Measuring water vapour permeability using remote-reading humidity sensors

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).