Preventing rainwater penetration is one of the most critical and challenging functions of an effective building enclosure design. This function becomes more complicated when considering wind-driven rain and the project-specific variations of magnitude, intensity, and duration of rainfall and wind speed. Since the directionality of wind-driven rain can change throughout a single storm, it is not always obvious what combination of environmental factors are contributing to water leakage in a building enclosure assembly. One of the design strategies for resisting rainwater penetration is a drainage approach, such as a rainscreen or cavity wall system, which relies on the outer surface of the enclosure assembly to deflect rainwater, coupled with a dedicated internal drainage plane to redirect water that bypasses the outer surface back to the building exterior. This strategy is commonly understood and implemented in exterior wall assemblies, but it is also important to consider components of mechanical systems that are part of the assembly, particularly when potential exists for rainwater to flow into them. If overlooked, the overall performance of the enclosure may be compromised.
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