In a stucco frame-wall assembly, what purpose does building paper serve?
Stucco is known to be a weather resistant building finish, but it is part of a system. In order for the wall to resist water penetration effectively, the system must be properly designed and detailed, then built according to plans. The main purpose of building paper is to keep water from contacting the substrate and structural support members—very commonly sheathing like plywood or OSB (oriented strand board) and wood or metal studs—so that these materials stay dry. Metal can rust and wood can rot. Also, wood is prone to expand and contract with changes in moisture, so it’s essential to keep sheathing dry to provide the plaster with a sound substrate. Minimizing the changes in moisture minimizes the stresses that might be placed on plaster from behind. In addition to structural considerations, excess moisture within a wall creates a potential for mold or mildew inside buildings. Building paper prevents moisture-related problems in stucco walls. Several industry documents, such as PCA’s
