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Community and Q&A

Interior Vapor Retarder with Continuous Rigid Foam

camdijion | Posted in Energy Efficiency and Durability on

Hello all,

I am building a home in Climate zone 6. On the exterior we used ZipR panels (1 inch foam) and taped seams. Interior will be Rockwool Comfortbatt. Whether or not it matters I’m not sure but the framing is 2×6 24 on center.  I have read a lot about smart vapor retarders, but don’t believe one is needed as I am vapor impermeable on the outside? I also wonder about the effectiveness of drying to the inside utilizing these diffusion changing membranes if I am using 5/8 drywall, and several layers of paint. Furthermore, I pay attention to humidity inside the home; (use dehumidifier in summer and humidifier in winter). So besides perhaps trying to gain another level of air tightness, (I also intended to pay attention to sealing drywall penetrations) is there any point in using one of these products?

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  1. dennis_vab | | #1

    In climate zone 6 as well. Bordering Utica. Will be finishing my home this summer. You don’t have enough rigid continuous insulation to waive using a class ii vapor retarder. You would need r-11.25 in order to use a class iii retarder (latex paint). The most cost effective class ii vapor retarder would probably be Certainteed Membrain.

    1. camdijion | | #5

      I am familiar with the product, maybe I am overthinking it, but if the house is humidity controlled, if CertainTeed membrane fluctuates between 1-35 perms, and 5/8 drywall painted with two coats of paint should have a perm rating of maybe 20? I wonder how much diffusion would actually occur if I am vigilant with an airtightness of the drywall. But either way, a fairly low cost insurance policy in the grand scheme of things

  2. MartinHolladay | | #2

    Your 1-inch Zip R sheathing has an R-value of R-3.6. But in your climate zone, if you want to install exterior rigid foam sheathing on a house with 2x6 framing filled with insulation, the minimum R-value for that continuous exterior foam sheathing is R-11.25. So your Zip-R is too thin. You made a mistake, in my opinion.

    For more information on this issue, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    The latest version of the International Residential Code, which may or may not apply in your area, allows people in your position to use an interior smart vapor retarder to lessen the risk that arises from too-thin exterior foam. For more information, see "Building Codes Update Vapor Retarder Requirements."

    1. matt2021 | | #3

      Dear Martin, without questioning any of the specifics in your comment to the OP, I am just puzzled by the assumption that ZIP R sheathing (which I am also planning to use on a project of mine) is “external” insulation. Being on the conditioned side of a home, isn’t it in fact internal?

      1. MartinHolladay | | #9

        The problem with Zip R sheathing is the same as the problem that comes from a continuous layer of exterior rigid foam: in either case, the foam layer prevents outward drying.

        With Zip R sheathing, it's possible for interior moisture to condense on the interior face of the rigid foam layer (especially if it is too thin to keep this interior surface warm enough); once these moisture droplets form, the water can drip to the bottom plate and cause rot.

        It's not inevitable, especially if you keep an eye on your interior moisture content, but it's a real risk.

        1. matt2021 | | #11

          Martin, thank you! Indeed, that makes a lot of sense.

          Are the recommendations, regarding the % of continuous insulation vs the % of cavity insulation, the same? In my case — walls in Central NJ, zone 5 — using R6 and R15 mineral wool or fiberglass, or R6 plus 1” of spray foam, and compressed fiberglass at ~R11, I’d be above the 41% mark only in the latter case; so, do you recommend that? (I should mention: I will have a rainscreen.)

          The OP, too, I wonder, could perhaps add a layer of spray foam to reach the right level?

  3. camdijion | | #4

    That's where I was a little confused as well, I was under the impression that the requirements are different depending on where the foam falls. I had seen numbers thrown out that say if the foam is to the exterior of the sheathing, then 30% of total wall R value should be in the foam. In one of the articles provided above it discusses wall sheathing as being the " interior face of the rigid foam" which mine is not. It seems to me as though the wrb impregnated OSB will dry just fine to the outside because it is in fact on the outside of the wall assembly.

