Examination of stability of boundary conditions in water vapor transmission tests

The diffusion of water vapor through construction materials is driven by the gradient of partial water vapor pressure. Traditionally, water vapor transmission tests (WVT), the 'dry cup', and the 'wet cup' tests have been conducted with 0 to 50% RH, and from 50% to 100% RH, respectively. Often, desiccants, saturated salt solutions or distilled water are used to generate the required RH. The stability of vapor pressure in WVT tests is of paramount significance because these values are used in calculating permeance. Dry cup tests performed with high permeance materials (>25 Perms) indicated a transient behavior for the two types of adsorbing desiccants; calcium sulfate and silica gel. Monitoring RH inside the dry cup was performed to examine the effect of low and high WVT on the performance of moisture sinks. The results indicate that for highly permeable construction materials, the use of vapor pressures corresponding to 0% RH in calculation of permeance is not appropriate.