What is a draught?
A draught is an unwanted flow of air in a room that most people find unpleasant, especially when the air is cold. Draughts are caused by gaps and cracks in the building envelope, but also by cold surfaces such as large windows. They cost energy and comfort: people turn up the thermostat, while the real problem is the moving air.
Causes
- Gaps and cracks: around frames, windows and doors, at the junction of floor and wall, around service penetrations and at loft and crawl space hatches.
- Cold downdraught: air cools along a cold window or poorly insulated wall, becomes heavier and flows across the floor as a cold stream.
- Pressure differences: wind against the facade and the stack effect in a stairwell push air through every opening.
- Ventilation: badly placed or wide-open trickle vents and supply valves blowing straight onto a seating area.
Finding draughts
- Choose a cold, windy day.
- Hold an incense stick or (carefully) a lighter flame along frames, doors and skirting boards. Where the smoke blows away, there is a gap.
- A thermographic survey or an airtightness test with depressurisation shows leaks even more clearly.
What helps?
- Draught-proofing: draught strips in windows and doors, automatic door bottoms or brush strips under doors, sealant or expanding foam tape along frame junctions.
- Insulation and better glazing: this reduces cold downdraught at windows and external walls.
- Keep ventilating: removing draughts is not the same as blocking ventilation. Never close all vents; ensure controlled ventilation instead.
- Mind combustion appliances: an open fire or gas heater without its own air supply needs fresh air. If in doubt, consult a heating engineer.
Related terms
- Draught strip
- Airtightness
- Natural ventilation
- Cold downdraught
- Sealant
- Thermal bridge
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