Wet and dry cup test with hygroscopic materials: what do we really measure?

Abstract

Wet and dry cup tests are widely used to characterize the vapor permeability (or the apparent vapor diffusion coefficient) of construction materials. From a theoretical approach, we examined the transport characteristics of such tests in detail. We precisely quantified the impact of the boundary conditions on the apparent diffusion coefficient and the time required to reach steady-state conditions, which is critical for accurate estimation of this coefficient. Finally, a major point concerns the physical meaning of the value measured through this type of test. For a homogeneous non-hygroscopic medium, the diffusion coefficient obtained represents the diffusion coefficient of vapor through the medium. For hygroscopic materials, where vapor diffusion, bound water diffusion, and sorption or desorption processes may be encountered, we demonstrate that the total water transport diffusion coefficient in the medium is strongly different from, yet proportional to, the apparent vapor diffusion coefficient. We validated and illustrated this analysis based on experiments with cellulose fiber stacks of different porosities and under different experimental conditions.