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Community and Q&A

Drywall Over OSB and Smart Membrane Performance

A423 | Posted in Green Building Techniques on

My detached garage wall construction from outside to in, consists of.
Everlast composite siding
R22 fiberglass insulation
Certainteed membrain
I would like to put 7/16 osb with 1/2 drywall on top for my interior walls. My question is will the osb under the drywall effect the performance of the membrain? I live by Lake Erie /Zone 5  I plane on keeping it heated. No A/C

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  1. Expert Member


    Membrain is a variable perm vapour-barrier that moves from less than 1, to 20 when damp. OSB, acts in much the same way starting at 2, and can climb to 12 when damp.

    So yes, the presence of a layer of OSB on the interior will thwart the smart membrane. I'd leave out the Membrain, and let the OSB do the job. But I'd be interested to hear what other posters think given it's potentially very wet environment, and you end up with almost the same permeability on both sides of the wall.

    1. A423 | | #2

      Thank you for your response.
      Am I getting it right to say.
      Sandwiching OSB between drywall and Membrain won't hurt anything it's just a waist of time and money to put both up. The Membrain has a potentially higher perm rating (20) for inward drying then OSB (12) The potentially higher perm of Membrain is lowerd to 12 by the addition of OSB. At that point the only thing the Membrain would be doing is acting as a air barrier.
      I do like the structural integrity OSB would add to the wall.

      1. huey_ce | | #3

        I agree with Malcolms comment that the OSB limits the capabilities of the smart membrane so you might as well save the money and skip the membrane.

        My advice would be to swap the OSB for plywood. In my experience OSB takes longer to saturate and dry than plywood. Once it is wet it likes to stay wet and starts to deteriorate. Seen some research that agrees with this which blamed it on the additional adhesives and broken structure of wood chips vs peeled veneer.

        I cant seem to find the link at the moment, but my anecdotal experience of leaving scraps of OSB and plywood outside at home support this.

        1. Expert Member
          MALCOLM TAYLOR | | #4


          Probably a valid concern in a high moisture environment like a garage.

      2. Expert Member
        MALCOLM TAYLOR | | #5



        1. A423 | | #6

          Thank's again for the sound advice and helping me make my mind up. The only other question I would have is, Do you have a recommendation ( does it matter) what type of plywood to use?

          1. huey_ce | | #7

            What is the purpose of the extra layer of sheathing on the inside face of the wall? I am assuming the exterior sheathing is taking care of any wall bracing requirements. Is it just some extra strength so you can hang tools everywhere?

  2. A423 | | #8

    What planted the seed in my head was looking at the wall with the garage doors. The wall is
    40' long and has (3) 10' wide doors in it. Tying the top plates, pony wall, header and
    rest of wall together with a sheet product would greatly strengthen the wall. Plus I may be putting side mount garage door openers in. So in a nutshell yes, extra strength. It should be okay without the interior sheeting since it is tied together on the outside. I just like the thought of added strength especially in that area. I have enough sheets of OSB to cover walls
    (I know just because I have it doesn't mean I need to use it). If I decide to I may just do the wall with the garage doors with plywood then membrain and drywall other walls. Hence me asking you guys your opinion so i can come to an conclusion based off your knowledge.

    1. Expert Member
      MALCOLM TAYLOR | | #9


      Plywood is an even more effective variable perm vapour-retarder than OSB. Wherever you use either OSB or plywood on the interior there is no point in using Membrain.

      1. A423 | | #10

        The only reason I mentioned OSB is because I have it. After are conversation I will not use OSB. If I decide to sheet, I'll use plywood and no Membrain. Is there any particular type of plywood that I should use?

        1. Expert Member
          MALCOLM TAYLOR | | #11


          1/2" CDX.

  3. A423 | | #12

    Thank you, I appreciate the education.

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