Last edited 27 Dec 2019

Condensation in buildings

Condensation on glass.jpg

Air will generally include moisture in the form of water vapour.

When air cools, it is less able to 'hold' moisture, that is, the saturation water vapour density falls, and so the relative humidity rises. When the relative humidity reaches 100%, the air will be saturated. This is described as the dew point. If the air continues to cool, moisture will begin to condense.

Typically this happens in buildings when warm, moist air comes into contact with cooler surfaces that are at or below the dew point (such as windows) and water condenses on those surfaces.

Moisture can also form as interstitial condensation - occurring within the layers of the building fabric - typically as a result of air diffusing from the warm interior of a building to the cool exterior and reaching its dew point within the construction of the building itself. For more information see: Interstitial condensation.

Condensation affects the performance of buildings, causing problems such as:

Condensation can be controlled by:

Some uses of buildings (such as swimming pools) can generate high levels of moisture and so specialist techniques may be necessary to prevent or mitigate the occurrence of condensation.

It is important that any systems introduced to limit condensation are properly installed and maintained to ensure continued optimal operation.

Condensation in buildings is regulated by Part C of the building regulations, and guidance about how to deal with common situations is given in Approved Document C (Site preparation and resistance to contaminates and moisture) and Approved Document F (Ventilation). Further guidance is available in BS 5250 Code of practice for the control of condensation in buildings.

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