GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Polyiso water absorption: Should the WRB be inside or outside of exterior foam?

airfix | Posted in Green Building Techniques on

I’m planning exterior foil faced polyiso with a rain screen between the foam and the siding.

My question concerns water absorption of polyiso. I read that polyiso absorbs water quickly and with this in mind does it mean that putting the wrb outside of the polyiso is a more favorable place to put it than putting it between the polyiso and the sheathing.

I assume in foil faced polyiso it only absorbs water at its exposed edges.  If those edges are all taped then it doesn’t matter.

I’m asking because I really want to use “outtie” windows (typically wrb outside of foam) but there are two other exterior finishes other than siding on my home such as EIFS (using EPS foam) & rock veneer that require my wrb to be inside  of my foam rather than the outside

Second question is it possible to flash a window as an outie window with the wrb on the inside face of the foam.  If so can someone point me to a detail.



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. Expert Member
    Dana Dorsett | | #1

    Absorbs water, yes, "...polyiso absorbs water quickly..." depends on what you mean by "quickly".

    Polyiso is NOT a sponge- nowhere near as wicking or absorptive as say, cellulose, or even fiberglass. But unlike EPS it is at least somewhat hygroscopic.

    Fully submerged in a pond polyiso will take on quite a bit of water weight over several days. If GLASS faced polyiso is fully submerged for 24 hours it takes on about 3% by volume, but foil faced fully summerged takes only 0.75 % Fully immersed for a couple of hours and allowed to drain for 10 minutes the glass faced goods take on less than 1.5% by volume, foil faced about 0.2%. See Figure 4 on page 4:

    Instead of complete immersion, spraying down one side of vertical sheet of foil faced polyiso with a hose for an hour or two it won't take up very much at all, probably less than 0.1 percent even if keeping the facer completely wet, and given time the tiny amount it takes up will dry. ( If your exterior walls are getting that much water you're probably in a boat below the water line. :-) )

    In a wall sheathing application, it's a complete non-issue. But in a flooded basement, it's a real issue, and even resting the cut edge directly on a potentially damp slab would be inadvisable (but 1/4" off the slab is fine if it doesn't flood.)

  2. airfix | | #2


    Thanks, good to know it is somewhat resilient to water in the short term. I was planning for the basement Polyiso to be above the concrete. The hope is that we've designed our house drainage system that the basement will never flood. We're not in a flood plane so that's a bonus and on a 20% grade so the water should flow around/under our home.

    Does anybody have tips or details on if I can do an "outie" window with the WRB on the inside of the foam rather than the outside as is typical with outie windows? It would help because of other exterior finish choices.


  3. brendanalbano | | #3

    The example of outie windows with mineral wool (figure 8) in this article should give you a starting point for your desired window/wrb configuration:

  4. GBA Editor
    Martin Holladay | | #4

    I suggest that you read these two articles:

    "Where Does the Housewrap Go?"

    "Installing Windows In a Foam-Sheathed Wall"

    This second article discusses ways to flash outie windows.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |