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Insulation: exterior rigid foam vs. Zip R sheathing

Jason Dennis | Posted in General Questions on

I understand the concept of using exterior rigid foam to increase R-value and decrease thermal bridging. I also understand that you have to use a foam thick enough to keep the exterior wall sheathing warm (above the dew point) to avoid condensation on the sheathing, and that the necessary thickness is based on your climate zone.

What I’m curious about is the difference in exterior rigid foam used OUTSIDE the wall sheathing and a product like ZIP-R sheathing, which has the foam between the sheathing and stud. I’ve seen several people state that they do not like ZIP-R because it “puts the foam in the wrong place.”

It seems to me that both methods would decrease thermal bridging and increase total R-value, but obviously ZIP-R would not keep the sheathing warm. So my questions…

1. Other than avoiding condensation on the sheathing when using rigid foam on the outside of the sheathing, is there another purpose in keeping the sheathing warm?

2. Is there a situation where ZIP-R would make more sense than OSB with exterior rigid foam on the outside?

3. If using OSB with rigid foam on the outside of the sheathing, is a WRB like Tyvek still used against the OSB or the foam? Or is the foam a WRB?

4. Assuming a typical 2×6 stud wall with R23 minieral wool batts in the cavities, what would be the best sheathing and rigid foam method and product for the exterior side of the wall in CZ3 (warm humid)?

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Replies

  1. Expert Member
    Armando Cobo | | #1

    I prefer to specify taped (with approved tape) plwd/osb and rigid foam on the outside. IMO, even though Zip-R has an ICC-ES report on structural, and you must follow installation instructions, I like to err on the side of caution for installation. In many areas, the builder and labor pool is uneducated about installation guides, and the subs want to get in and out in the least amount of time, so installation quality is something of a hit and miss. Having the taped plwd and taped rigid foam, with staggered seams, help cover the over-driven nails, and gives me a better chance of an effective moisture barrier.
    I don't see anyone cleaning the sheathing or rolling the tape on the regular bases, so most tapes are applied on dirty sheathing, with cracks, twists and bends that facilitate water leakage.
    I also feel that the nailing patterns are seldom followed to the tee, so I feel better on the structural issue, having the plwd nailed to the studs. I recognize that it could be a false sense of security since is all installation quality, but hey, that's reality. If all installations were perfect, I don't think it should be a difference on methods, but that's not our reality, unfortunately.
    Another issue is cost. In our market, for the same or less money you can buy 4x8 OSB + rigid foam, and with double the taping, gives me a chance of less moisture problems.
    One thing I have to say about Huber is their tapes are the best in the business.

  2. Expert Member
    Armando Cobo | | #2

    To continue answering your questions, I do specify Tyvek on top of the rigid foam, nailed with cap nails, because in our market, often takes a month or two to get a cladding installer, so protecting the foam from the sun is important.
    I happen to like 1" polyiso, even though in CZ3 like you, we're required to install R3. For cavity insulation, I like dense-packed cellulose. No voids, no gaps, no fitting, etc., and less cost than mineral wool.
    As you can see, installation education, quality and labor supply needs to be taken in consideration to the product to be specified.

    1. Jason Dennis | | #4

      Armando,

      Thank you for the insight and your excellent point on the structural issue. It’s very true that typically any product’s performance is only as good as its installation.

  3. Walter Ahlgrim | | #3

    In zone 3 no wall will have dew point problem regardless of how you build it.

    I am not sure insulating beyond what will fit inside a 2x6 wall makes economic sense in zone 3. Run a BEopt model to be sure. Adding the foam layer drives up cost and difficulty.

    I am not sold on the ZipR product for structural reasons. I do not care how many nails you use you cannot make a solid connection by nailing thru foam. If your home is in a hurricane, tornado or seismic zone I would avoid ZipR. I do like the regular Zip system covered with foam.

    Walta

    1. Jason Dennis | | #5

      Walter,

      I think you’re right about sheathing in CZ3. I imagine you could use any thickness of exterior foam that you want safely. My intention for using foam would just be to break the thermal bridge.

      On your comment about covering regular Zip with foam...I had wondered about this. So you tape or liquid flash the Zip like normal first and then foam on top of that? Any issues having the WRB underneath the foam?

      1. AlexPoi | | #6

        Putting the wrb under the foam makes lot of sense because it's better protected from physical damage and from extreme heat. The cons is that it's harder to integrate your wrb with outie windows. Innie are easy as they are in the same plane as your wrb.

  4. Zdesign | | #7

    For the little bit of extra money, tape your inner zip layer, and then tape your foam layer. Double water barrier for extra piece of mind.

  5. Walter Ahlgrim | | #8

    As a matter of fact I did build my house with regular Zip, 1 inch foam covered with vinyl siding in CZ4.

    On Zips website you will find drawing for zip + foam window flashing.

    I do not see a problem with foam on the outside.

    Walta

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