Balancing Hydrophobicity and Water-Vapor Transmission in Sol–Silicate Coatings Modified with Colloidal SiO2 and Silane Additives

This study investigates the optimization of sol–silicate façade coatings modified with colloidal silica and a silane-based hydrophobizing additive to enhance hydrophobicity while maintaining a high water-vapor transmission rate (V). The effects of the binder ratio between potassium water glass (WG) and colloidal silica (CS), the type of colloidal silica (unmodified or epoxy-silanized), and the concentration of the hydrophobizing additive (HA) were systematically evaluated. Water-vapor transmission was determined according to EN ISO 7783, and surface wettability was measured before and after accelerated UV-A aging. Dynamic viscosity was monitored for two years to assess long-term storage stability. The optimized formulation contained 7 wt % potassium water glass, 15 wt % colloidal silica, and 1 wt % hydrophobizing additive. It exhibited stable viscosity over time (≈19,000 mPa·s after six months), high water-vapor transmission (V > 6700 g·m−2·d−1, class V1), and an initial contact angle of 118°, which decreased only moderately after UV-A exposure. Coatings containing epoxy-silanized colloidal silica showed slightly lower transmission but still remained within the high V range suitable for vapor-open façade systems. The results confirm that balanced sol–silicate systems can combine durable hydrophobicity with long-term rheological and functional stability.