Water Vapour Permeability Permeability is a property of the material and is defined as the amount of water vapour which passes through unit thickness, normally one metre, in unit time under a given pressure. Typical units would therefore be: kg/(m.s.Pa) or g.m/(s.MN) where one Pascal = one Newton per square metre (Pa = Nm-2 ). The effectiveness of a vapour barrier is expressed as permeance which is the total rate of transmission through the actual thickness as applied. Permeance is measured in g/ (s.MN) or equivalent units. Materials such as Class O Armaflex which have a very high resistance to water vapour transmission, will have very low permeability values, i.e. less than 3.6 x 10-14 kg/(m.s.Pa) in the case of Class O Armaflex. In this manual the permeability values for Armaflex have been calculated from the relevant µ factor using a value for the permeability for air at 0 °C as 658 µgm/N.h.

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