What Is WRB in Construction?

February 21, 2024

In many homes that were constructed (or reclad) in the 1990s and earlier, builders may have not used a water-resistant barrier between the sheathing and siding. Fortunately, current residential building codes and standards now require that a WRB be installed behind exterior wall cladding.

WRBs can be mechanically attached, self-adhering or fluid applied. They are available in a wide variety of materials sold under dozens of trade names, including felt papers such as asphalt felt and Grade D building paper; plastic housewraps such as Tyvek and Typar; synthetic fibers woven into sheets, like engineered fabric or a product called Delta-Dry; and rigid foam insulation and sheathing assemblies. These products are often vapor permeable, and most must be properly lapped and sealed to prevent moisture intrusion into the sheathing or studs.

The most common mechanically attached WRB is the polyethylene film (6 mil or higher) used as an air/vapor barrier in wood-frame walls. Builders attach this to the sheathing and then install cellulose insulation on top of it. Unfortunately, UV sunlight, frost, and winds quickly damage this vapor-permeable film, leaving it exposed to the elements and creating a gap for moisture to enter.

Another alternative to the polyethylene film is a fully adhered membrane that offers superior performance in both the moisture and air barriers. These include membranes that feature rubberized asphalt bonded to either polyethylene or aluminum foil. Builders apply the membrane to primed sheathing, and then install insulation over it. When installed correctly, these types of membranes offer superior moisture protection and help improve energy efficiency by preventing air leakage through the sheathing and into conditioned spaces.


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