Last edited 17 Aug 2017

Condensation in buildings

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.

Condensation can occur on surfaces, or can be interstitial condensation, occurring between the layers of the building envelope, typically as a result of air diffusing from the warm interior of the building to the cool exterior and reaching its dew point within the building fabric. This interstitial condensation, when trapped within the structure by impervious materials is often erroneously referred to as Rising Damp.

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

Condensation can be controlled by:

Some uses of buildings 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 Approved Document C (Site preparation and resistance to contaminates and moisture) and Approved Document F (Ventilation) and further guidance is available in BS 5250 Code of practice for the control of condensation in buildings.

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