  4. maine_tyler | | #6

    >"I pay attention to humidity inside the home; (use dehumidifier in summer and humidifier in winter)"

    Be careful, with a tight house, you may not need/want to humidify in winter. Winter is when you get condensation on cold exterior wall parts so adding humidity is not 'paying attention to humidity' in a good way (from the building's perspective, your skin may think differently, but with a tight house still will probably be unnecessary).

    ZipR is confusing. You might try reading this:

    in which the advice is to follow the minimum exterior R value requirements.

    But if you don't (which you haven't) there is the code option to use an interior variable permeance membrane so long as your exterior insulation R value is above R 5. In other words, yes to your question that a variable permeance membrane at the interior would be good in your situation.

    >"I have read a lot about smart vapor retarders, but don’t believe one is needed as I am vapor impermeable on the outside?"

    What exactly do you think the purpose of a vapor retarder is? The point is not to just have some random (in your case exterior) component of your wall with low permeability. The idea is to reduce vapor drive from the interior to the cold exterior (given that you are in CZ 6). In other words, it needs to be at the interior.

    1. camdijion | | #7

      Thanks for the information, to your last part, looking back I did not word that correctly. Basically I was thinking that vapor drive from the interior posed little risk to the sheathing (which we were talking about earlier) because the foam was on the interior. But depending on the temperature of the interior face of the foam I understand how there could be condensation, and that's where I wondered if one would still be okay with no vapor retarder as the wall assembly could dry inwards. And I understand what you are saying that the risk is mitigated by preventing vapor drive with the vapor retarder. I'm not averse to using the product just more so asking questions. Vapor barrier/retarders/and placement seems to be an age old discussion. Maybe I did make a mistake and get a panel that was to thin. But I appreciate all the feedback and am glad to be a part of such an informative forum.

    2. camdijion | | #8

      Furthermore, foot in mouth, we framed/sheathed early summer, and I guess my memory didn't serve me right. I just checked with the builder and we did use the R6 panel :( I apologize..

      1. maine_tyler | | #10

        So that would allow you to use the interior smart retarder.

        "Vapor barrier/retarders/and placement seems to be an age old discussion."

        True, but the most recent IRC does now spell it out pretty well. In your case, a variable perm membrane on the interior is warranted it seems.

        The foam of the panel may be interior to the sheathing, but that doesn't make it interior to the wall framing. Perhaps a good, better, best type situation. If it were me, I'd use the smart retarder- which is 'better.'

  5. Expert Member
    Michael Maines | | #12

    The foam layer on Zip-R sheathing is vapor semi-permeable; when dry, the OSB is the least permeable part of the assembly, just as in an assembly without foam.

    In climate zone 6, if you want to rely on painted drywall as the vapor retarder, at least 35% of the wall's R-value needs to be in the foam layer, according to the IRC and Building Science Corp research. That's often cutting it close; a higher ratio of exterior-to-cavity insulation would be safer from risk of condensation. In your case, with R-23 cavity insulation, you would need at least R-12.4 exterior insulation.

    Because this is for long-term moisture resistance, I recommend using a conservative value for the foam; R-5 is reasonable for the polyiso used in Zip-R; even their thickest product, Zip-R12, is not really enough to be safe, plus for shear resistance it requires extra-large nails that pose an installation challenge.

    1. matt2021 | | #13


      I am embarrassed to ask: Are we talking about condensation on the interior side, on the ZIP R's foam, or the exterior, on the OSB? (In my case, I will have a rainscreen on the outside.)

      1. Expert Member
        Michael Maines | | #15

        Matt, no worries, there are no embarrassing questions here! In theory it could be either side but because in CZ6 vapor drive is almost always from interior to exterior, the "condensing surface of interest" is the interior of the polyiso. It will pass vapor over time but short-term, if it's cold, it's solid enough that moisture could condense.

