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Window flashing when using SPF instead of rigid foam

TimL_in_ND | Posted in General Questions on

The retrofit I’m planning will add exterior insulation in the form of SPF. Will add vertical furring strips suspended 1″ or more from the existing siding/sheathing for mounting the new siding. For windows, will use the window buck approach and ‘outie’ window mounting style unless another style has advantages for ease of installation, durability etc. ‘outie’ would match the existing appearance, could change for the right reason.

I’ve looked at numerous articles for flashing a window when installed in a wall with exterior rigid foam board. I’ve not come across an article that describes how to adapt these techniques for the SPF scenario.

The use of SPF that I want to use was described in #4 of “Six Proven Ways to Build Energy Smart Walls”
I didn’t see window flashing described in the above article for the above wall construction approach.

One option I see is to use 8″ to 12″ of rigid foam board on all 4 sides of the window opening and follow the provide flashing guidelines. The SPF would follow installation of the foam boards and would make a mechanical, continuous water barrier. Is there another alternative?



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  1. GBA Editor
    Martin Holladay | | #1

    The answer depends on a few variables, including:

    1. Are you using closed-cell spray foam or open-cell spray foam? (If you are using closed-cell spray foam, you don't have to worry much about water getting through the foam.)

    2. What layer is your water-resistive barrier (WRB)?

    As long as your window sill sheds water to the exterior plane of your spray foam, I imagine that everything will be fine. Here are links to two articles that discuss the technique -- you might get some clues from the photos:

    Deep Energy Makeover: One Step At A Time

    Brand New Appearance and Performance for An Older Duplex

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