TL-0002 — Substrate Preparation for Application of PERM-A-BARRIER® Products to Glass-Mat Faced Gypsum Sheathing Technical Letter

Glass-mat faced gypsum sheathing is used for direct mechanical application to structural framing as a backing for a variety of exterior claddings or as a soffit material. Its unique glass mat facing provides greater resistance to rain, heat and wind than regular paper-faced gypsum sheathing. The following are recommendations for substrate preparation when applying GCP’s PERM-A-BARRIER® products to glass-mat faced gypsum sheathing. Full membrane application guidelines are described on the product data sheets available for download at

PERM-A-BARRIER® Fluid Applied Membranes 

When installing PERM-A-BARRIER® fluid applied membranes over glass-mat faced gypsum sheathing, no surface treatment is required. PERM-A-BARRIER® fluid applied membranes have excellent adhesion to the surface of the glass-mat faced gypsum sheathing. In order to ensure a quality application, we recommend of the following joint treatment option:

  • To avoid deflection of glass-mat faced gypsum sheathing at panel joints, fasten corners and edges with appropriate screws as per manufacturer’s recommendations.
  • Completely fill the sheathing joint with PERM-A-BARRIER® Universal Flashing and Sealant and then install a scratch coat (approx. 15-30 mils) of Universal Flashing and Sealant with a margin trowel or similar onto the face of the sheathing approximately 1 in. (25 mm) on each side of the sheathing joint, ensuring the edges are tapered to prevent shadowing of the spray application.
  • Once the sealant is tack free, the PERM-A-BARRIER® fluid applied membrane may be applied.
  • This is the preferred joint treatment for all PERM-A-BARRIER® fluid applied membranes.

PERM-A-BARRIER® Wall Membrane



PERM-A-BARRIER® Aluminum Wall Membrane

PERM-A-BARRIER® Wall Flashing

PERM-A-BARRIER® Aluminum Flashing

PERM-A-BARRIER® Detail Membrane

In order to ensure a quality application we recommend the following:

  • To avoid deflection of glass-mat faced gypsum sheathing at panel joints, fasten corners and edges with appropriate screws as per manufacturer’s recommendations.
  • Gaps, voids, or sheathing joints exceeding 1/4" should first be repaired with a compatible sealant. Refer to Technical Letter 1: Chemical Compatibility of PERM-A-BARRIER® Self-Adhered Membranes with Other Materials for guidance.
  • When installing PERM-A-BARRIER® membranes that require primer, the surface of the sheathing must first be primed with PERM-A-BARRIER® WB Primer by roller, PERM-A-BARRIER® Adhesive Primer by spray, brush or roller, or PERM-A-BARRIER® Adhesive by roller. Coverage rates are provided in the data sheets for the primers, recommended in the following table and may vary depending on substrate and/or substrate roughness. | North America Customer Service: +1 (877) 423 6491

We hope the information here will be helpful. It is based on data and knowledge considered to be true and accurate and is offered for consideration, investigation and verification by the user, but we do not warrant the results to be obtained. Please read all statements, recommendations and suggestions in conjunction with our conditions of sale, which apply to all goods supplied by us. No statement, recommendation or suggestion is intended for any use that would infringe any patent, copyright or other third-party right.

PERM-A-BARRIER, BITUTHENE, and RIPCORD are trademarks, which may be registered in the United States and/or other countries, of GCP Applied Technologies, Inc. This trademark list has been compiled using available published information as of the publication date and may not accurately reflect current trademark ownership or status.

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GCP Applied Technologies Inc., 2325 Lakeview Parkway, Suite 400, Alpharetta, GA 30009, USA
GCP Canada, Inc., 294 Clements Road, West, Ajax, Ontario, Canada L1S 3C6
This document is only current as of the last updated date stated below and is valid only for use in the United States. It is important that you always refer to the currently available information at the URL below to provide the most current product information at the time of use. Additional literature such as Contractor Manuals, Technical Bulletins, Detail Drawings and detailing recommendations and other relevant documents are also available on Information found on other websites must not be relied upon, as they may not be up-to-date or applicable to the conditions in your location and we do not accept any responsibility for their content. If there are any conflicts or if you need more information, please contact GCP Customer Service.

Last Updated: 2023-11-22