Abstract:

Water vapor transmission (WVT) through building materials are determined experimentally using dry cup or wet cup tests. These measurements are typically affected by several sources of uncertainties including air layer resistance, transient behavior of the desiccant, masked edge, as well as temperature and relative humidity of the surrounding environment. WVT cup tests conducted with highly permeable membranes are affected by boundary layer effect on the top surface and by the transient behavior of the moisture sink on the lower surface. WVT cup tests conducted with low permeable materials having moisture storage capacity are lengthy, often requiring several weeks to determine single WVT point at specified set of conditions, which might not be sufficient for input into advanced heat, air and moisture (HAM) models.

The objectives of this study were to evaluate the transient behavior of calcium sulfate and silica gel in standard ASTM E96 (2005) dry cup WVT tests conducted with highly permeable construction materials, and based on experimental findings provide recommendations related to frequency of moisture sink replacement in the dry cup WVT tests to improve the current standard. Develop a novel approach for testing WVT of highly permeable materials. A multi-layer approach was developed and verified using highly permeable Class-P WRB membrane. The value of the multi layer approach is that it allows the determination of the Kirchhoff potential function from a single series of tests (continuous material function which can be used as a direct input into CHAMP-BES simulation tool). This leads to added benefits of this approach include: generation of a shortened period of testing, reduced resource needs, and application of the top layer as a buffer layer in isolating the boundary layer effects. An approach of shortening the period of WVT testing by reducing specimen thickness was examined with oriented strand board (OSB). The findings showed that reducing material thickness from 11.6 mm to 6.8 mm decreased the test duration from 900 to 300 hours. The minimum adequate material thickness was determined to be 6.8 mm, and the sensitivity of the dry and the wet cup WVT tests was comparable for OSB thickness greater than or equal to 6.8 mm.