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Opinions on This Wall Assembly

will4 | Posted in General Questions on

I am currently working on a project in Zone 6 with a wall assembly I am unfamiliar with. The wall  is designed with 1/2” Zip sheathing as the interior vapor barrier taped with Siga, the green WRB facing the insulation at the interior of the wall. There is a 11.75” space filled with dense pack cellulose before contacting the exterior sheathing which is 5/8” Zip taped with its proprietary tape.

Is there the possibility of moisture getting trapped in this wall system due to the low permeability of the Zip panels. I am understanding they are around 2 perms.

I would opt for using Intello as the vapor barrier but that is not an option. Would like to get some thoughts from others on this assembly.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    You didn't mention your climate zone, and it does matter a bit. This wall system is probably safe due to the interior layer of Zip acting as a vapor retarder and the hygric buffering capacity of the cellulose, but it must be aggressively air-sealed to do its job properly. In warm and mixed climates, the wall will probably do fine. In cold climates there will be some risk of condensation against the backside of the exterior sheathing and this could take some time to dry out. Vented rain screen siding would be a very good idea. But why use two layers of structural sheathing? With the Zip on the inside, you could use a fiber wood product like Gutex on the exterior that is far more permeable than Zip. And, why use an expensive product like Zip on the inside? Standard OSB has the same permeability and it can also be taped, though you do have to work a bit harder to get it to stick reliably. It seems like Zip is an expensive upgrade for this application if the only goal is to have a good surface to tape to. The WRB capacity is wasted. Taped OSB on the inside and Gutex on the outside of a double wall system works in all climate zones.

    1. maine_tyler | | #4

      "1/2” Zip sheathing as the interior vapor barrier taped with Siga, the green WRB facing the insulation at the interior of the wall."

      It sounds like you couldn't even tape the green with the proposed set-up anyways. (If I'm reading that right, the green is against studs).

    2. Expert Member
      MALCOLM TAYLOR | | #10

      "Taped OSB on the inside and Gutex on the outside of a double wall system works in all climate zones".

      Except those like the PNW where the exterior sheathing needs to function as shear-walls.

      1. Expert Member
        Dana Dorsett | | #13

        >"Except those like the PNW where the exterior sheathing needs to function as shear-walls."

        Why does the structural shear paneling need to be on the exterior in the PNW (or anywhere else)?

        >"...1/2” Zip sheathing as the interior vapor barrier..."

        Half-inch OSB on the interior delivers about the same structural capacity as on the exterior, no? Or is it presumed that the interior side studwall is strictly non-structural?

        There are many ways to build a double-studwall, and to be sure the framing & interior sheathing need to be analyzed (and perhaps revised) for shear strength but it's not a guaranteed fail by any means.

        1. Expert Member
          MALCOLM TAYLOR | | #24


          Shear walls, or as our code terms them "Braced Wall panels" have to be continuous between the foundation and roof, and connected to both. I suppose you could get an engineer to detail and sign off on some hardware connections at the floor levels and roof that would allow you to place the OSB on the interior of the walls, but that's going to entail a lot of gymnastics - especially with a double stud wall where the interior one is set back from the rim joist, or if you had raised-heel trusses.

          There is also the problem of construction sequencing if you were going to completely sheath the interior of the walls, and not interrupt them at partitions. How would the electrical rough in work?

          1. AlexPoi | | #27

            I'm going to build my house that way but I think it only makes sense if the interior wall is the load bearing one. That way it's pretty easy to connect the rim joist to the first and second story with a strap tie on the outside. You can use some hurricane clips with a good shear resistance to connect the roof to your wall as well.

            And with this kind of wall, you need to add a service cavity on top of the sheating. Otherwise, I don't think the sequencing would work as you wrote and air sealing the sheating would be really painful.

