Numerical simulation study on the hygrothermal performance of building exterior walls under dynamic wind-driven rain condition

Wind-driven rain (WDR) has a significant influence on the hygrothermal performance, durability, and energy consumption of building components. The calculation of WDR loads using semi-empirical models has been incorporated into the boundary conditions of coupled heat and moisture transfer models. However, prior research often relied on fixed WDR absorption ratio, which fail to accurately capture the water absorption characteristics of porous building materials under rainfall scenarios. Therefore, this study aims to investigate the coupled heat and moisture transfer of exterior walls under dynamic WDR boundary conditions, utilizing an empirically obtained WDR absorption ratio model based on field measurements. The developed coupled heat and moisture transfer model is validated against the HAMSTAD project. The findings reveal that the total WDR flux calculated with the dynamic WDR boundary is lower than that obtained with the fixed WDR boundary, with greater disparities observed in orientations experiencing higher WDR loads. The variations in moisture flow significantly impact the surface temperature and relative humidity of the walls, influencing the calculation of cooling and heating loads by different models. Compared to the transient heat transfer model, the coupled heat and moisture transfer model incorporating dynamic WDR boundary exhibits maximum increases of 17.6% and 16.2% in cooling and heating loads, respectively. The dynamic WDR boundary conditions provide more precise numerical values for surface moisture flux, offering valuable insights for the thermal design of building enclosures and load calculations for HVAC systems.