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The successful application of moisture simulation models to building envelopes requires accurate values of material transport properties. Unfortunately, although the presently-available database is reasonably voluminous, much of the information given is of limited use. This paper describes the specification of moisture transport data in terms of the differential permeability function and proposes its most appropriate form. The use of this function allows the accurate evaluation of permeability and total moisture flux appropriate to the humidity conditions to which a material is subjected in practice. It is also suggested that it allows the separation of the total moisture flux into its vapour and capillary liquid components, which is a prerequisite for the exact solution of the defining transport algorithms. This hypothesis has been tested using a prototype experimental procedure in which permeability measure ents are made under different barometric pressures.