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Community and Q&A

What is the proper window detail for the “Perfect Wall” system?

user-879876 | Posted in Green Building Techniques on

I am building in Kansas City, MO and plan to use the “Perfect Wall” concept of 2×6 walls with Icynene and 1.5″ of rigid foam on the exterior. I have not seen agreement on how to handle the window and door flashings. For instance, should the window flanges go over the foam at the window openings or should you line the window openings with a 2×4 so the window can be mounted into a solid piece of wood? This would provide a more stable connection, but negates the benefit of the thermal bridging provided by the foam. Secondly, should the building wrap go on the exterior or interior side of the foam? How should it be connected right at the window? I am leaning towards using Zip Wall sheathing (which I wouldn’t normally use solely as the building wrap because I believe physics beats chemistry – overlap is better than tape) under the foam and then building wrap over the foam. I know it is a bit overkill, but each system by itself seems to be inadequate.

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  1. Expert Member
    ARMANDO COBO | | #1

    Martin wrote a good article here in the GBA, see:
    I like to apply the WRB on top of the sheathing, install the windows, then the rigid foam, then a ventilated screen and finally your cladding.
    I would install cellulose in the cavity walls insted of spray foam for the same reason that Physics beats Chemistry.

  2. GBA Editor
    Martin Holladay | | #2

    I've written three relevant articles on the topics you raise:

    ‘Innie’ Windows or ‘Outie’ Windows?

    Where Does the Housewrap Go?

    Nailing Window Flanges Through Foam

  3. davidmeiland | | #3

    I would install cellulose in the cavity walls insted of spray foam for the same reason that Physics beats Chemistry

    +1 on that. I see very little benefit to the foam, especially low-R-value open cell, with a lot of risk and cost if/when something gets wet.

  4. jbmoyer | | #4


    The wall assembly you are describing is not "perfect" for the following reasons:
    - It has zero moisture buffering capacity. Follow Armando's advice and use cellulose for cavity insulation.
    - Zip System sheathing is expensive, and relies on tape rather than gravity for moisture protection, AND the fact that the company won't reveal the perm rating is a little concerning. Use regular OSB, tape the seams for an air barrier, use housewrap/15# paper as your WRB, and back-ventilate your cladding.
    - And last, but certainly not least, you're using petrochemical foams where you don't need to. Save money, eliminate fastening/finishing issues, reduce chemical plastics, and use a double wall or a Larsen Truss.

  5. user-879876 | | #5

    Thank you for the feedback. There may already be articles out there that addresses this question, but here goes...
    Why do you all seem to prefer cellulose to open cell foam? It seems to me spraying a wet substance into a wall is just a bad idea in general. Surely the wall insulation will settle over time leaving gaps at the top (physics working against it). It won't be able to fill cavities like an expanding foam. I would think newspaper will mold if it gets wet. Open cell will seal the cavites better, is hydrophobic, and will allow moisture to dry out if it gets in. It doesn't have the R-value of closed cell, but you give that up for a more forgiving substance if water does get it. I would certainly pick cellulose over batt, but not over foam.

  6. Expert Member
    ARMANDO COBO | | #6

    Cellulose has a much lower global warming impact. If it is installed correctly, it should not settle. It can handle a small amount of moisture when humid and releases it when dry. It’s cheaper. It’s recycle material. I do however use open cell foam on the joist/truss rim since it is very difficult to seal any other way; and when I design conditioned attics. I should add that sealing detailng is always more important than the kind of insulation you are installing.
    In KC, you need 1 1/2” min. (2” is better) rigid foam on the outside of the wall assembly and 5.5” of cellulose (better) or open-cell foam. If you are going to do a ventilated attic, then you need R38 BI cellulose or BI fiberglass over the ceiling and with healed trusses or stick frame roof over a 12” pony (knee) wall. If you are going to do an unvented attic, you need 8” of open cell under the roof decking and 2” of rigid insulation on top of the roof decking.
    The rigid foam outside of the sheathing and on top of the roof decking for a conditioned attic is for thermal bridging and to keep condensation to build on the inside of the wall and roof sheathing. You also should install your wall and ceiling sheetrock with an Airtight Drywall Aproach or ADA.

  7. user-879876 | | #7

    I agree with the global warming impact concern, but from a strictly performance based criteria, I still like the foam.
    I agree 2" of exterior rigid foam would be even better, but I am concerned about the structural loss of trying to attach things (siding, shingles, trim, etc.) through 2 or more inches of foam. I could see siding and shingles flying off much easier in high winds (I am in tornado alley). Unless there is some kind of excellent new technique for attachments through this amount of foam, I don't feel comfortable beyond 1.5".

  8. Expert Member
    ARMANDO COBO | | #8

    I believe Hardi's installation manual is for 1" rigid foam. Over that you need to get an engineer's report on it. There are few fastner companies that manufacture long screws for 4" rigid insulation, 1X4 furring strip, 1/2" OSB and 1 1/2" into framing members, and maybe some of those New England guys can tell you who they are.
    As far as the cellulose performance, I've designed houses with taped OSB, WRB, and taped 1" foam plus cellulose and end up with less than 1ACH50 every time... again is all in the air sealing and detailing.

  9. GBA Editor
    Martin Holladay | | #9

    The engineering calculations to determine how to attach furring strips through thick foam have already been made, so I think your worries are baseless. Here's more information: Fastening Furring Strips to a Foam-Sheathed Wall.

  10. cascadian76 | | #10


    Your articles about window detailing are fantastic, but I've also wondered about using 2x4s around the window if the foam is 1.5 inches thick. Obviously, this wouldn't work for other thicknesses of foam, but it seems that if the foam is 1.5 inches thick, nailing 2x4s to the outside of the wall would be easier than building a plywood box. Is there a reason not to do this?


  11. GBA Editor
    Martin Holladay | | #11

    In my article, Nailing Window Flanges Through Foam, I explain two methods that work with 1 1/2-inch foam. The first method I call the "picture frame" method; that's the method you propose. I have used the method many times myself, and it works.

    The other method for 1 1/2-inch foam is the Joe Lstiburek method: just nail or screw the window flanges right through the foam into the framing behind the foam. That method works too.

  12. cascadian76 | | #12

    Thanks Martin, this site is a wonderful resource.

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