            What I like about this wall though is the sequencing. First, you can focus on framing the building and waterproofing it as quickly as possible. Then you install the insulation, the sheating and air seal the building from the inside without having to worry about the weather or climbing on some scaffolding. You can even perform a blower door test and fix all the leaks you find from the inside. And when you're done you can rough in your service without having to worry about messing with your air barrier, insulation or structure.

            Seem much easier to me then taping the sheathing on the outside, stapling an house wrap and then installing 2 or 3 layers of insulation on top of it while you are 20 feet up in the air. Plus, rigid insulation isn't cheap.

          2. Expert Member
            MALCOLM TAYLOR | | #30


            Don't you mean it only make sense if the exterior wall is load-bearing?

  2. Expert Member
    Akos | | #2

    This is a really bad idea in cold climates.

    Walls without exterior rigid insulation for condensation control need a the warm side permeability to be lower than the cold side.

    Since 5/8" zip has lower permeability than 1/2" zip, you have essentially the opposite. At minimum you need a warm side vapor retarder (6 mil poly, membrain, intello etc.).

    This would still be a marginal assembly, OSB (ie zip) is still fairly low permeability and a thick wall like that will have a fair bit of moisture in the sheathing by spring. This will take a long time to dry due to the low permeability of the sheathing and can create mold risk.

    Thick dense packed walls work great in cold climate, you just have to follow a couple of rules.

    There must be a warm side vapor retarder.
    Sheathing needs to be highly permeable (CDX, gypsum sheathing, fiberboard or skip the sheathing completely).
    There has to be a rain screen behind the siding.

    Without the above items, the wall you are building could fail.

  3. Jon_R | | #3

    Conservatively comply with the recommendations in Table 2 A or B here. In your case, Zip inside and out doesn't.

    While following the guidelines, more vapor open is better.

  4. will4 | | #5

    The site resides in upper Washington state. Climate zone 6.
    Cold winters along with hot and dry summers in the valley where it is being built.
    The walls were all built off site ( on the east coast ) as panels and then shipped across the country to be set. I have been using different wall assemblies here and have not seen this detail before. I am also not accustomed to setting prefabricated wall systems. This is a first.
    Thanks for the feedback.

    1. maine_tyler | | #6

      Will, could you share the panel manufacturer?

      Just to say it, I don't think one could guarantee that you WILL have problems, but it carries greater risk. Do you think the panel manufacturer did some WUFI or similar on this system? Do they discuss climate zone usage?

      The zip on the outside may open up (vapor-wise) a bit compared to the zip on the inside due to RH differences at those layers (OSB 'smartness'), but likely not enough [edit: maybe not really at all, see below] to get the kind of perm ratio you really want for robustness.

      1. Expert Member
        Akos | | #7

        There has been a lot of study of double stud walls with OSB (ie ZIP) sheathing. North walls are generally on edge in zone 5 with controlled RH in the winter time and can have pretty high MC content if the occupants don't keep RH low.

        Cellulose helps a bit with moisture redistribution and mold control but still not a robust assembly. It relies on getting all the details right (low air leakage) and occupant behavior (low winter time interior RH).

        At the current price delta between OSB and CDX, I would not build this wall with OSB (or Zip) on the outside. You can use 5/8" zip on the interior as its permeability is pretty close to the vapor retarder definition in our code.

      2. Expert Member
        MALCOLM TAYLOR | | #8


        Does Zip open up the way standard OSB does? My understanding is that the wax coating 0n Zip which allows it to also act as an WRB, means it has a fixed permeance of 2 to 3. OSB can change for 2 to 12 when damp.

        1. Jon_R | | #11

          Or maybe Zip isn't reliably above 1 perm, wet or dry.

          1. Expert Member
            MALCOLM TAYLOR | | #14


            I took the perms from the list Martin published on GBA a couple of years ago. When you start looking around there seems to be claims that put it all over the place. This one has it at 12 to 16:

            What I can't find is what Huber says the permeance is.

          2. maine_tyler | | #15

            That 12 to 16 is for the coating alone.

