Rainwater and moisture control are key factors for maintaining the durability of wooden houses. Wall assemblies with sidings are installed on vented cavities to build durable wooden houses.Moisture condensation does not occur generally behind a vapor retarder in walls assembled with a vented cavity; however, it is reported that the condensation mechanism which occurs in the wall assembly due to the high humidity of the vented cavity by rain penetration in Japan, there are only a few studies that investigate hygrothermal behaviour considering effects such as rain penetration in the exterior system. To unravel the mechanism of internal condensation derived from rain penetration, labexperiments and field measurements were conducted. In the lab experiments, rain penetration from siding joints were quantitated using a water spray test. In the field measurements, for the exterior finishes of the experimental house, the sidings were installed on three types of vented cavities. To verify hygrothermal behavior within the wall assembly, intermittent long-term rain penetration into the vented cavity was reproduced for the experimental house. The measurements confirmed that internal condensation occurs with low ventilation performance, when moisture retained in the vented cavity is released into the wall assembly. This phenomenon is caused when the surface temperature on the sidings is increased because of solar radiation after rain. In conclusion, rain penetration through exterior finishes has a significant effect on the moisture behavior of wall assemblies. The obtained results verify that moisture condensation at the vapor retarder was caused by several factors including rain penetration, insufficient ventilation, and solar radiation. To maintain durability, it is important to ensure a ventilation performance and not retain moisture.
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