A Better Window Installation Method When Using Foam Sheathing CI?

Originally published by: Applied Building Technology Group, LLCDecember 12, 2016

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Both the American Architectural Manufacturer’s Association (AAMA) and the Window and Door Manufacturer’s Association (WDMA) recommend the installation of a window buck or ‘picture frame’ when installing a flanged window in walls sheathed with exterior continuous insulation, such as foam plastic insulating sheathing (FPIS). Common sense and laboratory testing say not only is this approach unnecessary, there are much better alternatives.

The downsides to the ‘picture frame’ method include:

It creates additional costs associated with framing material and installation labor.

It creates an unnecessary thermal bridge around each window, counteracting the use of the exterior continuous insulation. This issue is explored by Joe Lstiburek in his article “The Star Crossed Lovers of Building Science,” where he states:

“So what is my problem?  It is just that the wood “bump out” is a thermal bridge.  What is the point of continuous insulation if we run a thermal bridge through it at every punched opening?”

It also serves no structural purpose. Again, Lstiburek states in his article:

“Never needed this before. The building industry has only about 35 years of historical experience showing that this is not necessary… Testing and field experience show that “bump outs” are not necessary.  Did the window industry just “wake up”?  What problem are we solving?  Besides having to learn origami and apply the knowledge at window-to-wall interfaces and door-to-wall interfaces?”

To that last point, recent testing indicates flanged windows can be applied directly over exterior foam sheathing, thus eliminating the thermal bridge.

For guidance on better window installation methods in walls sheathed with FPIS, look at the standard method and rainscreen method installation guides created by the Applied Building Technology Group.

The standard method guide follows a more traditional installation approach, whereby an integral flanged window unit is installed with the flanges mounted directly over the foam sheathing meeting specifications appropriate for this application.  In particular, the foam sheathing should be minimum 15psi compressive strength rated. This method has been most commonly employed for foam sheathing thicknesses of up to about 1-1/2-inches thick, though it can be applied for thicknesses of up to 2 inches based on recent testing.

The rainscreen method uses the FPIS as the water resistant barrier (WRB), though the use of a separate WRB material layer. This is easy to do with appropriate installation and detailing. In this method, an integral flanged window unit is installed with furred cladding. 

For more information contact ABTG here or visit the Foam Sheathing Committee or Polysio web pages.