Condensation and Mould Risk Calculator

Why does condensation form on a surface?

Room air carries a certain amount of water vapour. When that air touches a surface colder than its own dew point, vapour turns to liquid: a wall sweats, a window fogs, a cold water pipe drips. Two things decide whether this happens: the dew point of the room and the temperature of the surface.

How is it worked out?

Vapour pressure and dew point are found from room temperature and relative humidity. If the surface temperature has been measured it is compared directly. If not, the internal surface temperature of an external wall is estimated from the U-value: t_s = t_i − U × R_si × (t_i − t_e), where R_si is the internal surface resistance.

A surface below the dew point has condensation. Above it, the relative humidity at the surface matters: 80 percent or more lasting for days can let mould grow.

Example

A room at 20 °C and 50 percent relative humidity, 0 °C outside. For an uninsulated wall with a U-value of 2 W/(m²·K) and a surface resistance of 0.25, the surface is at 20 − 2 × 0.25 × 20 = 10 °C. The dew point is 9.3 °C, so condensation is close but not yet present. Relative humidity at the surface is 95 percent, so mould risk is high. If room humidity rises to 52.5 percent the wall starts to sweat.

What can be done?

There are three routes: warm the surface (insulation, fixing the thermal bridge, moving furniture off the wall), lower the humidity (ventilation, fewer moisture sources) or raise the room temperature. The largest U-value shown is the insulation level needed to avoid condensation under these conditions.

Limits

This is a preliminary check of surface condensation. Interstitial condensation, two dimensional thermal bridge analysis and time dependent effects are not covered. For persistent mould and damp problems have a building physics assessment done.