Rainwater ingress is a major contributor to the early deterioration of building materials. Despite its importance, there is currently limited quantitative data on rainwater management in wall assemblies, particularly in ventilated façades. Key questions remain regarding the extent of water infiltration, the amount that penetrates the cavity, and its effect on hygrothermal performance. To address these issues, experimental watertightness tests were carried out to examine the influence of factors such as cavity depth, joint width between panels, out-of-plane panel positioning, and the dimensions of the supporting structure. A custom test setup was developed to measure rainwater infiltration into the wall cavity. The findings from these tests were then used to perform numerical simulations assessing the impact of rainwater ingress on hygrothermal behavior. One-dimensional hygrothermal simulations were conducted for ventilated façade systems in three different climates: Brussels (Belgium), Bergen (Norway), and Lisbon (Portugal). The results indicate a low risk of mold due to the high drying potential provided by the ventilated cavity. However, rainwater infiltration can significantly affect latent heat transfer and annual heat flux. For example, a 40 mm cavity results in a minimal heat flux increase of 0.4%, while a 20 mm cavity can lead to a rise of up to 10.5%. Although the use of a water-resistive barrier (WRB) decreases water penetration into the insulation, it also reduces drying potential, making the system more sensitive to moisture when installation defects are present.