Masonry Edge the Storypole Vol7 No1 : Page 23
Photos courtesy of Wiss, Janney, Elstner Associates, Inc Moisture within the wall system not only results in mold growth at discontinuities in the sheeting, but also can lead to deterioration of other elements within the wall system. Water condensing on the exterior face of polyethylene sheeting will wet the fiberglass batt insulation and furring. In the case of wood furring, this wetting can result in decay and mold growth ( Figure G ). In the case of metal furring, wetting results in corrosion ( Figure H ). If there are wood roof or floor joists or trusses that bear within the masonry walls and the ends of these members are not treated, decay can occur resulting in potential future structural problems. A B Figure A | Concrete masonry with batt insulation on the interior face of a concrete masonry backup wall that is covered by polyethylene sheeting and gypsum wallboard. Figure B | Mold growth between the gypsum wallboard and wood baseboard trim. This phenomenon is significantly worse as the rate of water penetration and absorption increases. Walls exposed to significant wind-driven rain typically have more problems than walls that are partially shielded by roof overhangs since they are exposed to greater volumes of water. Brick units with high cold-water absorption (11-13%, as allowed by ASTM C216) will absorb significantly more moisture than units with low cold-water absorption (8% or less). Poor tooling or fill in the joints also increases the rate of water penetration. Water readily penetrates through cracks or surface voids in the mortar joints. Typically, the volume of water penetration through the surface of the masonry is more related to the frequency of these voids, separations and cracks in the surface than it is to the absorption of the masonry units. Mortar joints that are acid-etched as a result of excessive cleaning will allow much greater water penetration than joints that are not etched. Flashing problems, especially the masonry copings, sills and caps at ledges, allow significant water to penetrate the masonry along the top. In multi-story buildings, water pene tra -tion can be increased both by the exposure to the heavier winds near the upper levels as well as by the rundown water from above on lower floors. All of these factors increase the rate of water penetration on the exterior surface of the masonry walls, which can result in greater storage of moisture within the masonry. In many cases, features of the wall system allow water to penetrate the backup wythe or the interior surface of the masonry directly. In multi-wythe walls, especially multi-wythe walls with a narrow air space less than 1", mortar bridges can form during construction ( Figure I ). Mortar bridges allow water that penetrates the exterior wythe to bridge across the air space and wet the CMU C D Figure C | Mold growth at perimeter of windows. Figure D | Water droplets on the exterior surface of the polyethylene sheeting. E F Figure E | Extended periods of rainfall allow masonry to absorb considerable moisture. Figure F | Water trapped at the base of wall can cause mold growth on the gypsum wallboard. G H Figure G | Water condensing on wood furring causes decay and can cause corrosion of fasteners. Figure H | Moisture can corrode metal furring strips within the wall. Figure I | Mortar bridges allow water within the masonry air space to wet the backup masonry. Figure J | Exposed masonry at parapet walls with surface applied roof flashings (no through-wall flashings) can contribute to moisture problems by allowing rain to directly wet the backup wall. I J masonryedge.com
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