Water resistive barriers (WRBs) come in several types. The words “mechanically applied” make the standard nailed or stapled water resistive barriers sound more hi-tech than they are. No. 15 asphalt felt (Type I asphalt-saturated felt, ASTM D226) qualifies as a time-tested, inexpensive, and reliable WRB with certain advantages. In addition to shedding bulk water behind the exterior cladding, asphalt-impregnated felt acts as a blotter, which means it holds back bulk water while allowing vapor to evaporate.
Today, WRBs include liquid-applied and self-stick membranes and those integrated into the structural sheathing, such as HUBER’s Zip System. Whichever WRB you choose, they all have the same purpose: to shed bulk water and reduce air infiltration through the exterior walls. In short, these products provide a second line of defense between cladding and framing to protect your building from moisture. By far, the most common type of WRB comes in a roll and gets nailed or stapled onto the exterior sheathing. This type is the so-called mechanically applied water resistive barrier. Given the many options, let’s look at the different products from which to choose.
|The International Residential Code (IRC) defines a water resistive barrier as: “A material behind an exterior wall covering that is intended to resist liquid water that has penetrated behind the exterior covering from further intruding into the exterior wall assembly.”|
Asphalt-impregnated felt ($) is the least costly yet effective WRB permitted by code. As the original “smart WRB,” the permeance of asphalt felt ranges from 5 to 60 perms depending on humidity. It can hold bulk water and then allow it to evaporate to the exterior. Its heavy-paper base was once made of recycled rags; today it is recycled cardboard. From the manufacturing perspective, it has green bona fides. Unfortunately, it acts only as a…