How do I find draughts in a room?

Draughts are worth finding before spending on anything larger, because sealing them is cheap and immediate. The difficulty is that most of them are too small to feel, which is what the smoke test solves.

Safety

Step by step

1. Pick a cold, ideally windy day.The bigger the difference between inside and out, the more obvious the air movement. A still mild day hides almost everything.
2. Set the room up to exaggerate the flow.Close all windows and external doors, turn the heating off, and run the bathroom or kitchen extractor fan. That slightly depressurises the house and pulls outside air in through every gap.
3. Use smoke or tissue to see the air.A lit incense stick gives a visible plume. A strip of tissue taped to a pencil works nearly as well. Move it slowly — a few centimetres a second — around every edge and hold still at each corner.
4. Work the obvious edges first.Around window frames and opening casements, along the bottom and hinge side of external doors, the letterbox, the keyhole, and where a frame meets the wall.
5. Check the floor.Gaps between floorboards, the joint between floor and skirting, and around pipes coming up through the floor. On a suspended timber floor this is often the single biggest source.
6. Check every penetration through a wall or ceiling.Extractor ducts, boiler flues, waste pipes, cables, aerial and broadband entries, downlighters in an upstairs ceiling, and the loft hatch. Each one is a hole someone made.
7. Check the fireplace.An unused open chimney moves an enormous volume of air. It can be blocked with a purpose-made balloon or plate — but only if the fireplace is genuinely disused and it still needs some ventilation left.
8. Feel behind fitted furniture and in cupboards.Under-stairs cupboards, the back of kitchen units against an external wall, and airing cupboards are common and rarely checked.
9. Separate cold surfaces from actual draughts.A single-glazed window feels cold because it radiates heat away, not because air is moving. The smoke tells you which you have, and they need completely different fixes.
10. Write down what you find as you go.A list per room, ordered by how strongly the smoke moved. That becomes the job list, and the strongest three or four usually account for most of the loss.
11. Leave the deliberate ventilation alone.Air bricks, underfloor vents, trickle vents in window frames, and anything serving a fuel-burning appliance. If you are unsure whether a vent is deliberate, assume it is.

Tips

• Do this before buying insulation or new windows. Draught-proofing is the cheapest saving per pound spent and it changes how a room feels immediately.
• A thermal camera attachment for a phone shows cold air tracks clearly, and many libraries and councils lend them free in autumn.
• Check again after windy weather. Some gaps only open up under wind pressure on one side of the house.

Common mistakes

• Blocking air bricks and underfloor vents — They keep a suspended floor dry and feed fuel-burning appliances. Blocking them causes rot, condensation and, with any gas appliance, a carbon monoxide risk.
• Testing on a mild still day — Without a pressure difference there is nothing to see. The whole test depends on air actually moving.

If it doesn't work

Questions people ask