Test Method to Measure the Relative Capacity of Wall Panels to Evacuate Moisture from Their Stud Cavity

Abstract

Rainwater penetration is the source of moisture that causes the greatest damage to building envelope assemblies. The building envelope should be designed to reduce the amount of rainwater penetration by deflection and drainage. Since it is not realistic to assume a perfect wall without any leakage, the envelope should have the drying capacity to tolerate defects that may arise from the design, construction, and aging of the exterior wall system. Systems with a greater capacity to evacuate moisture from the stud cavity are less likely to undergo moisture damage. A new testing method is developed and deployed to evaluate the relative drying capacity of six wood-framed wall panels of different configurations built into a test hut and tested within a large scale environmental chamber. The wall panels used plywood, oriented strand board (OSB), or fiberboard as sheathing, but did not include cladding. A uniform moisture source was introduced in a water tray set on a load cell at the bottom of each stud cavity. The protocol is based on the hypothesis that the potential for moving a water molecule from the bottom plate to the exterior of the stud cavity is independent of the previous journey of that molecule, i.e., whether it has traveled from the interior of the bottom plate to the surface of the plate or whether it comes from free water in a tray at the level of the bottom plate. For a given set of boundary conditions, this potential is a function of the characteristics of the wall panel, and is identified as the drying capacity of the panel or its drying by evaporation index (DEI). The value of DEI corresponds to the evaporation rate. The moisture response of wall materials enclosing the stud cavity and the evaporation rate of the moisture source were monitored. The results show that this index can be used as an indicator of the relative drying capacity of different wall systems.