Before you start: what to leave open

Draught-proofing is about stopping the air you don’t want, not all air. A home needs a steady supply of fresh air to keep condensation and mould down, and fires, boilers and stoves need air to burn safely. The gaps to seal are the accidental ones. The vents put there on purpose stay open.

Never block these

  • Air bricks in outside walls, especially those near ground level that ventilate the space under a timber floor
  • Trickle vents, the slotted vents at the top of many window frames
  • Extractor fans in kitchens, bathrooms and utility rooms
  • Wall or floor vents in a room with a gas fire, boiler, stove or open fire
  • Any chimney or flue that serves a working appliance

If one of these is letting in an uncomfortable amount of cold air, the cause is usually a gap elsewhere pulling air through it, often an open chimney. Seal the other gaps first and the vent will usually calm down. If it doesn’t, ask a Gas Safe registered engineer (for gas) or a HETAS-registered installer (for wood and solid fuel) before changing anything.

Fires, boilers and stoves

Any appliance that burns fuel inside the house needs air to burn and a clear flue to take the fumes away, including carbon monoxide, which you can’t see or smell.

  • Take any chimney draught excluder out before lighting a fire.
  • Don’t draught-proof a room with an open-flued gas fire or boiler without checking it has a fixed vent. If you’re unsure, ask a Gas Safe registered engineer.
  • Fit a carbon monoxide alarm in every room with a fuel-burning appliance. In England, landlords must fit one in any room with a fixed combustion appliance, apart from gas cookers.

Condensation and mould

Everyday life puts moisture into the air: cooking, washing, drying clothes, even breathing. In a draughty house much of it is carried away. Once the draughts are sealed it needs another way out, or it settles as condensation on cold windows and walls, where mould can grow.

  • Keep trickle vents open.
  • Use extractor fans when cooking and bathing, and leave them running a little afterwards.
  • Open a window for a few minutes each day, ideally on opposite sides of the house.
  • Dry clothes outside where you can, or in a room with the door shut and a window open.

If you see more condensation than before, or any mould, add more ventilation rather than undoing the draught-proofing.

Inside doors

Only draught-proof doors that lead outside, or into an unheated porch, garage or conservatory. The gaps round internal doors let air move between rooms, which helps keep condensation down and lets appliances draw the air they need. Fire doors need particular care; the door guide explains why.

Older homes

Houses built before about 1919 usually have solid walls of stone or brick that let moisture pass through them. Use materials that let walls breathe, such as lime mortar or sheep’s wool, to fill holes in walls, rather than cement or expanding foam. If your home is listed, check with your council’s conservation officer before any permanent work.

Sources

Last checked 23 October 2026.