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Exterior window jamb treatment for rigid foam?

welivehonestly | Posted in General Questions on

I am stumped. We are using 2-3″ of GPS exterior insulation on our addition/renovation. 

So if we put the WRB on sheathing below the GPS, and likewise install the windows with their flanges at the WRB, what exterior jamb/trim should be used if we end up using 3″ of GPS?

Layers atop the WRB/windows would be: 2-3″ GPS, 1/2″ furring strips, and a vertical steel siding which is 9/16″ thick.

The window manufacturer we are considering (Duxton) has a 3.5″ exterior trim. So if we do 2″ GPS, we’re at 3-1/16″ altogether with the furring strips and siding, and if we do 3″ GPS, so the 3.5″ trim should be OK, but if we’re at 4-1/16″ altogether. What do people do in this case? Build out the window frame to bring the flange out an extra inch? I know the thermal buck is supposed to be a solution to this, too, but I don’t yet understand how that would interact with the windows.


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hey Patrick,

    You are right that your windows need to be flashed to the WRB. The easiest way to do this, though not the only way, is to install the windows and the WRB in the same plane.

    That's a straightforward "innie" window and in your situation it will need some type of exterior extension jamb. Though there are some exterior extension jamb products available, most builders make their own and there are many ways to do that. Here are a few articles I would suggest to help you wrap you head around all of this:

    Where does the Housewrap Go?
    Innie Windows or Outie Windows?
    Installing Windows in Thick Walls

  2. Seabornman | | #2

    Or you can build your frame out to make an "outie" install. Attached pic shows that type. Underneath the Vycor is a full 2x4.

    1. GBA Editor
      Brian Pontolilo | | #4

      That's nice work Joel! Thanks for sharing.

  3. Expert Member
    AKOS TOTH | | #3

    I've made exterior extension out of some PVC trim. These mounted to the windows behind. Make sure the bottom trim is sloped to drain and that you don't block any of the vent holes on the window behind. The head flashing above the window extends past the trim/extension.

    One thing to watch for is thermal expansion especially with dark colors. Don't pin the jamb extension near the corners to structure so they can float a bit otherwise the corners will pull apart.

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