Sustainable Building Glossary

Hygroscopicity: Hygroscopicity is the capacity of nearly all materials to absorb and desorb water as a gas (water vapour) from and to the air as the relative humidity of the air changes. The effect of materials with good hygroscopic capacity can be to stabilise indoor air humidity, to reduce surface condensation and, in certain situations, to absorb moisture interstitially (depending on where in the structure the hygroscopic materials are situated). The consequences of this for design of insulation, vapour control and ventilation in both new build and in refurbishment are large. The hygroscopic capacity of a material is related to its equilibrium moisture content and it is sometimes measured as the percentage increase in water content in a material when the relative humidity increases from 50 % to 85 % with a constant temperature of 21° C.