Three numbers, not one
"Ideal" hides an argument, because three different goals point at three different settings.
Comfortable is around 20 °C (68 °F) for someone sitting still in normal indoor clothing. Move around and you want it lower; sit still for hours and you want it higher.
Safe is a floor rather than a target. Published guidance treats 18 °C (64 °F) as the minimum for an occupied living space, and 16 °C (61 °F) as the point below which a room is genuinely too cold to live in for long.
Cheap is as low as you will tolerate, plus a floor of about 13 °C (55 °F) to keep the plumbing out of trouble. That last figure has nothing to do with comfort — it is the number that stops pipes freezing in an empty house.
Most households end up somewhere between the first and third, and the outdoor temperature decides how far apart they sit.
Room by room
A single whole-house number is a compromise. If you have per-room control, these are the conventional targets:
| Room | Occupied | Why it differs |
|---|---|---|
| Living room | 20–22 °C (68–72 °F) | Sitting still for long stretches |
| Kitchen | 18–20 °C (64–68 °F) | Cooking adds heat, and you are moving |
| Bedroom | 16–18 °C (61–64 °F) | The body sheds heat as it settles |
| Bathroom | 20–22 °C (68–72 °F) | Wet skin loses heat fast |
| Hallway | 16–18 °C (61–64 °F) | Passed through, not occupied |
| Empty house, winter | 13 °C (55 °F) minimum | Protects the plumbing, not the people |
Why the same number feels different in two houses
The thermostat reads air temperature. You do not.
Roughly half of how warm a room feels comes from radiant exchange with the surfaces around you — walls, windows, floor, ceiling. In a well-insulated room those surfaces sit close to the air temperature and you lose little to them. In a poorly insulated room with single glazing, the wall behind you can be several degrees colder than the air, and you radiate heat into it continuously.
Which is why 20 °C is pleasant in one house and thin and draughty in another at the same reading. The three things that close the gap:
- Insulation, which raises the temperature of the inside face of the wall.
- Draught-proofing, because moving air strips warmth off skin exactly as wind does outdoors — the same mechanism as wind chill, indoors and at a smaller scale.
- Humidity in the 40 to 50 percent band. Very dry air makes a room feel cooler than it is; very humid air brings condensation.
The practical consequence is that improving the fabric lets you hold a lower air temperature at the same comfort, which is the only version of this that saves money without costing anything.
What the weather outside changes
Two things, and the second one is the one people miss.
How much energy the setting costs. Heat loss is proportional to the difference between inside and outside. Holding 20 °C when it is 10 °C outside is a 10-degree difference; holding it when it is −5 °C is a 25-degree difference, and costs more than twice as much per hour. Which is why a fixed monthly heating budget behaves so differently across a mild and a cold winter.
How far you can safely set back. On a mild night a deep overnight setback is free money. On a night near or below −6 °C the setback removes the margin protecting your plumbing, and the saving is no longer worth having. The overnight low is what tells the two nights apart.
A setting that works most of the year
If you want one arrangement rather than a project:
- Up and about: 20 °C (68 °F).
- Asleep: 17 °C (63 °F), set back an hour before bed and back up before you get out of it.
- Out for the day: 16 °C (61 °F). Letting a house go properly cold and reheating it is not cheaper for most buildings, because the fabric has to be reheated too.
- Away for more than a day or two in winter: 13 °C (55 °F) minimum, and lower is a false economy the moment a hard frost arrives.
- On a night forecast below −6 °C (20 °F): drop the overnight setback and hold the daytime setting.
The last line is the only one that needs checking against anything, and what it needs is the outdoor forecast rather than the thermostat. The money side of the same decision — what a setback is actually worth in cash — is in what thermostat setting for winter.