How do I attach Vapour Barrier to Tub?

Discussion in 'Shower & Bathtub Forum & Blog' started by lien, Jun 14, 2007.

  1. lien

    lien New Member

    Jun 14, 2007
    I have just installed a new Tub. Its a Basic american standard tub with a 1 inch flange.
    I would like to know how to properly install the Vapour Barrier and Cement Board! I am using 6 mil poly and was wondering if I should attach it to the flange with silicone and then have the cement board sit about a 1/4 inch above the flange? Should I then leave this gap between the CB and the flange open so any moisture can flow back into the tub or should I caulk the gap between the CB and flange? Also if I then tile and leave a 1/8 inch gap between the tub and the tile should that then be siliconed?

    Any Help would be Greatly Appreciated!!!!

  2. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    The best installation is for the vapor barrier to lap down over the flange, and then the backer board also laps DOWN OVER the flange, stopping about 1/4" above the tub deck. This gives the most secure tile installation, and is absolutely bullet proof. NOW, to make the backer board be plumb, not bowed out at the tub, you will need to shim the studs. I have even sheathed the wall with 1/4" plywood sometimes, rather that try to indivdually shim the studs. This work well to correct studs which are not plumb to each other horizontally.

    For lots of good info on this topic, visit our friends at the tile forum:
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  4. geniescience

    geniescience Homeowner

    Nov 27, 2005
    humid summers hot, humid winters cold
    what Jimbo said.

    and about the caulk or silicone, you have a good point, which nobody has a good answer for. Whether to seal it up with the world's best product, or whether to seal it with a temporary product that you remove from time to time (to let you to check out the situation) and then replace.


    p.s. you can notch a stud (e.g.1/8") where the tub touches it, if that works better for you. Depends on each situation.
    Last edited: Jun 15, 2007
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The goal of your outside sheath is to prevent liquid water from getting behind. If it does, the flange prevents it from going further. The vapor barrier protects the studs, etc. behind the wall from any vapor that goes through. It rarely accumulates enough to actually flow, but contantly adding water vapor to that unventilated space is prone to enabling mold. So, the goal is to prevent that water vapor from accumulating by blocking it with a vapor barrier. You don't need to seal the vapor barrier to the tub. You DO need to make sure that the tub is installed level so any liquid water that might seep behind the barrier can't flow where you don't want it, but that is the flange's job, not the vapor barriers.
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