Potential errors and improvements in estimating the humidity dependency of vapor permeability using average humidity during cup tests

Abstract

The cup method is a widely used technique for determining the vapor permeability of porous building materials. Despite the nonlinearity of the humidity gradient in a specimen during testing (due to the humidity dependency of vapor permeability), the measurement results are often regarded as representative of the average humidity values inside and outside the cup. In this study, we aimed to quantitatively clarify the errors associated with creating approximate functions for vapor permeability from cup test results using the average humidity, and we proposed suitable humidity conditions for the test as a countermeasure. We reinterpreted the measurement results in the literature using various building materials. Our findings confirm that the approximate functions using the average humidity of the test conditions significantly overestimate the vapor permeability of materials having strong humidity dependency (especially wooden materials) when a series of measurements were mostly performed using a desiccant or water. Even when intermediate humidity levels were included, errors still exceeded 20%. Our results indicated that a series of measurements with 20% humidity differences inside and outside the cup was necessary to create approximate functions without significant errors (less than 10%) when using average humidity. The findings are significant for preventing the overestimation of vapor permeability in hygrothermal simulations.