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Community and Q&A

OSB as Vapor Barrier

Akos | Posted in General Questions on

I’m running into an issue with a build here where I need a warm side vapor barrier on an overhang.

Figured I was being smart and skipped poly since 3/4″ OSB is around 0.5 perm, but now my inspector is asking for a spec sheet showing that it has been tested to¬†ASTM E96 / E96M.

I searched through the spec sheet of most major OSB manufacturers and none of them have a perm rating in their data sheets. Lots of into about spans and nailing but that is about it.

Anybody run into this and how did you get around it? I’m guessing I’ll have to pull up the floor and install the poly but it just seems silly.


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  1. Expert Member
    Michael Maines | | #1

    Akos, I don't think even Huber has done ASTM E96 testing, or if they have I don't think they report it. One reason is that different fibers (SYP vs aspen, primarily) have different characteristics. You could contact their technical services department; I do regularly and always find them responsive and helpful.

  2. Expert Member
    Akos | | #2

    Thanks Micheal,

    This is the case where just adding in the poly is less time (and argument) than anything else. Going in tomorrow.

    It is a shame that the info is not readily available, thick OSB makes for a great smart vapor retarder, the poly actually makes the assembly less robust.

    I understand that there will be some variability in the rating based on composition, they could still publish is maximum perm rating that covers all wood fibers.

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