By Karl Schaack, RRC, PE
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A variety of materials and technologies are currently being used in exterior wall construction of structures to form a membrane on the exterior face of the backup wall/substrate to function as a barrier to prevent air and/or moisture infiltration into building interiors. In general, regardless of the design intention of the wall assembly—whether permeable or impermeable, barrier wall or rain screen—the material is installed with the main intention of preventing the transfer of moisture (either in liquid or vapor state) to the building interior. Therefore, for the purposes of this article, the material will be referred to interchangeably herein either as a weather-resistive barrier (WRB) or air barrier.
WRBs that are currently available consist of a variety of material options, including sheet goods (mechanically attached or adhered), spray-applied polyurethane foam, fluid-applied products, sheathing with pre-applied barrier, and rigid board insulation. There can be multiple issues associated with the installation of these products or the subsequent installation of exterior cladding/finishes applied over the top of the WRB that can affect the overall performance of the WRB/air barrier and the entire wall assembly.
WRB materials have product-specific properties that must be tested to ensure they will meet rigors encountered during their service life and under the buildings’ imposed loads to be considered approved air barrier materials and assemblies. An air barrier assembly is a collection of materials and accessories (e.g., sealants, tapes, transition membranes) assembled together to form a continuous barrier to air or water infiltration into the conditioned space. It is understood that the air barrier materials/assemblies are tested in a laboratory setting that is much different than real-world applications commonly encountered in the construction industry. The purpose of this article is to highlight several conditions that have been repeatedly encountered during the installation of these WRBs that could adversely impact the overall building performance.
Fluid-Applied Air Barriers
For fluid-applied air barriers, pretreating joints in sheathing is a standard procedure that commonly utilizes gun-grade sealant, which is compatible with the air barrier, or a “trowel-grade” consistency air barrier material. The material can be applied with a bulk gun over the respective sheathing joint and then is often smeared or haphazardly tooled by a technician with a flat-blade spatula. This practice can result in heavy applications of the product with profiled (thick) edges, in lieu of feathered edges. When the fluid-applied air barrier is applied over the heavily treated joints—particularly during spray-applied applications—the heavy-build treatment can act as a shield, blocking the fluid from reaching the surface of the sheathing, unless the applicator takes the time to apply the material in a multidirectional method (Figure 1). Even during roller applications, the highbuild edges can act as a bridge when the roller is brought across the material, and the sheathing facer—located directly along the sides of the sealant—does not receive a proper application of the fluid material.
Pretreating heads of fasteners used to secure sheathing to framing is commonly recommended by air barrier manufacturers when the fastener is overdriven and fractures the fiberglass facer on the sheathing. Even when the fastener head is properly seated flush with the surface of sheathing, the formed indentions in the head of a Phillips screw can draw the fluid material down into the indentations, pulling the material from the screw head and resulting in small voids around the perimeter of the head (Figure 2). Since holes in the air barrier due to inconsistencies at fastener heads are found to be very prevalent, pretreating of each and every fastener head is highly recommended so as to alleviate judgment calls by the applicator on which specific fasteners need treatment. Furthermore, since sheathing joints are required to be pretreated, applying additional material over fastener heads at that same time does not involve significantly more time and material compared to the effort required to go back and search and identify voids at heads and then to perform respective repairs (apply additional material). See Figure 3.