            Huber seems to cite very vaguely (across a few different products) this:

            which I kind of take to mean ONLY the substrate in this case.

          3. Jon_R | | #20

            So Huber's irresponsibly vague to the point of being useless document doesn't contradict the 1 perm wet&dry measured by Dupont (Huber says 1 perm minimum and then lower this to account for the facer).

            Conclusions: "don't equate Zip and typical OSB" and "2-3 perms is definitely too high for > 7/16 Zip".

        2. maine_tyler | | #12

          Good question/point. It seems I'm making erroneous assumptions. I figured since the zip coating is (somewhat significantly) higher perms than the osb substrate, it would act similarly to osb. But even the substrate for zip seems to differ from most other basic osb's, from what I'm reading.

          Either way it's not reliably high perm and, I agree, probably risky. I'm not endorsing the wall.
          To some degree I'm just trying to think of what hidden mechanisms may be helping such walls despite our thinking them risky, since double stud walls seem to often reject that analysis in the real-world*. Probably has more to do with the ability to dry in the spring once it warms up. Rainscreen gaps being very helpful for that. And most probably don't use Zip. Not sure if there's much data on double stud with specifically zip out there?

          *at least certain types of double-stud walls deemed risky by modeling. I have no idea if this specific stack up has case studies showing it to buck such analysis.

  5. Expert Member


    Before installing these panels I'd get a disclaimer signed by the owner limiting your liability for problems that may arise due to the risky stack-up of components they contain. If they are as you have described, it's a head-scratcher why anyone would manufacture something with such a p0or vapour profile.

  6. maine_tyler | | #16

    Riddle me this:
    How is this fundamentally different (moisture management wise) from Joe L's ideal double stud wall?
    Specifically the Zip? Cus he 'allows' exterior osb (not ideal though, I agree).

    1. Expert Member
      Akos | | #17

      Joe's ideal wall has painted drywall (5 perm)+ middle OSB (2 perm), so the overall permeability is 1.4. This is less than the permeability of the exterior OSB sheathing, thus the wall should work.

      1. maine_tyler | | #18

        I assume on Will's project there would also be painted drywall. So I still don't see how it's different. Not trying to be difficult, just trying to wrap my head around the differences.

        You point out that he specs interior zip which is thinner than the exterior, and that's a fair point, but my point is that we're dealing with minutia, so far as I can tell. Could one just add vapor retarding paint to make up for this?

        I don't think anyone who has read significantly on cold climate double stud walls, including Joe's ideal one, would come away thinking exterior osb is indeed 'ideal.' But nonetheless Joe appears to allow for it is his detail.

        Personally, I wouldn't build it because I'm a bit risk averse. I built a double wall for myself and used intello on the exterior of the interior wall with plywood as the structural exterior sheathing followed by a healthy rainscreen gap.

        1. creativedestruction | | #19

          "How is this fundamentally different (moisture management wise) from Joe L's ideal double stud wall?"

          Air barrier is more protected in Joe's mid-wall location, and it nearly always stays above dewpoint there, whether you're air conditioning or heating. Those two factors are crucial to the drying patterns you would see in the wall. I've run similar stackups through WUFI -- helps to see the effect of what Joe calls the 'vapor throttle'.

        2. Expert Member
          Akos | | #21

          I guess I assumed the interior finish will be T&G or similar, otherwise zip on the interior makes no sense. Even for pre-fab there are cheaper alternatives.

          You definately can install drywall or coat with barrier paint. The 5/8 exterior OSB is still questionable.

          According to this:

          5/8 OSB is around 1 perm, not what you want on the outside of a thick wall. 7/16" is better, they even say it allows for drying of green wood, which would be similar to spring time drying of accumulated moisture.

          1. Expert Member
            MALCOLM TAYLOR | | #23


            OSB can open up to 12 perms when damp.

