A water-resistive barrier (WRB) is a material installed between the sheathing (or studs if there is no sheathing) and the siding. It is designed to prevent water from reaching building components that could be damaged by moisture. Builders should assume that siding intallations aren’t truly waterproof, and that some water will find its way through or around the siding (at least once in a while). Without a WRB, sheathing and other parts of the wall assembly would be much more susceptible to damage.
A WRB can be one of a variety of materials sold under dozens of trade names. There are two types of building paper, asphalt felt and Grade D building paper; plastic housewraps such as Tyvek and Typar; rigid foam insulation; liquid-applied sealers; Zip System sheathing; and a unique product called Delta-Dry.
Most WRBs are vapor-permeable. Whatever type is used, it’s essential that wall assemblies have the ability to dry out.
The International Residential Code requires builders to install a layer of number 15 asphalt or paperbacked stucco lath over the wall sheathing or studs. However, builders can still meet this requirement with some “other approved water-resistive barrier.” A number of materials have met the requirements of the International Code Council Evaluation Service (ICC-ES) for use as a substitute, including plastic housewraps, liquid-applied WRBs, Grade D building paper, and certain wall assemblies incorporating rigid foam insulation.
Common WRBs, including most housewraps, asphalt felt, and Grade D paper, offer only so much protection against water. Eventually, water will get through. George Tsongas, a former professor of mechanical engineering at Portland State University, says, “In fact, they are not moisture barriers. If you get any significant amount of water behind the siding, the building paper will not hold back water — not even 15-pound felt. All the papers will…