Water-Resistive Barrier (WRB)

No exterior finish is perfectly waterproof. Wind-driven rain, sprinkler overspray, and small cracks let some water through stucco, brick, or siding, which is exactly why builders install a water-resistive barrier (WRB), a continuous water-shedding sheet between the wall's outer cladding and the sheathing or framing behind it. The WRB catches that water and drains it back out so the wood structure stays dry. Common materials are asphalt-saturated felt (building paper), Grade D paper used under stucco, and modern house wraps.

The 2021 International Residential Code (IRC), in Section R703, requires a water-resistive barrier behind most exterior wall coverings. Behind stucco, the code calls for the equivalent of two layers of Grade D paper, because wet stucco bonds to and can compromise a single layer as it cures. The barrier laps shingle-style, each upper piece over the one below, so gravity carries water down and out rather than behind the layer.

In Arizona, most production and custom homes wear three-coat stucco, so the WRB lives behind that stucco and ties into the weep screed at the bottom of the wall, which lets the drained water escape. During monsoon season, that drainage path is what keeps a wind-driven downpour from soaking the wall framing.

A WRB is not the same as a vapor barrier. A WRB blocks liquid water from the outside while still letting the wall dry; a vapor barrier controls water vapor moving through the assembly. Both manage moisture, but they sit in different places and solve different problems. Getting the WRB lapped and flashed correctly at windows and the base of the wall is one of the most important defects-prevention steps in the whole exterior.