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Moisture and dew point

Warm air can hold more water vapour than cold air. When moist indoor air cools down inside a wall, it can reach the dew point and the water condenses. That is why walls have a vapour barrier on the warm side and a wind barrier that lets moisture out.

RH=vvsat⋅100 %RH = \frac{v}{v_{sat}}\cdot 100\,\%relative humidity

Symbols

RHRHrelative humidity%
vvactual water vapour contentg/m³
vsatv_{sat}saturation content at the temperatureg/m³

Example

Indoor air at 20 °C with 8.6 g/m³ (saturation 17.3 g/m³):

RH≈50 %RH \approx 50\,\%. Cooled to about 9 °C, the air is saturated and condensation forms.

The vapour barrier always goes on the warm side of the insulation.
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