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.