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Community and Q&A

Zip R Sheathing and Interior Vapor Retarder

Emel | Posted in General Questions on

We are just about to send our home plan out to be bid but had a question on Zip R before we do. We are looking at having metal siding over a rain screen, over Zip R sheathing. For the interior, we are planning to use Rockwool insulation, followed by a smart vapor retarder like Membrain, and then drywall. By code we have to have some sort of vapor retarder on the inside even if it’s kraft paper over fiber batts. Paint alone would not count. We are in zone 4, where it gets very cold and at times, humid in the winter, but also very hot and humid in the summer.

My question is: Will vapor build up and create condensation on the inside of the drywall/Membrain if the Zip R insulation is so tight it doesn’t allow it to escape? We have family with mold sensitivity and are hoping to build with extra attention to anything that would cause moisture issues.

Thank you,

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  1. Expert Member
    Akos | | #1

    What is the R value of of the Zip and the batts you are looking to build with.

    Depending on the combination, the only warm side vapor retarder you need is painted drywall.

    1. Emel | | #2

      I went back and forth on this with the inspector and he said code requires a vapor barrier of some kind unless it’s a vapor barrier specific paint. Because we are wanting a low toxin & VOC paint, I don’t know if that would be possible? Is there a combination you are aware of that would usually work and I can try taking it to him?

      1. Deleted | | #3


      2. Expert Member
        Akos | | #4

        Take a look at Table 2 here:

        In zone 4, if you go with Zip R5 with a 2x6 stud wall, you don't need any interior vapor retarder other than drywall. 99% chance this is in your local code as well but your inspector might not be familiar with that section as exterior insulation is rare down there.

        These are condensation controlled assemblies, the way this works it the exterior rigid insulation warms up the inside surface enough that you don't get condensation thus no additional vapor retarder is needed.

        The only time you would need a vapor retarder is if you use less insulation (say you are building a 2x8 wall with Zip R5). In that case the ratios in the table are not met so you must include more vapor control.

        1. Emel | | #5

          Thank you for sending that information. First of all we had our zone wrong...we are zone 6 bordering on zone 7 according to the map. We have a farm and we are zone 4 for hardiness so we thought it was the same for building which is obviously not the case.

          Not sure if that really changes anything other than what I need to label this thread.

          1. Expert Member
            Akos | | #7

            It works the same way in zone 6. As long as you have enough exterior rigid (R7.5 for 2x4 and R11.25 for 2x6) you don't need a separate vapor retender.

            If you are building with 2x6, the R12 Zip might be a challenge to use as it needs a special nailer that can shoot 4" nails, in that case it is probably simpler to use either R6 or R9 Zip and include a warm side vapor retarder.

  2. maine_tyler | | #6

    I was wondering about calling zone 4 cold... :)

    "Will vapor build up and create condensation on the inside of the drywall/Membrain if the Zip R insulation is so tight..."

    It's not the drywall or interior vapor retarder that would see moisture issues but the inside of the zip R. But using an adequately thick layer makes it safe.

    You may find this discussion useful. Especially note the Expert response (by Kohta) at the end.

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