Damp, drips and mould: myth or fact?

The questions

1. Question 1 of 6Myth or fact: a window left tilted open all day is the best way to stop mould in winter.

   

   • Fact
   • Myth: open wide for a few minutes instead

   Show answer

   Answer: Myth: open wide for a few minutes instead

   The Umweltbundesamt says not to leave windows tilted: it wastes heat and cools the wall by the window, which can lead to mould. Open several windows wide for a short time instead.

   Source: Umweltbundesamt: Wie lüfte ich richtig? (German, updated 13 Dec 2024)

   Stop condensation and mould in winter

2. Question 2 of 6How often, and for how long, does the Umweltbundesamt suggest airing in winter?

   • Once a week for an hour
   • 2 to 3 times a day, about 5 minutes per room
   • Never in winter
   • All night

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   Answer: 2 to 3 times a day, about 5 minutes per room

   In winter, 2 to 3 times a day for about 5 minutes per room is enough, ideally with windows on opposite sides so the air flows straight through (Umweltbundesamt).

   Source: Umweltbundesamt: Wie lüfte ich richtig? (German, updated 13 Dec 2024)

   Stop condensation and mould in winter

3. Question 3 of 6What indoor humidity does the Umweltbundesamt aim for?

   • 10 to 20 percent
   • 40 to 60 percent
   • 70 to 80 percent
   • As high as possible

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   Answer: 40 to 60 percent

   Aim for 40 to 60 percent; a cheap hygrometer shows which rooms need more airing. The US EPA says below 60 percent, ideally 30 to 50.

   Source: Umweltbundesamt: Wie lüfte ich richtig? (German, updated 13 Dec 2024); US EPA: A brief guide to mold, moisture and your home

   Stop condensation and mould in winter

4. Question 4 of 6Small black dots near a cold window frame are a classic sign of what?

   • Rising damp
   • Condensation
   • A roof leak
   • Old paint

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   Answer: Condensation

   Condensation is the most common cause of damp, and you can recognise it by small black dots near cold surfaces like window frames (CSE).

   Source: Centre for Sustainable Energy: Condensation, damp and mould (reviewed Dec 2025)

   Stop condensation and mould in winter

5. Question 5 of 6Your wardrobe stands flat against an outside wall. Good idea?

   • Yes, it insulates the wall
   • Better to pull it away so air can circulate behind it

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   Answer: Better to pull it away so air can circulate behind it

   Mould often grows on outside walls and behind big furniture (Umweltbundesamt). Pull furniture away from the wall so air can circulate (CSE).

   Source: Umweltbundesamt: Wie lüfte ich richtig? (German, updated 13 Dec 2024); Centre for Sustainable Energy: Condensation, damp and mould (reviewed Dec 2025)

   Stop condensation and mould in winter

6. Question 6 of 6Which heater does the CSE say to keep out of a damp home?

   • Radiators
   • Portable gas heaters: they make the air very moist
   • Electric fan heaters
   • Underfloor heating

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   Answer: Portable gas heaters: they make the air very moist

   The Centre for Sustainable Energy says portable gas heaters make the air very moist. Keep the home heated often enough to stay dry in cold weather.

   Source: Centre for Sustainable Energy: Condensation, damp and mould (reviewed Dec 2025)

   Stop condensation and mould in winter