Rainwater leakage through façades poses a significant risk of moisture damage in building envelopes. This requires testing of new wall assemblies adjusted to prevailing local climate conditions. Existing standardized laboratory tests for watertightness, particularly against wind-driven rain, are often resource-intensive and involve extreme conditions primarily applicable to high-rise buildings. Moreover, these tests rely on a pass/fail criterion, limiting quantitative data collection under prevailing wetting scenarios. To address these challenges, a small-scale laboratory test setup was developed to replicate rainwater runoff wetting based on historical wind-driven rain intensities in Swedish climates. The aim of the setup design is to enhance efficiency, ease of use, and real-time monitoring of moisture conditions during wetting and drying phases. To verify its performance, the moisture performance of a commercial coating system with aerogel-based coating mortar (ACM-system), new to the Swedish market, was tested. ACM-systems, typically 50 mm thick, comprise multiple layers of ordinary coating mortars and 40 mm of ACM, known for its low thermal conductivity of 30–50 mW/(m·K). Various surface water-repellent properties, with and without a horizontal crack measuring 1 ± 0.5 mm in width, were applied. The results show that the setup and integrated monitoring system gives replicable quantitative data for measuring moisture conditions during wetting and drying phases.