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.

One example is the picture frame method, whereby an integral flanged window unit is installed with the flanges mounted directly to a picture frame installed flush to the thickness of FPIS (see detail at right).  This method is suitable for foam sheathing thickness between 3/4-inches to 1-1/2-inches to match common lumber dimensions.

Another example is the window buck method, whereby an integral flanged window unit is installed with the flanges mounted directly to a buck installed flush to the thickness of FPIS (see detail at right). This method is commonly employed for foam sheathing thickness greater than 1-1/2-inches thick.