Application of Rain Intensity Dependent Rain Admittance Factor (RAF) in Hygrothermal Performance Assessment of Wall Systems

Wind-driven rain (WDR) is one of the main moisture loading sources on the exterior enclosures. The direct impact of wind-driven rain on the hygrothermal performance of building envelope has been well documented. Rain admittance factor (RAF) and rain penetration values characterize the amount of water reaching the exterior surface and the exterior surface of the water-resistive barrier respectively based on measured horizontal rain intensity. In common RAF factor calculation from horizontal rainfall data procedures, such as ASHRAE 160, RAF values are not affected by the intensity of the rainfall. However, a previous study shows RAF coefficients are sensitive to the rainfall intensity. Thus it is important to investigate how the sensitivity of using horizontal rainfall intensity dependent RAF factors and the subsequent rain penetration relates to hygrothermal performance assessment of building envelope components. This study is based on five years of WDR and horizontal rainfall data collected at different orientations of façades at a two-story test building in a mild coastal climate. The data is categorized into two sets based on rain intensity. The correlation between the measured moisture content on the sheathing board of a building envelope at different points utilizing RAF values based on the proposed approach and the overall measured RAF values is studied using WUFI simulation. Results show that an average percentage difference between the moisture content values of a sheathing board using RAF values of the rain intensity dependent approach and the overall RAF measured value can be as large as 9 %