The Foam Sheathing Committee (FSC) serves as a manufacturing-driven organization focused on developing solutions to building code issues and promoting the proper technical use of foam sheathing to the construction industry.

Foam Sheathing Committee Compendium

A growing collection of technical, educational and training documents for Foam Sheathing Committee members from the Applied Building Technology Group.

Foam Sheathing Committee (FSC) Members:

Atlas RoofingDowGAFHunter PanelsJohns ManvilleKingspan Insulation LLCOwens CorningRMAX




  • There are many acceptable ways to mount and detail windows for support and weather resistance.
  • This installation best practice provides only a representative solution for integrating windows with foam sheathing.
  • It is the responsibility of the user to verify the appropriateness of any specific detail for their specific conditions.


  • The installation approach featured in this presentation:
    • Is a “window buck” installation concept with window flanges mounted directly over a limited thickness of FPIS.
      • Represents a common method for installing windows in walls with generally more than 1-1/2 to 2-inches-thick FPIS.
    • Uses FPIS as the water-resistive barrier (WRB).
      • Refer to DrJ DRR 1410-05 and the FPIS manufacturer’s installation instructions.
      • Use of a separate WRB material layer is also common and acceptable with appropriate installation and detailing.
  • The installation approach shown includes windows with integral mounting flanges.
  • Integral mounting flange windows:
    • Are sometimes referred to as “integral nailing flange,” “integral fin,” or “integral mounting fin.”
    • An integral flange is extruded with the frame and forms one continuous piece around the perimeter.
    • A mounting flange is typically about 1½” wide and is set back about 1” from exterior window face. Fasteners are installed through the pre-punched holes in the flange.

Foam Plastic Insulating Sheathing (FPIS) Products

  • Three types of FPIS:
    • Expanded Polystyrene (EPS) - ASTM C578
    • Extruded Polystyrene (XPS) - ASTM C578
    • Polyisocyanurate (Polyiso) - ASTM C1289
  • R-values ranging from R-4 to more than R-6 per inch.
  • Come in many thicknesses, compressive strengths, and densities.

Typical FPIS Applications

  • Often used as exterior continuous insulation (ci) on buildings to comply with energy codes or for improved performance.
    • Can be used as an air-barrier (AB) and water-resistive barrier (WRB) per manufacturer’s code approvals and instructions.
    • Proprietary FPIS products are also available as a structural insulating sheathing composite for wall bracing.

General Installation Guidance

  • DrJ Best Practices
  • Window, FPIS, WRB, or Flashing manufacturer’s installation instructions
  • An approved design
  • The following general installation guidelines

Key Principles

The intent of any acceptable detail for integrating windows with FPIS is:

  • To provide adequate structural support to the window unit.
  • To prevent water penetration at the window-wall interface by flashing to direct water onto the exterior surface of the WRB layer and/or cladding and away from the window opening.
  • To provide adequate drainage at the window sill for any incidental leakage of water that may still penetrate into the rough opening.

Framing Methods

  • There are four typical methods for window framing. This program covers the “Lumber Window Buck” method.

Window Buck Installation - Sill

Window Buck Installation - Jambs

Window Buck Installation - Header

Steps 1-3

Step 1: Frame Window Opening

  • Frame walls as required by the applicable code.
  • Ensure window rough opening is square and true.
  • Ensure appropriate framing in accordance with window installation method selected and support for FPIS edges is provided.

Step 2: Verify and Install FPIS

  • FPIS material must comply with:
    • ASTM C578 (EPS, XPS)
    • ASTM C1289 (Polyiso)
  • Wind pressure resistance
    • See ANSI/SBCA FS-100 for guidance
    • Only required when FPIS not used as oversheathing

Step 2: Verify and Install FPIS

  • Drive nails flush and snug with the surface of the insulation board.
  • Do not overdrive nails.
  • Do not underdrive nails.
  • Many FPIS manufacturers recommend use of cap nails.

Step 2: Verify and Install FPIS

  • Follow manufacturer’s installation guidelines
  • While not prohibited, avoid placing vertical joints in the sheathing over a window head where practical.
  • See FPIS Installation Instructions program.

Step 3: Verify Flashing and Sealant Materials

  • Ensure chemical compatibility of all sealants and flashings with intended substrates; refer to sealant and flashing manufacturer’s data.
  • Use flashing tape and sealants recommended by the window and FPIS/WRB manufacturers.

Steps 4-7

Step 4: Apply Sill Flashing

  • Apply all flashings in shingle fashion (e.g., jamb flashing overlaps sill flashing and head flashing overlaps jam flashing).
  • Overlap and seal sill flashing at center of sill if a multi-piece sill or pan flashing is used.

Step 4: Apply Sill Flashing

  • Alternatively, use a manufactured sill pan to simplify sill drainage installation.

Step 5: Apply Jamb Flashing

  • Apply flashing at jambs

Step 6: Apply Head Flashing

  • Apply flashing at head

Step 7: Apply Sealant

  • Apply sealant at jambs and head (or as required by manufacturer’s install instructions).
  • Sill is left open to allow the cavity below the window to drain to the exterior.

Steps 8-10

Step 8: Install Window Shims at Sill

  • Apply setting blocks and/or shims between the window buck and window frame.
  • The window frame must be anchored to the wood rough opening as required by the window manufacturer or in accordance with an approved design for sill support.

Step 9: Install Window

  • Install window plumb, level, and square per manufacturer’s instructions.

Step 9: Install Window

  • The window frame must adequately bear on the wood sill particularly if using a non-structural flange window.
  • Providing adequate sill support is good practice and often required by window manufacturer installation instructions.

Step 10: Verify Window Fasteners

  • Window flange fasteners must penetrate a minimum of 1¼" into framing members per IRC 2015.
  • Follow manufacturer installation requirements for size and spacing.

Steps 11-16

Step 11: Install Window Shims

  • Apply shims between the rough opening and window frame.
  • Anchor the window per the manufacturer’s installation instructions.

Step 12: Apply Jamb Flashing

  • Install flashing over the nailing flanges of the jambs to provide a final layer of protection against water intrusion.
  • The sill is not sealed, allowing for drainage of the rough opening, back to the exterior.
  • Where applicable, install drip cap per manufacturer.

Step 13: Apply Head Flashing

  • Apply head flashing.
    • Typically, butyl flashing tapes are used for this purpose.
  • Overlap window head flange and jamb flashing.

Step 14: Tape Head Flashing

  • For extra durability and protection, terminate the top edge of the head flashing tape with the FPIS manufacturer’s approved joint tape.
  • Typically, acrylic tapes are used for this purpose.

Step 15: Apply Sealant

  • Air seal window around entire perimeter on the interior with sealant or expanding foam made for this purpose.

Step 16: Install Cladding

  • See Installation Instructions “Attachment of Exterior Wall Coverings Through Foam Plastic Insulating Sheathing (FPIS) to Wood or Steel Wall Framing.”

Additional Reading