The water vapour permeability is a material property used in calculations of the hygrothermal performance of buildings. The standard test method (the 'cup test'), little changed for decades and based on measuring weight changes, has been shown repeatedly in round-robin comparisons to have poor accuracy and little consistency between laboratories. Here we describe a new approach in which the primary measurement is of the humidity difference across the test sample, which is monitored continuously using sensors that are remotely readable. The box-in-box (BiB) apparatus described is smaller and simpler than that of the standard cup test. The BiB test is of shorter duration and is carried out without disturbance to the sample. New results on calcium silicate sheet, brick ceramic and autoclaved aerated concrete are compared with published vapour permeability values obtained by the standard test (considered to be of lower accuracy).
Water-vapour permeability (WVP) data on brick, stone, plaster and cement-based materials from some some seventy publications are reviewed and assessed. Almost all sources use standard cup-test methods or close variants. Comparisons of WVP values from different sources on similar materials confirm that reproducibility between different laboratories is poor. Some deficiencies of cup-test methods are discussed, including uncertainties arising from the use of saturated-salt humidistats and desiccants. There is some evidence that the water-vapour resistance factor decreases with increasing mean relative humidity across the test specimen; and weak evidence that the resistance factor increases with volume-fraction porosity. The contribution of liquid film flow to the measured permeability is discussed. It is concluded that (1) available data are inadequate to establish the fundamental physics of WVP; (2) vapouronly permeability data for engineering purposes should be obtained in dry-cup tests at low humidity; and (3) research studies should aim to integrate the WVP into the framework of unsaturated flow theory.
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