How do I ventilate a house properly in winter?

A household puts several litres of water into the air every day through breathing, cooking, washing and drying clothes. In summer it leaves through open windows without anyone thinking about it. In winter it condenses on the coldest surface it can find, which is how mould starts.

Safety

Step by step

1. Air wide and briefly rather than narrow and long.Windows fully open for five to ten minutes, ideally on two sides of the house at once. The air exchanges quickly while the walls, floor and furniture — which hold most of the heat — barely cool.
2. Do it after the moisture-producing activities.After showering, after cooking, after drying washing. That is when the moisture is actually in the air, and removing it then is far more effective than airing at a fixed time.
3. Use the extractor fans properly.On before you start and left running for fifteen to twenty minutes after. Most people turn them off with the light, which is the point at which they are still needed.
4. Shut the door on the moisture.Bathroom and kitchen doors closed while the moisture is being produced, so it goes out of the fan rather than round the house to condense in a cold bedroom.
5. Put lids on pans.A large pan boiling uncovered puts a surprising quantity of water into the air. Lids reduce it substantially and save energy at the same time.
6. Deal with drying washing indoors.A load of wet washing releases a couple of litres into the house. Dry it in one room with the door shut and the window open or a dehumidifier running, not spread across the living room.
7. Leave the trickle vents open.The small sliding vents in window frames are there to provide continuous background ventilation. Closing them for winter is exactly backwards.
8. Keep furniture off cold external walls.A gap of a few centimetres lets air move behind a wardrobe or sofa. Pushed flat against a cold wall, the surface behind it stays cold and still, which is where mould appears first.
9. Consider a dehumidifier where airing is not enough.In a house that runs persistently damp, a dehumidifier removes moisture without losing heat, and running costs are modest against the cost of dealing with mould.
10. Never block deliberate ventilation.Air bricks, underfloor vents and any vent serving a fuel-burning appliance stay clear. They are not draughts to be fixed.

Tips

• Airing while the heating is off, then letting it come on afterwards, avoids heating air you are about to throw out.
• Condensation on the inside of windows in the morning is the signal that the house needs more ventilation, not that the windows are faulty.
• A cheap hygrometer tells you where you actually are. Somewhere around 40-60% relative humidity is a reasonable target indoors.

Common mistakes

• Leaving one window on the latch all day — It cools the room fabric continuously while exchanging air slowly. Brief wide airing removes more moisture for far less heat.
• Closing trickle vents to keep the heat in — They provide the background ventilation that stops moisture building up. Closing them is a common cause of winter condensation and mould.

If it doesn't work

Questions people ask