Ventilating properly and avoiding mould

In short: Mould forms when building parts stay damp. With short intensive airing, a suitable humidity and warmth at the walls it can usually be prevented.

In more detail: Warm air can hold more water vapour than cold air. If humid warm air meets a cold surface (outer wall, window reveal), the water condenses and mould grows.

In Depth

• Humidity: The target is about 40 to 60 % relative humidity. A hygrometer (a few euros) shows the value. Permanently above 60 % is risky, above 70 % especially.
• Intensive airing: Several times a day open windows fully for 5 to 10 minutes (cross-draught with several windows, briefly turn down the heating). Permanently tilted windows cool the walls and ventilate poorly.
• Moisture sources: Cooking, showering, drying laundry, houseplants and breathing. Air immediately after showering and cooking, lids on pots, do not dry laundry in the flat or only with ventilation.
• Furniture: Place cupboards a few centimetres from outer walls so air circulates.
• Heating: Heat all rooms moderately, do not let the bedroom cool completely, close doors to colder rooms.
• Thermal bridges: Corners, window reveals and shutter boxes are colder. Mould appears there first. Insulation or insulation boards help.
• If mould is present: Wipe small spots (under about half a square metre) with water and cleaner or alcohol, fix the cause. For larger areas a specialist firm, respiratory protection (FFP2) and gloves. No chlorine cleaner on wallpaper.
• Tenants: Report mould to the landlord in writing (defect notice). Whether building condition or ventilation behaviour is the cause is often clarified only by an expert report.
• Technology: Ventilation units with heat recovery ventilate automatically.

See also: reducing heating costs, tenancy agreement