Condensation in buildings occurs when water changes from its gaseous phase into its liquid phase in response to changes in temperature and/or pressure. If this occurs on an exposed surface, it is called surface condensation. If it occurs in concealed voids in a building structure, it is called interstitial condensation.
The deposition of condensate may result in: absorption or adsorption. If the materials wetted by the condensate are water tolerant, the condensate may later be lost by evaporation leaving no permanent damage behind. If the materials wetted by the condensate are susceptible to water damage, the condensate may cause progressive deterioration.
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Control of condensation requires balance. Water need to be removed, either by ensuring evaporation rates prevent harmful accumulations of condensate or by ensuring condensate is drained harmlessly away. Barriers such as vapour checks work within limits but rely on the vapour pressure differential remaining permanently as assumed in design.The appropriate provision of ventilation, drainage and heating can provide flexible methods of controlling condensation risk.