UPDATED on September 18, 2013
By now, almost all builders know the importance of installing a water-resistive barrier (WRB) behind siding. Most types of siding leak, so it’s a good idea (and a code requirement) to install a WRB to protect your wall sheathing from any wind-driven rain that gets past the siding.
A WRB can be vapor-permeable, like Tyvek, or vapor-impermeable, like foil-faced polyisocyanurate. As long as the wall assembly is designed to dry out when it gets wet, either vapor-permeable or vapor-impermeable WRBs work well.
In most cases, WRBs are not air barriers. (The notable exceptions are Zip System sheathing and liquid-applied WRBs.) If you want to create an air barrier on the exterior side of your wall, it usually makes more sense to create the air barrier at the sheathing layer, not at the WRB.
Although there are at least six categories of WRB — asphalt felt, Grade D building paper, plastic housewrap, liquid-applied WRBs, rigid foam, and Zip System sheathing — building codes calls for just one: asphalt felt. (One other category — three-dimensional vapor-impermeable barriers — is represented by a single product, Delta-Dry. Although it is often used as a WRB, Delta-Dry has not received code approval as a substitute for code-required WRBs.)
In section R703.2, the International Residential Code requires builders to install a layer of number 15 asphalt felt or paperbacked stucco lath over the wall sheathing or studs of every new home. The requirement includes a qualification: if you don’t want to use number 15 asphalt felt, you can use some “other approved water-resistive barrier.”
The code requirement calling for walls to be covered with asphalt felt is rather odd, because every asphalt felt manufacturer declares unequivocally that the product is intended for roofs, not walls. When builders install asphalt felt on walls as required by code, the…