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Doubling exterior walls in rehab, moisture concerns

S_Hensley | Posted in General Questions on

This question is similar to the post here: 
(, but is a bit different.  I have a newer brick facade house with existing faced R13 insulation and existing drywall.  I would like to add another wall on the inside of all exterior walls, and I would like to stay away from spray foam/foam board as much as possible.  

My preference would be to leave the existing drywall and vapor barrier (the facing on the existing insulation) and simply build my second wall, insulate it, install vapor barrier and drywall and be done.  Am I asking for moisture problems by having a double vapor barrier and the potential for moisture to be trapped between the old wall and the new?  

If that were a likely problem, my plan would be the above plus a tear-out of existing drywall and vapor barrier.  

I’d love to hear your thoughts… Thanks!

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  1. Expert Member
    AKOS TOTH | | #1

    The double vapor barrier in this case would be a bad idea. A vapor in the middle of the wall works in most climates. If you go with the 2nd stud wall, skip the interior VB but keep the one in the existing walls.

    It might be simpler to get your hands on some reclaimed fiber faced roofing polyiso (check Craiglist). You can install directly over the existing drywall and hang the drywall through it with long drywall screws. Commercial drywall places stock drywall screws up to 6" length, you can put a fair bit of insulation.

    2" of continuous polyiso would have the same assembly R value as a 2x4 wall with mineral wool batts, and you would loose less of your interior space. You would have to extend/move your electrical boxes, which is not a huge job.

  2. Expert Member
    Dana Dorsett | | #2

    >"...existing faced R13 insulation..."

    Not all facers are the same.

    An asphalted kraft facer is a "smart" vapor retarder that runs about about a half-perm when bone dry, but 5 perms or more when the moisture content is high enough to support mold. If a kraft facer is the only "vapor barrier" it isn't going to create a "...double vapor barrier..." problem.

    Foil facers usually run less than 0.1 perms and would qualify as a true vapor barrier, which could indeed create a moisture trap.

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