We are going to re-insulate and re-side our 1947 home with new Roxul ComfortBatt, IsoClad 2 1/4" foam boards and some basic kaycan vinyl siding.

I have been looking at how to flash a window and have concluded that I'll need to remove an existing window, flash the frame and re-install it. All of the guides I have read use an air barrier that is not included in the foam board, meaning they can cover the window frame and cut the wrap in order to fold it into the frame and flash properly.

Like this:

enter image description here

What I am wondering is, what is the correct way to flash a window when using an air barrier that is stuck to the foam board insulation?

I was thinking of buying the smallest roll of House Wrap and just cutting a section out to cover up the width of the window, but if this is not necessary i'd rather not spend the time/money.

I can't find anything in the Isoclad manufacturer's website or the product manual.


  • Air barrier? perhaps you are referring to Water resistance barrier - WRB ? "air barrier that is stuck to the foam board insulation" Please explain.
    – Alaska Man
    Aug 30, 2017 at 18:13
  • @Alaskaman The air barrier in this case is the plastic layer on the foam board insulation. If you follow the link, you'll see that it is an "air barrier/non vapor barrier" insulation. So I think in this case its House Wrap, just stuck to the foam board. Aug 30, 2017 at 18:29
  • Are you insetting or outsetting your windows? (i.e. are they going to be on the inside with a ledge on the outside, or on the outside with a ledge on the inside?) Also, what's your wall stackup going to be? (i.e. which order are the various wall layers in) Aug 30, 2017 at 22:26
  • @ThreePhaseEel The walls will be: Drywall, furring, roxul 2x4, Isoclad 2 1/4 (with air barrier included), furring, siding. The window will be slightly inset as the isoclad will add about 1/4". We have about 2" of old wood siding underneath our current aluminum siding, so the thickness of the wall after adding the foam boards won't change much. Aug 31, 2017 at 10:06
  • @MatthewGoulart -- I take it that the air barrier layer in the isoclad will be on the outside? (I will admit that it is odd to hear of a rigid foam product that has an explicit air barrier layer, as the foam itself is normally a barrier) Aug 31, 2017 at 11:39

1 Answer 1


You got it. Just install a piece of housewrap over your window, going past the window, maybe 4"+. Then seal thw housewrap to your foamboard using housewrap tape, or flashing tape.

Then, refer back your drawing there and cut as necessary.

Remember, you can hide all bad cuts with the tape.

Alternately, just flash the entire window frame in wide flashing tape and walk away. A tiny bit more expensive, but much better job.

Good luck with your project.

  • 1
    I think it will be easier to just flash the entire window frame. Thanks for the reply! Aug 31, 2017 at 10:07

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