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Thread: Backer board in bathroom

  1. #1
    DIY Member Semon's Avatar
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    Default Backer board in bathroom

    Shower exterior wall I live in east Tn.
    I put up Hardie board on the inside of the exterior wall of the shower and going to put tile over that. I plan to use Aqua Defense on the Hardie board before the tile.
    Should I put a vapor barrier between the Hardie board and the insulation?

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    In the trades Dana's Avatar
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    It depends- what is the stackup of the exterior wall, from siding to the Hardie backer, and the type of insulation? (If fiber, does the insulation have a facer? If yes, what type, and which side of the assembly is the facer?)

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    DIY Member Semon's Avatar
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    Quote Originally Posted by Dana View Post
    It depends- what is the stackup of the exterior wall, from siding to the Hardie backer, and the type of insulation? (If fiber, does the insulation have a facer? If yes, what type, and which side of the assembly is the facer?)
    Hadie board, glass insulation, 1/2 " brown board, brick.

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    In the trades Dana's Avatar
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    Does the fiberglass have a facer (if so, which side), and is there rosin paper or roofing felt on the brown board, or is it an ashpalted fiberboard?

    Is there an air space between the brick & fiberboard- if yes how deep, and is it vented/open at the top, with weep holes at the bottom?

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    DIY Member Semon's Avatar
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    Fiberglass has facer but it is just stuffed in.
    No paper or felt .
    Brown board is just that pressed fiber from the 60s.
    Brick space is about 2". Weep holes yes, vented at top I think.

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    In the trades Dana's Avatar
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    The facer is still a vapor retarder, even if it's not a very effective air barrier if it's just stuffed in there. Vapor diffusion is all about vapor pressure and cross sectional area- so if it's just stuffed in there it's not a continuous vapor barrier, but it's still slowing the overall drying.

    If it's a kraft-facer (~0.5 perms) I think you're fine. If it's a foil facer (<0.1 perms) it's not the greatest situation, but adding another vapor barrier on the interior wouldn't fix it.

    With a vented 2" cavity and a highly permeable fiberboard sheathing you can skip the interior vapor barrier- the assembly can dry just fine passing any shower moisture that makes it past the ~2 perm AquaDefense into the masonry cavity through that stackup.

    If you DID put up an interior vapor barrier like poly you might create problems where none previously existed, particularly if that wall gets direct sun:

    Brick can retain a TON of rain & dew moisture, then release it with intense bursts of extreme humidity into the cavity when heated by the sun. The venting to the cavity eventually clears it, but with a vapor barrier like poly blocking all drying toward the interior you could end up with events (usually in the summer) where condensation forms on the poly and dribbles to the bottom, creating moldy sections on the subfloor or stud plate. If that moisture is allowed to freely adsorb into the Hardi then dry more slowly toward the interior through semi-permeable AquaDefense, nothing bad happens.

    Only if that shower is seeing 2+ hours/day of use would it call for putting something in there, but not poly- maybe Certainteed MemBrain, which changes vapor-openness with changing humidity, or a ~0.6-0.8 perm siding underlayment type product. At typical 4-person household showering duty cycles, fuggedboudit- the brick would be putting more moisture into the wall than the shower will through AquaDefense.

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    DIY Member Semon's Avatar
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    Thanks for the info.

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