Experimental and numerical analysis of the hygric performance of earthen buildings after façade hydrophobization treatment

Earth is used extensively for buildings worldwide, as its thermal properties contribute to a favorable indoor thermal environment. However, earthen materials are sensitive to moisture ingress, which can result in structural damage and undesirable indoor humidity conditions. Through hydrophobization treatment, the capillarity of earth can be significantly reduced while hygroscopicity remains largely unchanged, thereby possibly improving the hygric performance of earthen buildings. In the present study, experiments and simulations were conducted to investigate the effect of hydrophobization on earthen materials and buildings. The experimental results show that the bulk density, open porosity, and pore-size distribution of rammed earth changed by only a few percent after hydrophobization treatment. The thermal properties (thermal conductivity, specific heat capacity, solar reflectivity and longwave emissivity) and hygroscopic properties (sorption isotherm and water vapor permeability) also showed limited changes (<20 % in most cases). In contrast, capillarity, represented by the capillary absorption coefficient, was reduced by over 80 %. Numerical simulations further showed that relative humidity and moisture content at the exterior and middle positions of earthen walls strongly decreased after hydrophobization treatment. The largest moisture flux discrepancy between untreated and treated walls was 652.1 g/(h·m2). Furthermore, the parametric analysis indicated that façade treatment dominated the hygric state of the exterior and middle positions of earthen walls, while ventilation was most important at the wall interior and in the indoor space. Therefore, combining façade hydrophobization treatment and ventilation is recommended to improve the performance of earthen walls and maintain a favorable indoor environment.