Why Your House Feels Draughty, and Where From

Look up before you look down

A heated house behaves like a chimney. Warm air is lighter, it collects at the top, and it pushes out through every gap in the ceiling. That leaves the house slightly short of air, and cold air is drawn in low down to make up the difference.

So the cold you feel at floor level is the symptom. The cause is above your head.

The attic hatch. Usually a thin board resting in a hole with no seal at all, and ENERGY STAR lists it among the common attic leaks.

Recessed ceiling lights. Each one is a cup with slots in it opening into the loft. A dozen of them adds up to a serious opening.

Where pipes and cables go up. Behind bath panels, in airing cupboards and above kitchen units, there are usually holes cut far larger than the pipe passing through them.

The chimney. An open fireplace with the damper up is a permanent hole in the roof, and closing it costs nothing.

Make the house suck, then walk round

Shut the windows and outside doors. Switch on every extractor fan in the house, the cooker hood and the tumble dryer if it vents outside. That pulls the house slightly below the outdoor pressure, so every leak starts pulling inward hard enough to see.

Now walk round with a lit stick of incense, or a strip of tissue taped to a pencil, and hold it near the edges of things. Where the smoke bends, air is moving.

Work the ceiling first, then the places where floors meet walls, then where the house meets the ground floor, then the windows and doors last. That is the order of how much they matter.

Write down what you find rather than fixing as you go, because the big ones are usually awkward and the small ones are tempting.

Do not do this with an open flued appliance burning. Depressurising a house on purpose while a chimney is trying to draw is the one situation where this test is a bad idea.

Does it get worse the moment the fan starts?

This is the part that belongs to the heating rather than the building, and almost nothing written about draughty houses mentions it.

Ductwork that runs through a loft or a crawlspace leaks. What leaks out of the supply side is air the house has to replace, and it replaces it by pulling outside air in through every crack. So a house with leaky ducts is at its draughtiest exactly when the system is running.

The test. Stand somewhere you normally feel it, with the system off, and note how it feels. Then set the fan to ON at the thermostat and wait a minute. If the draught arrives or gets noticeably worse, the ducts are doing it.

The other half is the return side. A leak there pulls loft air straight into the system, which is a draught you never feel because it comes out of the vents already mixed in.

Shut doors make it worse. A room with a supply vent and no way back pressurises and pushes its air out through the walls, while the rest of the house goes short and pulls more in.

Sealing duct joints with mastic and wrapping them properly is the fix, and it usually improves the heating bill at the same time: where the heat is going.