How to Install an Air Barrier Beneath Cement Plaster Lath

Air barriers are also called water-resistive barriers and serve two purposes: preventing moisture from corroding the wooden portion of the wall and to prevent leakage of air from inside the house. Building codes contain two different specifications for water-resistive barriers, and the one applying to cement plaster finishes applied over wood bases is the more restrictive. The code requires the equivalent of two layers of Grade D Kraft building paper, a strong formulation, instead of one layer or asphalt felt. While the definition of equivalent is not specified, it generally includes modern house wraps and does not include asphalt felt.

Things You'll Need

• Barrier material
• Nail gun
• Sealing tape
• Lath

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Instructions

1. • 1

     Unroll barrier material from the bottom to the top of the wall, nailing it to the building studs every six to 10 inches. Install over weep screed, a flat piece of metal with an angled flange that drains trapped moisture away from the wall. Unroll over window and door openings.

   • 2

     Tape all vertical and horizontal seams with waterproof sealing tape.

   • 3

     Install lath over the air barrier, mounting sheets horizontally, not vertically, by nailing it to studs every 18 inches. Cement plaster can then be applied, with the plaster going through the lath to adhere to the air barrier.