Guide to Rainscreen Window Installation for Foam-Sheathed Walls

Originally published by: Applied Building Technology GroupApril 24, 2017

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Window installation practices for foam plastic insulating sheathing (FPIS) can follow many approaches.  The Applied Builidng Technology Group recently developed new, step-by-step window installation guides to help the industry follow best practices appropriate for a given window and FPIS application.  These guides offer a standard method, a window buck method, a picture frame method and a rainscreen cladding method for mounting window units into walls with exterior foam sheathing or continuous insulation. Taken as a whole, these best-practices help to ensure constructible, code-compliant, durable, and high-performance wall assemblies using exterior foam sheathing for continuous insulation.     

The best approach for a given application with FPIS depends on a number of factors:

  • thickness of foam sheathing
  • size or configuration of window
  • whether or not the foam sheathing is used as an approved water-resistive barrier system
  • type of window frame and mounting approach (e.g., integral flange or block frame). 

In addition, the installation must consider an appropriate structural anchorage and support condition for the window unit, as well as an appropriate flashing method to ensure continuity with the water resistive barrier or drainage plane of the wall.

The rainscreen cladding method features window flanges and furring mounted directly over any thickness of FPIS (see image at right).  The intent of this approach 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.

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