Volume 26, Issue 6 (November 1998)
An Alternative Procedure for the Analysis of Data from the Cup Method Measurements for Determination of Water Vapor Transmission Properties
The cup methods are used extensively to determine the water vapor transport characteristics of building materials. The dry cup method and the wet cup method prescribe two standardized procedures for this purpose. The dry cup method gives the results as an average value of the property at a mean relative humidity of 25% and the wet cup at 75%. This information is inadequate for detailed hygrothermal analysis of building components. The complete dependence of water vapor permeability on relative humidity is required by most of the current computer models used for hygrothermal analysis. This paper presents an extension of the cup methods in which a series of dry cup and another series of wet cup measurements are suggested at varying relative-humidity levels outside the cup. Also, a detailed procedure for the data analyses to define the dependence of vapor permeability on relative humidity is presented. The methodology is applied successfully to three materials: perlite insulation board, calcium silicate insulation board, and plywood sheathing.