            This article contains a list of materials and their permeance:

          2. Expert Member
            Akos | | #28

            If you look at the chart here:

            Depending on the type of OSB, it won't start to open up until saturated. I think this is the reason for increased mold risk. The thicker the osb, the more moisture it takes for this to start increasing the risk.

            With CDX you can get drying staring even at pretty low cavity RH, combined with a rain screen, it is a much more robust assembly.

            With the high lumber costs, currently CDX is ~15% more than OSB. For a high performance build, it is a no brainer upgrade.

    2. Expert Member
      MALCOLM TAYLOR | | #22


      This is the commentary from the BSC site:
      "Drying: Cellulose and fiberglass insulation allow drying to occur relatively easily, so drying is controlled by other more vapor impermeable enclosure components such as the plywood or OSB sheathing on the exterior of the interior frame wall. Installing a vapour barrier on both sides will seal any moisture into the stud space, resulting in low drying potential, and possibly resulting in moisture-related durability risks."

      That's the fundamental difference. Zip is a vapour-barrier. Plywood, and OSB are variable perm vapour-retarders. Joe's wall allows drying in both directions. The Zip wall the OP is installing appears to have very little drying potential in either.

  7. maine_tyler | | #25

    Yeah I wish the info on both osb perm ratings and especially zip was a little more clear. I think part of the problem is that it depends on the specific manufacturer and perhaps even batch to a lesser degree (with osb). Because I'm reading different numbers for that too. The problem with zip might be they don't want us to know.

    Something I read in passing leads me to believe it's the binder in zip (not so much the facer) that reduces permeance. But why it wouldn't act as a smart retarder as much as standard osb, who knows (maybe for the same reason, or maybe the facer?).

    I don't see the location of the interior zip retarder layer as issue for cz6 but maybe I'm wrong.

    1. Expert Member
      BILL WICHERS | | #26

      I've always thought the Zip "green stuff" on the face is the vapor barrier part. I've never really looked into it though. Their OSB IS generally regarded as "something better" compared to regular OSB, which would seem to imply they are using a binder that holds up better in the presence of moisture. This is a good thing in some ways, but it's entirely possible that whatever they are using also reduces the vapor permeance of the OSB part of their product too. Physics and engineering are all about tradeoffs, and often times a gain in one area is a loss in another.

      I doubt Huber doesn't want us to know though -- they pitch their product to architects and engineers, and those are people who don't like maybes. I know that I, as an engineer, get a bit annoyed with what I called "partially specified products", which are products that have things missing in their datasheets that are actually important parameters. Luckily, this kind of thing is unusual. It's more likely Huber just hasn't thought to publish that info because they don't think of it as a primary parameter for their product, or they do know people would like to know that info, but they don't publish it because they don't want to GUARANTEE that info -- and if that's the case, it's probably because they don't control and/or test for that particular parameter in their manufacturing process. You don't want to specify something that you don't control, and can't guarantee.

      I agree that wall with Zip on both sides isn't a good idea. I think some people thing "a little mositure proofing is good, so more must be better", but that's not always the case. Joe also has been known to say that such assemblies would be great if we could ensure water never gets in, but since we can't do that -- water always gets in eventually -- it's better to make sure water always has a way to get OUT.


      1. maine_tyler | | #29

        The 'read in passing' about the binder turns out to be the article by 475 Malcolm linked to in post #14, which was a bit of speculation by 475.

        If anyone can find solid technical documentation of zip's wet and dry cup testing, it'd be interesting.

        Zip wants to sell their product.
        If the permeance characteristics of the product isn't it's strong (selling) point, it's not surprising they wouldn't make that info readily available (unfortunate, but not surprising). That seems to be the simple explanation to me.
        I agree with Jon that there is seemingly some intentional obfuscation. And other than us whining on a forum, how's it hurt them?

        The green wrb facer is listed as 12-16 perms as mentioned above, so seemingly not the component that is reducing it's overall perm rating (unless it's the adhesive interface between the facer and the osb or something).

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