Experimental verification of conditions within the cups in the cup method of water vapor permeability measurement

Abstract

Abstract Vapor transmission coefficients of building materials can be determined using the cup method, known from ASTM and ISO standards. The measurement principle is simple, but the method is known to be problematic. In this paper, problems concerning the discrepancy between assumed and real values of relative humidity inside the cups are investigated. This concerns especially dry cups, in which desiccants are expected to produce relative humidity close to 0%. Interestingly, there are studies which suggest a much greater value should be used. To verify these claims, experiments were conducted for cups filled with calcium chloride (CaCl2), and saturated aqueous solution of potassium nitride (KNO3), covered with hemp-lime concrete samples. The study confirms earlier reports of a discrepancy between actual and assumed values of relative humidity inside the cups and shows that they might introduce significant error in the measurement of water vapor transmission coefficients.