Beyond Averages: Predicting Façade Algae Risk Through Coupled CFD-HAM Wetting and Drying Patterns

Heat, air, and moisture (HAM) models are essential tools for understanding moisture accumulation mechanisms and assessing renovation measures in building restoration projects, particularly for heritage structures. Among the primary factors affecting hygrothermal performance, wind-driven rain (WDR) is the most significant source of moisture in Belgium, while evaporation serves as the dominant drying mechanism. However, traditional HAM models often simplify these processes by applying uniform values for WDR and the convective heat transfer coefficient (CHTC) across the façade, neglecting their spatial and temporal variability. This limitation can lead to inaccuracies in predicting moisture-related risks and surface degradation. In this study, a coupled computational fluid dynamics (CFD) and HAM model is employed to enhance the accuracy of WDR and CHTC representation. steady Reynolds-averaged Navier-Stokes (RANS) equations and Eulerian multiphase CFD simulations are used to account for turbulent raindrop dispersion and spatial variations in wind flow. The coupled model is applied to a cubic low-rise building located in a suburban coastal region of Belgium. At this site, local climate data including wind speed, wind direction, temperature, humidity, and rainfall were collected. Additionally, WDR was measured at six points on the south façade to facilitate comparison with the simulation results. The findings highlight the limitations of conventional approaches, which may underestimate critical rain loads while overestimating drying potential. The improved model captures the intricate interplay between WDR exposure and drying mechanisms, providing more accurate predictions of surface degradation risks such as frost damage, salt crystallisation, and algae growth. This study underscores the importance of spatially resolved modelling in advancing hygrothermal performance assessments and guiding effective renovation strategies for building durability.