Wetting and drying of aerogel-based coating mortars in Swedish climates

Abstract: Aerogel-based coating mortars (ACMs) have emerged as energy-efficient wall finishes with thermal conductivities of 30–50 mW/(m∙K). These coating mortars represent a promising alternative to traditional thermal insulation materials for retrofitting uninsulated building envelopes particularly in culturally significant structures. Although previously used in Central Europe, their moisture absorption under rainwater wetting, early-stage drying, and long-term hygrothermal performance in other climates remain inadequately explored. This gap in knowledge presents challenges in designing moisture-safe solutions and evaluating in-field thermal insulation performance in regions characterized by high moisture loads and limited drying potential. Therefore, an investigation was conducted to increase knowledge of the moisture performance of coating systems with ACMs. This investigation combined field and laboratory-based measurements with numerical hygrothermal simulations to study their moisture absorption under diverse wetting scenarios and evaluate their drying performance. Furthermore, the impact of surface water-repellent properties and surface cracks was assessed. The laboratory studies employed a newly developed small-scale test setup designed to simulate runoff wetting caused by typical wind-driven rain intensities in Sweden. Moreover, two supplementary capillary suction experiments under zero (free suction) and elevated hydrostatic pressure (created by Karsten tube) were conducted to explore additional wetting scenarios. A 15-month field test in Gothenburg, Sweden, combined with hygrothermal simulations were utilized to evaluate the early-stage drying and long-term hygrothermal performance of the coating system with ACM for four Swedish cities. The laboratory measurements demonstrated minimal moisture absorption in the undamaged coating system with ACM, even during prolonged 24-h runoff wetting. As expected, applying water-repellent paint (sd = 0.01 m) to the exterior of the coating system effectively reduced the water absorption while maintaining the drying capacity. Conversely, coating systems with a 1 ± 0.5 mm wide surface crack had 3–5 times amplified water intrusion due to hydrostatic pressure from the created water film on the surface. This could increase the risk of local moisture accumulation. Capillary suction tests of the ACM revealed a substantial increase in water absorption after repeated wetting exposure. Meanwhile, the same tests on the complete coating system showed a consistently stable water absorption. Field measurements indicated that the built-in moisture in the ACM dried out within six months. Hygrothermal simulations for four Swedish cities revealed an early-stage drying period ranging from 134 to 336 days based on the climate and time of application. Over time, the ACM exhibited no hygroscopic moisture accumulation; however, walls highly exposed to wind-driven rain could experience elevated relative humidity within the ACM, thereby resulting in an average increase in thermal conductivity of up to 9%. The findings show that the examined coating system with ACM presents a moisture-safe solution for retrofitting external homogenous concrete and masonry structures, preventing moisture accumulation from rainwater wetting. However, considering the information regarding the anticipated early-stage drying time and the moderate elevation in thermal conductivity is crucial while evaluating the in-field hygrothermal performance of the coating system.