Multilayer test method for water vapor transmission testing of construction materials

Water vapor transmission (WVT) measurements conducted with highly permeable materials are complicated by the instability of boundary conditions, more specifically by changes in vapor pressure and air velocity on either or both sides of the specimen. Such effects pose a greater challenge in determining the water vapor transport characteristics of thin, flexible, and highly permeable construction materials. This article presents a novel approach to WVT testing that improves some of these issues. The approach denoted as ‘multilayer test’ involves testing simultaneously several vertically stacked material layers each separated by air gap with equal thickness. Wireless relative humidity and temperature sensors mounted on the opposing surfaces of each layer provide continuous temperatures and relative humidity monitoring near the specimen surfaces. The test is conducted in a controlled environment with the set-up kept on an analytical balance. Change in the weight of the system is determined automatically at predetermined time intervals, and fluxes are calculated. Two approaches are used in determining the permeance of each material layer. The results of both approaches are compared. The multilayer WVT tests provide several benefits over traditional ‘Dry’ or ‘Wet’ cup WVT test method in that; the boundary layer effects due to moving air above the specimen surface can be accounted for, and simultaneous testing of multiple specimens provides greater statistical confidence. The method is particularly advantageous in cases when the transport coefficient has a strong dependence on moisture content. Using data from a single multilayer test, a continuous transport function can be derived. This article highlights that the multilayer test approach leads to significant reduction in experimental effort, resources required for testing, and test durations, and improves the precision of the material property data.