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Does exterior rigid foam require plywood box around window framing?

Justin_DeSilva | Posted in General Questions on

We’re renovating a 1950s home in Massachusetts. The home is constructed of 2X4 framing with pine sheathing wrapped in tar paper and sided with cedar shingles. I’d like to increase the r-value of the home by replacing the cedar shingles and tar paper with a peel and stick membrane, 2″ pink rigid foam, furring strips and HardiePlank lap siding. Harvey slimline windows will also be installed.
I understand the thickness of the wall will grow to about 7-8″ deep. When creating window framing details I’m thinking we should position the windows towards the center of the wall. Should the outer perimeter of the window jamb be framed with plywood to seal and provide a firm area for the window to sit?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Justin,
    There are several different ways to install windows in a wall with 2 inches of exterior rigid foam. All of the various approaches are explained in this article: "Installing Windows In a Foam-Sheathed Wall."

    By the way, green builders usually avoid XPS, since XPS is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see "Choosing Rigid Foam."

  2. Justin_DeSilva | | #2

    Thanks Martin,
    I read "Installing Windows In a Foam-Sheathed Wall" and several other articles. The article with the most detailed illustrations was Building Science Corporation's "BSI-085: Windows Can Be A Pain*—Continuous Insulation and Punched Openings".
    https://buildingscience.com/documents/insights/bsi-085-windows-can-be-a-pain

    Interesting point regarding the manufacturing impact of XPS foam. Deciding between rigid insulation is more complicated then I expected. I assembled a simple table of rigid insulation, R value and price per sq/ft. I see Polyiso outperforms EPS, XPS and mineral wool. I read mineral wool may be the most environmentally friendly but it's R value isn't very high and it's weight is heavy, especially at 4" thick.

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