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Community and Q&A

Window frame insulation

user-5538665 | Posted in Green Building Techniques on

My windows are installed in the middle of a 14″ window buck. I am planning on using rigid insulation on the exterior side of my frame. Is there any value to do the same to the interior side? I was not planning on it, but thought I would see what you all say.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Drew.

    I'd have to see a drawing of your assembly to fathom a guess. But I will offer this: I spoke with an architect recently--and hope to publish the detail on GBA once we have the chance to get some photos--who is using ZIP R to make her window bucks so that they are thermally broken from the rough opening framing. Seems like a good idea and creates a water resistant and compatible surface for flashing tapes.

  2. Expert Member
    Michael Maines | | #2

    I second the idea of Zip-R on the exterior. In fact I think it may be the best use for Zip-R.

    As for insulating the interior of the buck, it depends on your assembly. If your assembly is thermally broken except for a 1/2" to 3/4" buck, with exterior-insulated bucks and innie windows it's probably not worth the effort--the isotherms at the inside of the window won't change. But if you have thermal bridges that are bypassing the window frame, it might be worth the effort.

  3. Expert Member


    I'm wondering how practically you would do it? Once the interior casing is installed, a typical vinyl window has maybe 3/4" of frame showing. Adding any appreciable amount of foam behind the casing would reduce that to zero.

    1. Expert Member
      Michael Maines | | #5

      Malcolm, European tilt/turn windows, which I assume (perhaps incorrectly) Drew is using if he has 14" walls, have big, beefy frames that can (and should) be "over-insulated," meaning covered with insulation on the exterior.

      At the interior there is usually only 1/2" to 5/8" of space between the outside edge of the frame and the window hinges, which doesn't allow space for insulating the frame itself, but if the window buck has a second, inner buck (usually 2x material) that leaves space to insulate the buck itself.

      1. Expert Member
        MALCOLM TAYLOR | | #7


        I see that from the pic Drew has posted. That's a nice detail.

  4. user-5538665 | | #4

    I like the idea zip r for other bucks I do. Attached is an image of the first window I have put in. 23/32 ply with 2” XPS window buck. The intello is on the outside of the inside wall and then come in between the studs and buck. It will be taped to the frame. This is a retrofit Larsen truss type construction. Mento will be on the outside of the outside wall and taped to the outside of the frame.

    1. Expert Member
      Michael Maines | | #6

      Drew, I would guess that you would save a little money in energy costs, depending on your location. If you are using new XPS you will never repay the carbon debt, though--the blowing agents in XPS are potent global warming agents.

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