Moisture permeability data: Mathematical presentation

The successful application of moisture transport models to building envelopes requires accurate values of moisture permeability. Unfortunately, although a considerable amount of information is available, much of this is in a form which is of limited use to a designer. This paper examines the presentation of permeability data in terms of the concept of differential permeability. This allows permeability values to be predicted accurately for the humidity conditions to which materials are exposed in practice. Several suggested mathematical forms of the permeability-humidity functions have been compared by applying them to experimentally determined permeabilities for four representative building materials. From this comparison the functional form which would provide the most appropriate basis for a standardised system of data presentation has been identified.