Short answer: because it gets no direct sun. A room loses heat through its walls, windows, floor and ceiling at a rate set by their construction, and that is the same whichever way it faces. What changes with orientation is what comes back. A south-facing room is warmed by the sun for much of the day; a north-facing room in the UK never is. So it needs the radiator to do all of the work, all of the time.
The BTU Calculator handles this with a 15 per cent uplift for a north-facing room. This guide explains what that covers, what it does not, and shows the arithmetic.
What the uplift is for
Radiator sizing works out the heat a room loses on a cold day and picks a radiator that can replace it. Our guide on how many BTUs a room needs walks through the method: area × U-value for each surface, plus air changes, scaled by the room temperature.
That calculation does not include the sun. On a south, east or west face, direct sunlight on the windows and walls offsets some of the loss for part of the day, which is why those rooms feel warmer than the arithmetic alone predicts. A north face gets none of that. It is also the face that tends to stay damp, holds a colder surface temperature and, in much of the country, takes the prevailing weather.
The 15 per cent factor is the trade’s conventional allowance for that difference. It is applied to the whole calculated requirement, before the safety margin.
Worked example
Take the 1930s semi lounge from our sizing guide: 4 m × 5 m × 2.4 m, one external wall of 220 mm solid brick, a 2 m² single-glazed window, a suspended timber floor. The heat loss works out at 63.8 W per degree, and at a 22 °C living room target that is 1,404 W before any factors.
| Step | Working | Result |
|---|---|---|
| Base requirement | 63.8 × 22 | 1,404 W |
| North-facing uplift | × 1.15 | 1,614 W |
| Safety margin | × 1.15 | 1,856 W |
That is 6,332 BTU/hr, against 5,507 BTU/hr for the identical room facing south. A single-panel radiator that would have been adequate on the south side is a double-panel on the north side.
Add French doors and the calculator applies a further 20 per cent: a large glazed opening that is also a door loses heat through the glass and through air leakage around the frame every time it is used.
What the factor does not fix
It is worth being clear about what the uplift is, because it is often reached for as the answer when it is not.
It does not reduce the heat loss. The 15 per cent compensates for the missing solar gain. It does nothing about single glazing, an uninsulated solid wall, or a draughty suspended floor, and in the example above those account for the large majority of the number. Replacing the single glazing with a double-glazed unit takes the window loss from 12.8 to 5.8 W per degree, an 11 per cent cut in the whole requirement, which is most of what the north-facing factor adds back. Fix the fabric first, then size the radiator for what is left.
It does not warm a cold wall. A north-facing solid wall can be cold enough to the touch that a room feels chilly at the thermostat’s set point, and it is where condensation and mould show up first. A bigger radiator helps by keeping the air moving and the surface a little warmer; insulating the wall is the fix.
It is a rule of thumb, not a measurement. The factor is a conventional allowance, the same one used across the UK trade, and it errs generous. For a new heating system or a heat pump, a heating engineer’s full heat loss calculation to BS EN 12831 handles orientation and solar gain properly and supersedes it.
When to tick the box
Tick north-facing in the BTU Calculator when the room’s main external wall and window face north. A room with its window on a north wall and a second external wall facing east is north-facing for this purpose. A room whose only external wall faces west is not, even if the house as a whole is described as north-facing. The factor is about the room’s exposure, not the front door’s.
Factors and the worked example are as implemented in BTU Calculator v1.1 and were computed from that code on 11 September 2026. The U-values are typical figures for the construction types named, not measured values. Plan CD is not affiliated with any heating manufacturer or certification body.