. There isn't — and the law never said there was.

The law has two numbers, not one.

We checked what the rules actually say. The Housing Health and Safety Rating System — the statutory basis for excess cold — works off roughly 21°C in living rooms and 18°C in bedrooms, judged for a vulnerable occupant. And it asks whether the heating is capable of reaching those when it's −1°C outside. That's a property of the home, not a reading you must hold at 3am.

The World Health Organization puts the floor at 18°C. Nobody, anywhere, requires an empty bedroom to be 21°C through the night.

Warmer isn't healthier.

Here's the part the timeclock gets backwards. A cooler bedroom is better for people, not worse. To fall into deep sleep the body has to drop its core temperature by about a degree, and a room held too warm blocks that. The evidence points to roughly 16–19°C as the sweet spot for sleep; push warmer and sleep quality starts to fall.

So the 'correct' temperature depends on the room and the person in it. A warm living room at teatime and a cool bedroom at midnight isn't a failure — it's how a healthy home behaves. The real danger is sustained cold for the vulnerable, not a bedroom that sits a few degrees below the living room.

The real enemy is water, not cold.

So is the danger even about temperature? Mostly, it's about humidity. Damp and mould — the hazard the sector is chasing hardest — isn't caused by a room being a degree cool. It's caused by moist air meeting a cold surface.

Mould needs sustained humidity of around 80% at a surface. That happens when a wall, a window reveal or the patch behind a wardrobe drops below the dew point and water condenses on it. The room's air can read a perfectly safe humidity while a cold corner quietly sits at 100%.

That's why mould comes back in the same spot. It isn't a heat problem you fix by blasting the whole house. It's a dew-point problem — cold surface, damp air — that you fix with ventilation, insulation and a little targeted warmth exactly where it's needed.

It's a balance, and the conditions decide.

So compliance isn't a number you pin to the wall. It's a balance between what the law asks and what's actually safe in this room, for this person, right now. Sometimes that means less heat — a cool empty bedroom, a better night's sleep. Sometimes it means more care — a vulnerable resident, or a cold damp corner that needs warmth and air before mould takes hold.

Same as the road. Sometimes you ease off the limit; sometimes the conditions mean you slow right down. The number is where the judgement starts, not where it ends.

The answer.

Hold a safe, efficient setback that keeps surfaces above the dew point. Bring rooms up to the right temperature — 21 or 18 — only for the people about to use them. Watch the humidity, not just the thermostat. And do the heavy lifting in the cheap, clean windows of the grid.

That needs what a timeclock has never had: who's in, what the surfaces are doing, what the air is holding, and what the heat costs right now.

The point.

There is no single correct temperature. There's the right temperature for that room, that person, that moment — and a surface kept dry enough that mould never starts.

Chase a fixed number and you lose on all three: comfort, health and carbon. Read the conditions, and you win all three at once.