The moment the heating goes on, the windows fog up in the morning and that stale smell settles into the corners. Ventilating your home in autumn is exactly what decides whether the house gets through winter without mould patches behind the furniture. And it isn't about leaving a window ajar all day: it's about opening it fully, briefly and several times a day. Here are the minutes, the frequency and the detail for every room.
Why so much water condenses in autumn
Warm air holds far more water vapour than cold air. When you heat the house to 20 or 22 degrees, that air loads up with vapour from the shower, from cooking, from laundry drying indoors and from our own breathing: a family of four releases close to ten litres of water a day inside the home. When that loaded air meets a cold exterior wall or the window pane, it cools down and releases the water as condensation. On that damp surface, mould takes just a few weeks to appear.
Ventilating swaps the loaded air for air from outside. Even if a rainy day reads 90 % relative humidity, in absolute terms that air carries far less water: once inside and warmed up, its relative humidity drops and it can take on vapour again. That is why ventilating works on grey days too.
Fully open for a few minutes, never a window on the latch
A tilt-and-turn window left tilted barely renews the air, yet it chills the lintel and the reveals for hours. That is exactly where the black patch shows up later. What works is opening fully, turning the radiator valve down while you do it and turning it back up when you close:
- September, with 15 degrees outside: 10 to 15 minutes.
- October and November, between 5 and 10 degrees: 8 to 10 minutes.
- Frosty days: 4 to 6 minutes is plenty.
Faster still is cross-ventilation: two windows facing each other and the internal doors open. In three or four minutes the air is completely renewed and the walls barely notice. Three or four times a day is enough in almost any flat: when you get up, after the shower, after cooking and before going to bed.
Humidity between 40 % and 60 %
Ventilating by guesswork doesn't work well. A ten-euro hygrometer tells you when you need to open up: below 40 % the air dries out your throat and the woodwork; sustained above 60 %, the risk of mould shoots up. Put one in the bedroom and one in the bathroom, the two problem rooms.
A useful clue: if the glass is fogged on the inside first thing in the morning, humidity is already high. Wipe the condensation off with a cloth before it runs down to the frame, because repeatedly soaked wood and plaster are mould's favourite spot.
Room by room
- Bedroom: this is the room with the most vapour built up at dawn. Open the window as soon as you get up and fold the duvet back so the mattress can air.
- Bathroom: window open right after your shower, with the door shut so the steam doesn't spread into the hallway. Run a squeegee over the shower screen.
- Kitchen: extractor fan on while you cook and lids on the pans; boiling uncovered for half an hour puts almost a litre of water into the air.
- Living room: pull the sofa and the big cabinets a few centimetres away from the exterior wall so air can circulate behind them.
- Laundry drying indoors: always in a room with the window open, or with the door closed and the radiator on, never in the bedroom with everything shut.
The mistakes people make most
Switching off the heating in rooms you don't use and keeping the door shut backfires: those walls stay cold and all the moisture in the house condenses there. Better to keep them at 16 or 17 degrees and ventilate them once a day. Nor does a dehumidifier make up for not ventilating: the machine removes water, but it doesn't renew stale air.
And a classic at this time of year: painting over a patch of mould. If the ventilation isn't fixed, it comes back within a month.
With these habits the house goes into winter dry and free of that shut-up smell. And if you'd like to give the season a warmer feel, take a look at our selection of home essentials.