vapor barier, cement board, etc

Discussion in 'Shower & Bathtub Forum & Blog' started by FJK, Dec 4, 2007.

  1. FJK

    FJK New Member

    Messages:
    73
    Location:
    Illinois
    I've searched this form for the last year on the subject matter of vapor barriers & cement board installation for a 3 wall alcove tub installation & still can not find answers to the following questions:

    1) It is stated to run the vapor barrier over the tub flange (meaning tub side, not wall side). What about the wall surface that the height of the tub covers? No vapor barrier there?
    2) Use either felt or plastic. If you use plastic, what mil thickness? BTW, which is better, felt or plastic?
    3) Shim studs to be plumb. How exactly? A continous shim, the length of the stud, unless tapered is going to follow the stud. So, how is this done & with what?
    4) What cement board thickness should be used, if you are going to eventually meet with 1/2" drywall & you want a flush cementboard to drywall joint?
    5) Does it matter if the last column or row of tiles slightly canaliver off the cementboard onto the drywall?

    Specific guidance would be appreciated.

    Thanks, FJK
  2. frenchie

    frenchie Jack of all trades

    I've searched this form for the last year on the subject matter of vapor barriers & cement board installation for a 3 wall alcove tub installation & still can not find answers to the following questions:

    1) It is stated to run the vapor barrier over the tub flange (meaning tub side, not wall side). What about the wall surface that the height of the tub covers? No vapor barrier there?

    No.​


    2) Use either felt or plastic. If you use plastic, what mil thickness? BTW, which is better, felt or plastic?

    Raging debate, no clear answer, either works. Kerdi is better than either.​


    3) Shim studs to be plumb. How exactly? A continous shim, the length of the stud, unless tapered is going to follow the stud. So, how is this done & with what?

    I like to go the full length, makes for straight walls. I used to rip down ply for this (1/8), now I buy trim stock, 2 x 1/8th - ask for "lattice". The nice thing with this option is, you can get it different thicknesses, sometimes the tub's not perfectly centerd on the walls...

    4) What cement board thickness should be used, if you are going to eventually meet with 1/2" drywall & you want a flush cementboard to drywall joint?

    1/2". Except you're shimming the walls, so it won't be flush. You can't switch to 3/8 wonderboard, that's too flexible. Switch to 5/8 sheetrock, or buy some bullnose tile.​

    5) Does it matter if the last column or row of tiles slightly canaliver off the cementboard onto the drywall?

    I usually do it the other way 'round: run the wonderboard 2-3" past the tub, then tape the seam, and skim the wonderboard that isn't under tile.

    Obviously, it only works with the 5/8 sheetrock system. If you go for the bullnose option, it's okay that the tile ends on rock, and it's not like you've got a choice, anyways...​
  3. wryghtman

    wryghtman New Member

    Messages:
    3
    If I was to install felt paper than cement board 1/8 of an inch above the tub and than chaulk the tub seam with silicone how would the moisture escape from behind the backer board?:confused:
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,262
    Location:
    New England
    Good point, but you need to consider that not much moisture should get behind the tile into the backer board. Some will migrate through the grout between uses. Keeping the relative humidity down and helping things dry out by the proper use of a ventilation fan is also a big contributor. Having the tub set properly (i.e., level) so water doesn't' pool around the edges helps immensely. The goal of that barrier is to keep it from spreading into the walls and condensing and initiating rot.

    You can put a layer of vapor barrier below the tub, just lap the section above the tub onto the tile flange.

    So that the cbu doesn't bow out when trying to go over the tub flange, depending on its height and the size of the tile, you can either stop it above the flange if most of the tile would still be supported, or notch the studs to allow the flange to fit flush, or shim them so things fit over. Notching may be the easiest since you don't' need extra materials.

    As long as the outer tile is in the dry zone, you can lap them over the drywall. Thinset and drywall make a very good bond, it just you don't want it to perpetually be getting wet. Getting a nice surface on cbu so that it looks like drywall can be done, but normally, it isn't necessary. What you don't want is the joint in the wet area.

    Check out www.johnbridge.com for tiling help.
  5. frenchie

    frenchie Jack of all trades

    The whole point of the tar paper, is that it prevents moisture getting back there in the first place.
  6. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    The vapor barrier should slightly overlap out over the tub flange. As with any flashing, you want a continuous protected path where any amount of water will have an unimpeded vertical downward path to "safety". We have to assume the some water might make it past the tile, through the backer board, and that's where the vapor barrier finally gets to do its job. Maybe 20 years down the road, but you want it to be there.


    If the vapor barrier went behind the tub , then the caulking is your last line of defense, and that is not proper. Caulking can be a first line of defence, but does not take the place of proper flashing.
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