TL-0018 — Use of GCP Applied Technologies PERM-A-BARRIER® Systems with Stucco Technical Letter

The stucco and air barrier industries, both recognize the need for drainage between an air barrier assembly (used in lieu of building paper in exterior walls) and a stucco veneer. Building science sources/guides recommend that a drainage space be provided between the stucco and the air barrier drainage plane. GCP Applied Technologies (GCP)supports this recommendation and offers the additional information below.

GCP allows the use of traditional stucco and lath exterior veneers over PERM-A-BARRIER® Systems under the following conditions:

• The GCP air barrier material and assembly is appropriate for the intended wall design (by others) in consideration of air barrier type (permeable vs. impermeable) and climatic conditions
• Because stucco is a relatively absorbent cladding, proper wall design requires drainage of water down and out of a wall assembly. If it is expected that the stucco could become bonded to the air barrier assembly, GCP strongly recommends drainage between the stucco and the PERM-A-BARRIER® System.
• While ultimately the type of method used to provide for this drainage is up to the designer, it has been shown this can often be accomplished via a number of different ways:
  • A layer of conventional building paper/felt between the stucco lath and PERM-A-BARRIER® System.
  • A type of metal lath or other method that stands the stucco away from the PERM-A-BARRIER® System and creates a drainage channel behind the stucco.
  • Or similar means to facilitate proper drainage between the stucco assembly and the PERM-A-BARRIER® System.
• Regardless of which drainage method is incorporated, an air and water tight seal should be achieved at any fasteners through the air barrier to provide for air and water tight performance as required per project specifications.

GCP is continuing to evaluate the use of stucco and lath exterior veneers with our PERM-A-BARRIER® Systems to determine best practice techniques.

Please refer to the GCP PERM-A-BARRIER® Stucco Details for additional information about the use of Stucco Veneers with our PERM-A-BARRIER® Systems. GCP PERM-A-BARRIER® Aluminum Flashing is not included in the GCP PERM-A-BARRIER® Stucco Details due to a requirement per ASTM C926 Standard Specification for Application of Portland Cement-Based Plaster1 indicating that no aluminum flashing should be used with stucco veneer. All flashing material used with stucco veneer shall be comprised of corrosion-resistant materials.

¹ ASTM C926-06 Standard Specification for Application of Portland Cement-Based Plaster, paragraph A2.1.2