GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Need some advice on flashing a window in relation to the housewrap and exterior foam

scadjacket | Posted in General Questions on

Need some advice on flashing a window in relation to the house wrap and exterior foam.

Purchased a house that was a foreclosure and had been empty for several years. Just started the remodel and when replacing a rotted window we found termite damage. So we took the wall down to the studs and started over.

New OSB sheathing, Tyvek, new construction window flashed with a slopped sill and tape flashing. Basically followed the steps for an ‘innie” window.

I was about to start to install the siding when someone made the suggestion of installing 1” exterior foam on the outside of the sheathing, then using a rain screen behind the siding.

I went and purchased the rigid foam but now I can’t figure out exactly how to flash the window?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    If you try to switch horses in midstream (as you are doing in this case, by deciding to add rigid foam on the exterior of your walls after your new windows are already installed), things can get a little tricky.

    That said, you should be able to come up with details that work. You will probably need metal flashing at the sill, to cover the rigid foam and the furring strips and to conduct the water from the sill area to the exterior of your siding. You will also need exterior jamb extensions and head extensions; these can be made of cellular PVC or a durable species of wood.

    For more information on this topic, see Installing Windows In a Foam-Sheathed Wall.

    There is one other topic you didn't ask about, but it's worth mentioning: 1 inch of exterior rigid foam won't work in all climates. If you live in a cold climate, you'll need thicker rigid foam. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. scadjacket | | #2

    Thanks Martin, I appreciate you taking the time to respond.

    When I got your response I hadn't fully finished taping the top and sides, I had only placed in the window with a few screws to start leveling.

    So I pulled the window back out and followed the instructions from Dupont on installing a bump-out window.

    Now I at least have a window on the same plane as the rigid foam.

    In this case, how would I handle flashing the window to make sure water flows to the outside plane of the rigid foam?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "How would I handle flashing the window to make sure water flows to the outside plane of the rigid foam?"

    A. I'm not sure of what you did, but the usual answer to your question is the following: "By installing flashing on the sill of the rough opening before the window is installed."

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |