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3 Condensation

Condensation is a process whereby water is deposited from air containing water vapour when its temperature drops to or below dewpoint, temperature at which air becomes saturated with water vapour.


3.1 Visible/Surface Condensation

Surface condensation is condensation occurring on visible surfaces within the building. It results in deterioration of surface finish, mould growth, etc. In general, windows present the lowest interior surface temperature in most buildings and provide the best guide to allowable values of indoor humidity for avoiding condensation.


3.2 Concealed/Interstitial Condensation

Interstitial condensation is condensation occurring within or between the layers of the building envelope. The migration of water vapour through the building envelope by diffusion or air leakage brings the moisture into contact with concealed surfaces. This gives rise to serious deterioration of materials. The allowable humidity level to avoid interstitial condensation is much lower than that for surface condensation.


3.3 Transient Condensation

Transient condensation is condensation occurring if the moisture content/vapour pressure of air increases more rapidly than the temperature of surfaces and materials. Examples are:

(a) A door of an air-conditioned space is opened unintentionally for several minutes, the incoming moist air causes condensation on most room surfaces.

(b) In a cold massive building, a sudden influx of warm moist air causes condensation because vapour diffuses faster than the increase of temperature in the massive wall. The condensation becomes interstitial as the surfaces warm up.