Where and How to Apply Pecora XL-Flash


There are several components that make up the four-step approach to a complete air & vapor barrier protection for your building envelope. One of those components is Pecora XL-Flash. Pecora XL-Flash is a unique one-part, fast curing, non-sag elastomeric STPU (silyl terminated polyurethane) gun grade, trowelable liquid flashing & joint sealant. This product has many of the strengths of a two-component sealant but with the user-friendly ease-of-application properties of a one-component sealant. Pecora’s XL-Flash is specially formulated for the Air and Vapor Barrier Industry. It is resistant to ultra violet rays, ozone and airborne contaminants. It is designed to achieve high tensile and tear strengths, abrasion resistance and an average ultimate hardness of at least 55, yet still withstand 25% total joint movement. XL-Flash utilizes low VOC formulation that achieves primerless adhesion to most substrates. For this week’s blog post, we go in-depth on how to apply this product.

The purpose of an air barrier system is to provide a continuous seal throughout the building’s envelope. This is achieved by ensuring all gaps and seams are caulked and sealed up tight. On any project, you are going to have multiple penetrations coming through your concrete wall or sheathing. These penetrations may include window openings, electrical boxes, seams in the sheathing, PVC piping, cables, conduit, etc. These penetrations need to be sealed in order to have a continuous seal.

When it comes to fasteners on a sheathing wall application, any fasteners that are installed flush with the face of the sheathing just needs to be detailed with the air barrier coating. There is no need to hit all of those screw heads with flashing which can be very time consuming. Any fasteners that are countersunk of misaligned need to have flashing applied. Any joints, seams, or gaps need to be first sealed with the XL-Flash.

The following are instructions on where and how to apply the XL-Flash.

Sheathing Penetrations:

*Ensure all penetrations are firmly secured prior to installation of air & vapor barrier system

  1. All substrates must be clean, dry, and frost-free of contaminants such as dirt, dust residue, oil and grease.
  2. Fill all seams and static joints with the XL-Flash Liquid Flashing and Joint Filler. Joints greater than ¼” will require a sealant backing. Sealant should be applied flush with exterior face of wall.
  3. Apply a minimum ½” bead of XL-Flash around penetration and onto adjacent sheathing.
    1. For larger gaps, apply spray foam around penetrations and shave excess foam flush with sheathing prior to sealing with Pecora XL-Flash. See DWG# PEN-002.

Sheathing Joints & Fasteners:

  1. All substrates must be clean, dry, and frost-free of contaminants such as dirt, dust residue, oil and grease.
  2. Fill all seams and static joints with the XL-Flash. Joints greater than ¼” will require a sealant backing. Sealant should be applied flush with the exterior face of the wall.
  3. Extrude and tool XL-Flash over the sheathing joint in a minimum 2” wide band centered over the joints
    1. XL-Flash thickness should be applied at 20-40 wet mils.

Inside/Outside Corners of Sheathing:

  1. All substrates must be clean, dry, and frost-free of contaminants such as dirt, dust residue, oil and grease.
  2. Fill all seams and static joints with the XL-Flash Liquid Flashing and Joint Filler. Joints greater than ¼” will require a sealant backing. Sealant should be applied flush with exterior face of wall.
  3. Apply a minimum ½” fillet bead of Pecora XL-Flash to all inside corners.
  4. Apply 20 – 40 wet mils of Pecora XL-Flash and extend a minimum 2” over the inside/outside corner areas. Liquid flashing must be applied to both sides of the corner.

Masonry Penetrations:

*Ensure all penetrations are firmly secured prior to installation of air & vapor barrier system

  1. All substrates must be clean, dry, and frost-free of contaminants such as dirt, dust residue, oil and grease.
  2. Fill all seams and static joints with the XL-Flash Liquid Flashing and Joint Filler. Joints greater than ¼” will require a sealant backing. Sealant should be applied flush with exterior face of wall.
    1. Large voids should be filled flush with mortar and allowed to cure.
  3. Apply a minimum ½” bead of XL-Flash around penetration and onto adjacent masonry substrate.

Expansion Joints Liquid Joint Sealant:

  1. All substrates must be clean, dry, and frost-free of contaminants such as dirt, dust residue, oil and grease.
  2. Apply Pecora 890NST Silicone Sealant or approved equal and appropriate sealant backing in all expansion joints greater than ¼” wide. Sealant should be applied flush with exterior face of wall.
  3. Apply 20 – 40 wet mils of Pecora XL Flash and extend a minimum 2” over each side of the joint.
    1. XL Flash Liquid Flashing may be applied immediately after installation of silicone based joint sealant.

These air barrier systems are very important because they provide energy efficiency as well as weatherproofing to a building’s envelope. To learn more about our complete air barrier systems, please visit our website at https://www.pecora.com/air-vapor-water-barrier-products/.

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