Flushing wet air out of the house
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Warm air holds far more water than cold air. Heat a closed room and moisture is drawn out of the carpet, plaster, timber, furniture and us. Turn the heating off, the room cools, and the air can't hold it any more. It picks the coldest surface it can find: our single-glazed windows, or the slats under our mattress.
The fix turned out to be free: open the windows when the heating goes off. Wide open for half an hour reliably removes a third to a half of the moisture the room is holding above the outside air — we've measured that seven times now. But simply leaving a couple of windows open for a few hours, such as overnight, has beaten it every time we've compared the two. Slower per hour, but it keeps working through the small hours when the outside air is at its driest.
Our sliding windows turn out to have a catch built into the sill that locks them open a few centimetres and stops them going any further. I had lived with those windows for years without understanding what it was for — it means a window can stay open overnight without leaving the house open.
Curtains matter too. Closed, they insulate nicely while the heating is on. But as the room cools they trap damp air against cold glass, which is exactly where you don't want it. So we draw them while heating and open them as the room cools.
How we measured any of this is the next post — it turned out our air conditioners had been keeping the records all along.
Do you purge your rooms in winter, or seal them up tight?
Coming up
Did going all-electric cause our mould? What we found under the floor.
Wet windows, and the tool that turns them into a dehumidifier.
The sensors that had been recording all this in every room, for months.
Related Posts
- Previous — why our heating never brought fresh air in: Our heating doesn't breathe 3 Aug 2026
- Next — what the wet windows were actually telling us: Wet windows: the free dehumidifier 7 Aug 2026
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I expect that a 24/7 exhaust fan would suck away the hot air when I want to keep it, yes?
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even better would be a Heat Recovery Unit, which will draw the moist air and keep the heat. But $$$
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