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furring strips vs osb exterior of rigid foam

carsonb | Posted in General Questions on

my builder would like to put a layer of osb on the exterior of rigid foam instead of using the typical furring strips I see.  The idea was this would be easier to make flat and install windows/etc. onto which would save more on labor than the cost of the extra layer of OSB.  So the assembly is drywall -> mineral wool insulated 2×4 framing -> ZIP sheathing -> 2″ polyiso -> OSB -> homeslicker rainscreen -> cedar siding.  Any potential issues with this?  Would all seams on both layers of OSB need to be taped as well as the foam, or only on the first ZIP sheathing layer?  This seems like it may run into the same issues found with SIPs as it is effectively a site built SIP at this point, but for walls maybe not all that bad.  
thanks for any help

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  1. ohioandy | | #1

    Carson, at today's OSB prices many of the old calculations have to be rethought.

    If you builder wants to save labor by mounting doors and windows to an exterior OSB layer, he's effectively moved the main weather barrier to that layer, leaving the underlying ZIP to function only as a very expensive air barrier.

    And with exterior foam at 2" thickness, he'll still have to box out all the penetrations with ripped-to-width 2x material to allow structural hanging of the windows.

    As for taping the layers, the wisdom I've read here is to choose one layer and make sure it's done robustly and properly; taping other layers is great if you can afford it but has diminishing returns.

    1. carsonb | | #2

      thanks Andy, I was skeptical there would be any real labor savings there. In this scenario, would the second layer of OSB need its own WRB? As for the zip being an expensive air barrier, it seems to be a well thought out and effective one though. Would you recommend just using regular OSB and taping it then? I have read most tapes do not stick as well to OSB, and then we are getting into primers and expensive tapes. Or taping the foam? But that seems more risk prone that the sheathing.

      On the other hand, if it were just one layer of OSB, 2" of foam, and then furring strips, what would be the recommendation there? Would the foam itself become the WRB and just tape the OSB really well?

      1. ohioandy | | #5

        Yes, ZIP is a great air barrier; it's just that it's ALSO a weather barrier, and it's all-in-one characteristic is what justifies its high price. If you have other barriers to the exterior, it becomes harder to justify the ZIP. My 2 cents.

        And here's another 2 cent solution: yes, you can tape plain OSB as an air barrier. I have found ZIP tape to be great for this, without need for primer. No warranty and no proof of concept other than the "backyard tape test" on this site, and my own experience working with these acrylic tapes.

        Have you considered using ZIP-R? Seems like your builder might find this to be an ideal solution. Your plan to use cedar (shingles?) would be well suited to this route.

        To your question, yes, the foam can be taped as it's own WRB, but you gotta have supreme confidence in your tape. Alternately, you can overlay housewrap (between the foam and the firring strips) although this has its awkwardness. Or even put the housewrap behind the foam.

        There's no right answer. To be sure, there are plenty of wrong answers! Your decision is based on your relationship with cost and risk, and what your builder is willing to do.

  2. Expert Member
    Akos | | #3

    Since exterior rigid has become code here, the building assemblies have standardized to the following.

    housewrap (skip for ZIP)
    rigid insulation
    furring strips (skip for vinyl)

    This means you can use standard flanged windows nailed to the sheathing, keeping the rest of the construction pretty standard.

    I prefer to go with CDX as it hold nails much better and is more durable in the long run.

    It never hurts to tape the seams of the foam, but your WRB should be at the same plane as your windows. This makes the water management details much more straight forward.

    1. carsonb | | #4

      thanks Akos, so then this would be an "inbetweenie" window, with the foam and siding well outside of the window? Is there a potential moisture issue with using the extra sheathing layer + homeslicker as the builder wanted vs just furring strips? One complication- the siding is cedar shingles. So I would imagine the horizontal furring strips would be more of a pain.

      1. Expert Member
        Akos | | #6

        Yup, inbetweenie window. You can get extensions for most to add a bit of extra depth on the outside but with 2" of foam it you'll have to add trim.

        There is nothing wrong with the extra layer of sheathing, but in that case I would make that the WRB surface and bring your windows out to that.

        For the complexity of doing all that, might as well just go with R9 Zip and call it a day.

        1. carsonb | | #7

          I'll inquire about zipr again, but it was my understanding that there were structural issues with the 2" R12 zipr and locally here I got quotes much higher than I've seen the cost listed in other areas. The supplier here didn't even know what it was when I asked, I seemed to be the first.

          1. JC72 | | #9

            +1 for ZIP-R but do you really need 2" of foam or is that more of a want?

            With the thicker foam the sheathing loses the ability to provide sufficient racking resistance so mechanical bracing/strapping within the framing itself is required. Not a big deal IMO. Especially if you're not in a high wind, hurricane, or earthquake zone. Huber (Manufacturer of ZIP) has all the technical requirements,.

            Resident blogger Carl Seville used ZIP-R + Rainslicker on his Atlanta home

          2. carsonb | | #11

            2” was a want to get towards a “pretty good house”. Honestly with the cost of lumber doubling though, just upiing the amount of PV is looking very tempting.... the average lumber price for a new build has gone up $16k, and that buys a lot of pv panels.

  3. AP1210 | | #8

    Wouldn't it be simpler to use Zip-R then your rainslicker? Drywall>Insulation>Zip-R (with 2" foam)>Rainslicker>cladding.
    Maybe I am missing something, but that seems the easiest, least labor intensive method.

    1. carsonb | | #10

      My understanding was the extra layer of osb was required for shear strength vs the 2” zipr. So it would be wall cavity-osb-zipr-rainslicker. Zipr may be easier than separate osb and foam and that was my suggestion, but that’s up to the builder. Homeslicker is also fairly expensive at roughly $1 sqft and now osb has reach that in this crazy climate so its a question if just the foam with furring strips would be worth $2sqft of labor.

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