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Water-resistive barrier behind cement backer board for shower

etting | Posted in General Questions on
I’m installing a 30″x60″ steel tub and a glue-up acrylic surround attached to Durock cement board, with one end of the tub against a double-stud exterior wall that has Tyvek between the outer and inner walls (as well as another WRB behind the exterior sheathing).  The walls are insulated with Roxul rockwool.  An article from Building America Solution Center, says, “cement board is water resistant but not waterproof. You must coat it with a fluid-applied waterproofing or apply a water-resistant barrier behind it that drains.”   In the discussion of a 2010 article from GBA,, several contributors say they install poly behind the cement board.  
I was considering a waterproofing membrane on the shower side of the cement board, but they don’t seem to be compatible with adhesives for a glue-up tub surround.  RedGard, for one, explicitly says so.  Would regular Tyvek behind the Durock work well?  Should it hang over the tub flange, hidden by the surround, to “drain” into the tub?  The regular Tyvek doesn’t have any drainage channels built into it, but neither does the poly or tar paper people mention using.  I’m in central Arizona, so there’s plenty of drying available.
Perhaps also relevant, the tub surround instructs to use a 1/4″ v-notched trowel to spread the adhesive.  I’ll use DAP DynaGrip.  I gather that the fairly thin surround does best the more uniformly and smoothly the adhesive is spread.  If the adhesive were spread to make a continuous layer, such as with a finer trowel, would it act as a waterproof membrane?

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  1. Expert Member


    Unlike tile installations, the glue up tub surround (acrylic or fiberglass) is your waterproofing. If you are worried about the caulked joints you can use Durock's liquid membrane behind them as back-up, but if you don't trust the surrounds design to keep water out, it's probably not a good choice to use.

  2. etting | | #2

    Thank you, Malcolm. The title of the article from BASC is "Cement Board Installed behind Tile and Panel Tub and Shower Enclosures," so I take it to apply to panels like those I'm using, and BASC seems to be a respected source. Would there be any harm or benefit in using Tyvek behind the Durock?

    1. Expert Member
      MALCOLM TAYLOR | | #3


      No harm at all, and in this situation where you have an impermeable surface in the surround, it's probably better choice than poly. You can tuck the edge into the tub flange, but the Tyvek should never see enough moisture for it to actually drain. What's interior vapour-retarder are you using on your double walls?

      1. etting | | #5

        Thank you, Malcolm. I don't expect the Tyvek will actually need to do much, but if it can't do any harm, I'll use it. I have plenty left over. The wall assembly is drywall with latex paint, Roxul, Tyvek between the two walls to stop air infiltration, Roxul, another WRB I can't recall at the moment, and SmartSide panels on the exterior.

        1. Expert Member
          MALCOLM TAYLOR | | #6


          So any moisture that did get back there has good drying paths. Sounds safe to me.

          Having never done one, what I don't know is if latex painted drywall is a sufficient vapour retarder on a double wall.

          1. etting | | #7

            Thanks again, Malcolm.

  3. Deleted | | #4


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