        Responding to one of your comments above--the wall could dry toward the interior if there were condensation, but it would be fighting vapor drive to the exterior. It's better to keep moisture from condensing in the first place.

        1. matt2021 | | #16

          Michael, Thank you, also for the encouraging words!

          I'm zone 5, and will be using R6. I am going to make sure I have a 1" layer of spray foam added to the interior of the ZIP sheathing, before adding any fluffy insulation to the cavity.

          1. Expert Member
            Michael Maines | | #17

            Oh, I thought you were the original poster. Zip R-6 with flash-and-batt or flash-and-fill on the interior is a good option from a building science perspective; I'm detailing a renovation with that system right now. It would be better to use products with lower environmental impact if possible.

          2. Expert Member
            Akos | | #18

            No need for any of that spray foam. Both field experience and building science has sown that 2x6+r5 with an calss I or II interior vapor retarder works just fine. There have been a lot of these walls built around me (zone 5) for a long time.

    2. camdijion | | #24


      I thank you for the informative response. I did take the time to read up on the code. I did have a question which maybe you can answer. A little off topic but still in the same realm of the discussion. I have seen pictures of houses in Climate zone 6-7, where dense packers are packing against siga/intello membranes, but also quite a few where they just have isulweb or similar product. I would assume if they are following the code, that there must be R11.25 (2x6 wall) or R7.5 (2x4 wall), then that would be fine. If however there was not, and code aside, the cellulose should absorb any water vapor being driven towards the exterior and dry out at a later date? In this situation if there was no vapor retarder (right or wrong) that the dense pack would be far superior to my mineral wool.

  6. Expert Member
    Akos | | #14

    This this a pretty good summary of the wall (closest equivalent I can find):

    Toronto is zone 5, Montreal is zone 6 and Edmonton is zone 7.

    Lot of text but helpful to understand the assembly.

  7. matt2021 | | #19


    In fact, my walls will be 2x4. So, R5 + whatever insulation I manage to put into the 3.5" cavity. Does the same applies?

    By the way, one reason to add a 1" of spray foam all around would have been to increase air sealing, as I am not sure how well the contractor will work with flashing and taping. In contrast, for me, a reason not to add spray foam to the walls is that I don't like the idea of "glueing" the sheathing and the studs all together, in case ever a repair is needed.

    1. Expert Member
      Akos | | #20

      Take a look at the table 2 here:

      2x4+R6 is fine with painted drywall only. No need for even a smart vapor retarder.

      Spray foam is one of those items of last resort. It is expensive and comes with its own risks. You only use it when there is no other option.

      If you are installing Zip R you are already taping the sheathing seams. Your only two major leak points are now the top (ceiling to wall connection) and the wall to foundation joint. Those can be addressed with either tape, liquid flash or a small amount of spray foam from the inside.

      1. matt2021 | | #21

        Thanks, Akos! I am taking this advice (and the documentation) at heart. I like the principle, "Spray foam is one of those items of last resort. It is expensive and comes with its own risks. You only use it when there is no other option." I will supervise the folks who will work on the room, and make sure they tape all the sheathing seams. At the foundation, maybe I will use the (not cheap) SIGA tape (and, as you suggest, liquid flash or spray foam). But I have been wondering about how much installing the furring strips for the rain screen will create points of air penetration, namely, at every place where a nail will go through the sheathing. Am I worrying excessively?

        1. Expert Member
          Akos | | #22

          "Am I worrying excessively?" Yes.

          A nail hole with a nail in it will not leak a measurable amount. If you want even better, aim to hit the studs for your strapping.

          1. matt2021 | | #23

            Thanks, Akos! And, yes, absolutely I am going to ask the workers to follow the stud lines for the strapping. (I might end up being the most annoying client they ever had, or, maybe, we will have some fun together in paying attention to details in helpful ways.)

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