vapor barrier back to back?

Discussion in 'Remodel Forum & Blog' started by M3, Mar 8, 2009.

  1. M3

    M3 New Member

    Messages:
    39
    Location:
    Minnesota
    I have two bathrooms that utilize a common wall. One has already been redone and I did use a vapor barrier, cement board, tile, etc... Now I'm redoing the other and am wondering if I should put on a vapor barrier, or if that will potentially trap moisture in the wall...? I could put a pvc vent to/through the attic as a precaution. What is best to be done?

    The areas that face this adjoining wall are: the back of the shower which is already done (which would get most of the spray), and the valve system of the shower that I'm currently redoing (which usually doesn't get too much spray). The new shower will also be cement board & tile...

    what do you think?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,896
    Location:
    New England
    I'd put a vapor barrier on the new remodel's walls.
  3. SeattleSoxFan

    SeattleSoxFan In the Trades

    Messages:
    36
    You don't need a vapor barrier in a shower, you need it to be waterproof! I'm sure you wouldn't skimp on one wall of a shower because you didn't expect it to get much spray, would you? :) Anyway, keep the water out of the stud bays and waterproof the shower. The "vapors" should be taken care of primarily by ventilation.

    I don't know what your shower is going to be made of though... If it's tile, use a membrane on top of the surface (schluter or nobel), if it's a slip-in unit just use PVA primer and use a fan with a timer switch.
  4. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    The point is to not allow moisture into the stud wall period. So you shouldn't be concerned with trapping moisture because there won't be any in there because you're going to build it right.

    Yes double it up.

    Tom
  5. GabeS

    GabeS Remodel Contractor

    Messages:
    294
    Location:
    Brooklyn NY
    If you double it up, and some moisture works it's way into the wall, then it will never get out and that's a bad thing.

    I would be skeptical about doubling up the vapor barriers. Putting it one wall only would kind of defeat the purpose because the moisture would get to the backside of the wall with the vapor barrier (pointless).

    So what's the solution? I've never done this setup, so I couldn't tell what I did in this specific scenario. Black tar paper allows vapor to escape to the outside but also protects from the outside in. You might need to do more research.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,896
    Location:
    New England
    What you don't want are two vapor barriers in the same wall covering - like a membrane on top of the cbu, then one against the studs, or say it was an insulated, outside wall, the barrier on the insulation then another right on top of it behind the cbu. An inside wall stud cavity will have enough leaks where having one on each side of the stud should still allow enough air exchange to mitigate any moisture that might get in there in the first place...after all, you should not get much from within the shower through it into the wall structure in the first place. Just don't place one on either side of a solid sheet like cbu or drywall, or one on top of the other. Then, if there are any leaks, there's no air exchange, and it can remain trapped.

    If you are really worried, drill a couple of holes in the studs to an adjacent stud cavity outside of the shower area...then it could migrate out eaiser, but it shouldn't be a problem if you do it right in the first place there shouldn't be any there.

    Personally, as mentioned earlier, my preference is to avoid all of this and use a surface membrane and make the whole shower waterproof, rather than water resistant. Kerdi would be my first choice, but there are other quality products that can do similar, but not necessarily the same thing.
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