Masonry Edge the Storypole Vol7 No1 : Page 24
Mitigating Moisture weather due to stack effect. Stack effect occurs due to the difference in density between the warm exterior air and cool, air-conditioned interior air. This difference causes positive pressurization at the lower floors of the building and negative pressurization at the upper floors. Figure K | Closed-cell polyurethane spray-foam insulation applied in a continuous layer on the exterior side of electrical conduit backup wall. This allows water to be in direct contact with the wood furring as the backup units absorb moisture. Another common problem occurs on masonry walls that extend above the roof to form parapets. Often, the masonry on these parapet walls is exposed on the roof side ( Figure J ). Masonry exposed on the roof side of the parapet can absorb water that can wet the backup wythe if there is no through-wall flashing. Typically, through-wall flashing cannot be provided at these locations because it creates a bond break at the base of the parapet, which prevents the parapet from functioning as a cantilever. Moisture problems are also significantly worse in situations where evaporation to the interior is restricted. The polyethylene sheeting will effectively prevent moisture from evaporating to the interior spaces. This allows moisture to be held within the walls. Moisture problems are typically significantly worse when vinyl wall-coverings are used on the interior surface of the gypsum sheathing. Not only does the vinyl wall-covering act as a vapor retarder, preventing drying to the interior, but the moisture that condenses on the exterior surface of the vinyl wall covering will facilitate mold because the wall-covering paste and the paper of the gypsum wallboard are food sources. sion joints must be provided at recom mend ed spacing to avoid cracking problems. Proper cleaning techniques must be used to prevent mortar joints from being etched. Flashing must be properly installed, especially below masonry copings, sills and caps at horizontal ledges in the masonry wall. The masonry on the roof face of the wall must either be covered or flashing must be provided to prevent water migration to the masonry below (note that the structural continuity must be maintained). Insulating the interior face of well-constructed multi-wythe or single-wythe masonry walls can be effective if moisture migration during warm weather is either blocked or drying to the interior is facilitated. Moisture migration can be blocked by using closed-cell polyurethane spray foam insulation bonded to the interior face of the masonry ( Figure K ). The spray foam insulation functions as an air and vapor barrier. Also, because it is bonded to the interior face of the masonry, it eliminates the air space onto which condensation could otherwise occur. To work effectively, spray foam insulation must be installed to form a continuous layer across floor lines and extending behind electrical outlets. Studs containing electrical conduit would be positioned to the interior of the insulation. It is important to note that because this system prevents drying to the interior, it can allow walls to remain wet for longer periods of time. As a result, potential for efflorescence is increased. Preventing Moisture Problems in New Construction These problems can Because (spray foam insulation) is bonded to the interior masonry face, it eliminates air space onto which condensation could occur Moisture problems are significantly worse in these masonry wall systems if there is excessive air infiltration. Air infiltration will carry moist air from the masonry air space to the interior at gaps in the wall system. Gaps exist around win -dows and through voids in the masonry backup wall. Air infiltration is especially a problem in buildings where the mechanical system creates negative pressurization. Negative pressurization occurs when the air pressure on the inside of the building is less than that on the exterior of the building. Negative pressurization often occurs in taller buildings at the upper floors during warm be prevented in new construction by modifying the wall system design and employing quality assurance on the masonry workmanship. The wall system can be designed as a masonry veneer wall, an insulated masonry cavity wall, or, in the case of older buildings, as an uninsulated masonry mass wall. Veneer walls have a masonry veneer, an air space and water resistive barrier/air barrier on the exterior surface of the backup wall. Water that penetrates the exterior wythe is prevented from wetting the backup due to the presence of the air space and water resistive barrier. In insulated masonry cavity walls, rigid insulation will not only help to prevent water from bridging across the wall system, but causes the dewpoint to occur within the rigid insulation, where it will not result in condensation. Many older buildings used multi-wythe wall systems without air spaces or water resistive barriers. However, these wall systems are effective because they were not insulated on the interior face. Instead, plaster was applied directly to the inside face of the walls. This allows moisture within the masonry walls to evaporate to the interior and exterior. Older mass masonry walls that have been retrofitted with insulation on the interior to increase thermal performance often develop the moisture problems described above. Regardless of the wall system used, good masonry workmanship helps to prevent problems. Mortar joints must be properly filled and tooled. Expan -Moisture migration during warm weather can be facilitated if galvanized steel furring is attached to the interior face of the masonry, unfaced insulation is installed between furring strips and the interior face is covered with a smart vapor retarder. Smart vapor retarders have variable vapor permeance depending on the relative humidity of the air adjacent to the warm side of the membrane. When relative humidity is high, these function as vapor permeable membranes allowing moisture to pass through. When relative humidity is low, generally 30% or lower, these membranes function as a vapor retarder preventing moisture migration. Therefore, during warm weather when the air within the wall system is often warm and humid, moisture is allowed to dry to the interior preventing moisture accumulation. During cold weather when the interior relative humidity is 30% or less, the membrane functions as a vapor retarder preventing interior air from entering the wall system and condensing within the masonry. For this system to function properly, the interior surface must be finished masonryedge.com 24 MASONRY EDG E / thestorypole Vol 7 No 1 Masonry Technology | Innovation
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