Topic
Condensation and mould
Condensation forms wherever a surface is colder than the dew point of the room air. Lower the humidity or warm the surface and it stops.
- At 20 °C and 60 % RH
- Dew point about 12 °C
Warm air holds more water vapour than cold air. When room air touches a cold window or an outside wall, it cools, and if the surface is below the dew point water condenses on it. Repeated condensation is what feeds black mould.
Use the dew point calculator to see how cold a surface has to be before it gets wet, and the humidity you need to keep it dry.
Persistent damp patches, tide marks or damp that does not follow the weather can have other causes such as leaks or penetrating damp. A dehumidifier treats the air, not the building.
Guides on this topic
- How to treat damp walls: condensation, penetrating or rising dampHow to treat damp walls: tell condensation, penetrating and rising damp apart with simple checks, the right fix for each, and when to call a surveyor.How-to
- How to stop condensation on windowsHow to stop condensation on windows: dew point on cold glass explained, then ventilation, heating and humidity steps that keep panes clear overnight.How-to
- How to remove black mould from walls and siliconeHow to remove black mould from walls, ceilings and bathroom silicone: cleaning steps, safety points and the humidity changes that stop it coming back.How-to
- Dew point explained: why walls and windows get wetHow air temperature, relative humidity and surface temperature decide where condensation forms, with a worked UK winter example and a dew point calculator.Guide
- Electric dehumidifier vs moisture traps and crystalsElectric dehumidifier vs moisture traps, crystals and dehumidifier bags: grams a week against litres a day, and where each one has a place.Comparison
- Dehumidifier vs air purifier: what each one doesDehumidifier vs air purifier: one removes water from the air, the other filters particles. When each makes sense, and when a combined unit is worth it.Comparison
- What humidity should you set a dehumidifier to?Dehumidifier settings by room and season in % RH, why 50 to 60 % suits most UK homes in winter, and why setting it lower only adds running time.Guide
- Damp walls in a bedroom: causes and fixesWhy bedroom walls go damp or grow black mould: overnight moisture, cold outside walls and furniture against them, with fixes ranked from free to professional.Fix