Property management
Dew Point & Condensation Risk
Condensation is not bad luck - it is any surface sitting at or below the air's dew point, and mould follows a few degrees above that.
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How it works
The Magnus formula converts temperature and relative humidity into the dew point. Compare it with your coldest surface (window reveals, external corners, behind wardrobes): at or below the dew point water forms; within about 3 degrees, sustained surface humidity exceeds 80% - the mould growth zone referenced by BS 5250.
Frequently asked questions
At what humidity does mould grow?
Mould germinates where the SURFACE relative humidity stays above roughly 80% - which happens when a cold surface sits within ~3 degrees of the room air's dew point, long before visible droplets.
How do landlords prevent condensation mould?
Lower the dew point (ventilation and controlled heating cut moisture) and raise cold surfaces (insulation, air movement behind furniture). Post Awaab's Law, measuring and documenting both is the defensible position - this calculator gives the numbers.