A study on correction factor method for estimating water vapor permeability in earthen building materials

Water vapor permeability/diffusivity is one of the fundamental properties of building materials that influence the hygrothermal performance of building envelope. This plays a key role in overall moisture management and durability of wall components. The cup method, as described in EN ISO 12572, is widely used in building physics for the experimental determination of water vapor permeability in building materials. However, this method presents certain limitations, including difficulties in applying a wide range of humidity gradients and the long duration required to complete the experiments. This paper presents the laboratory results obtained using the dry and wet cup methods, with an extended range of applied humidity gradients from 20% to 95% of relative humidity. Compressed earth bricks have been selected as the reference material for this study. Water retention curves were also determined, employing sorption, desorption, and osmotic techniques. These complementary approaches enable the characterization of moisture equilibrium states over a broad suction range and humidity. Hence, the correction factor method was evaluated using its key input parameters such as porosity and clay fraction, and water retention curve, and the results were compared against experimental measurements obtained from the standardized cup method. The results indicated that while various tortuosity models can reasonably predict water vapor permeability, they exhibit limitations when applied to unconsolidated material with higher porosity values (above 50%). The correction factor method has demonstrated strong potential for assessing water vapor permeability of earthen building material across a wide range of suction levels and saturation degrees. • A complete water retention curve (WRC) was obtained through vapor equilibration and osmotic methods, providing the basis for moisture transport analysis. • The water vapor permeability of compressed earth bricks was determined through dry-cup and wet-cup tests in accordance with standardized procedures. • The correction factor method effectively predicts the water vapor permeability of earthen materials across wide ranges of relative humidity and suction. • The selection of appropriate tortuosity models, as well as the consideration of porosity and clay fraction, plays a crucial role in the accuracy of the correction factor approach.