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Community and Q&A

Rigid foam and windows on a new house

Christa Campbell | Posted in General Questions on

We made the decision to use 1.5” exterior rigid foam after our new house walls were framed so we don’t have the option of expanding our rough window openings and building plywood frames to accommodate our ‘outie’ windows, as described in your article on installing windows over exterior rigid foam. We plan to install vertical cedar siding, and you advised us to use horizontal furring strips for attaching the siding. We are assuming that we can use a ‘retro-fit’ method for installing the rigid foam. There are a few details that we can’t seem to find regarding fastening the windows to the frame and fastening furring strips to window and door framing. Some retrofit diagrams show the rigid foam butting up to 2x framing that has been added around each window rough opening. Other diagrams show the rigid foam extending all the way to the rough opening with the window flange installed and fastened through the foam into the wall framing. Which method do you recommend?

For either of these two methods, we are assuming that we create a 1x blocking/furring frame around the window and fasten our window framing to that. We are also assuming that we install the house wrap over the rigid foam and use current methods for flashing around these windows. Are these assumptions correct?

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  1. GBA Editor
    Martin Holladay | | #1

    You can either nail the window flanges through the 1.5 inch foam, or build "picture frames" of 1.5-inch-thick lumber around the window rough opening, and butt the foam to the picture frame. Either method will work.

    Joe Lstiburek has had success nailing window flanges through 1.5-inch-thick foam. Here is a link to an article that discusses his technique: Nailing Window Flanges Through Foam.

    Regardless of which method you choose, it is essential that the rough opening be flashed before the window is installed. The rough sill needs to direct any leaks to the exterior of the wall. The rough jambs need to be flashed so that any water that dribbles down the rough jambs is directed to the sill pan, and then out. The two lower corners are the most vulnerable corners of the rough opening.

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