Why is building paper used on the face of sheathing in a steel stud backup wall? The brick masonry in these locations is designed with a cavity wall, so why is it necessary to protect the face of the sheathing?

The Brick Industry Association (BIA) recommends the use of building paper whenever brick veneer is applied over steel stud backup walls (BIA Technical Note 28B) and over wood stud backup walls (BIA Technical Note 28). The BIA technical notes refer to these elements as moisture barriers.

BIA Technical Note 28B states: “While the drainage cavity acts to divert most water penetrating the brickwork, a moisture barrier should keep out any water which finds its way across the cavity via ties, mortar bridging, or splashing. Individual pieces of moisture barrier should be installed with their edges and ends lapped at least 6 inches. Sheathing or rigid insulation with an inherent resistance to moisture penetration may also serve as a moisture barrier when all edges and joints are completely taped or sealed.”

The face of the sheathing needs to be protected because water may bridge across the face of the collar joint cavity in the steel stud backup walls. Some mortar droppings may occur at the base of the cavity and on wall ties, even in walls that are built with reasonable care. Water may also bridge the cavity and blocking at windows, window lintels, and at a variety of other locations.

The sheathing is protected from water penetration by providing a barrier on the face of the wall. If any water does bridge across the cavity, it reaches the face of the building paper and flows down the wall into the flashing and out of the wall without causing harm.

The sheathing can absorb water when mortar bridging is present within masonry veneer systems where there is no building paper on the surface on the sheathing. This water absorption can lead to deterioration of the sheathing, mold growth, and corrosion of the steel studs.