GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Waterproofing Shower Wall Cavities and Subfloor

JennL | Posted in Expert Exchange Q&A on

We are building a new home in CT (climate zone 5). In the bathrooms, we have chosen to go with one-piece fiberglass tub/shower units with cultured marble panels on the walls above the unit. I am very sensitive to mold, so we plan to moisture proof the walls and subfloor prior to installation. I see lots of information related to tile showers, but what is the best method for waterproofing behind and below a one-piece fiberglass unit? We will be doing a pre-drywall blower door test. Am I correct that this should be done before the moisture proofing is in place? Thank you

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    PETER G ENGLE PE | | #1

    No special moisture proofing is required for a FG shower insert. The insert is waterproof (or should be).

    1. JennL | | #2

      My thought was that the moisture proofing would protect the framing, insulation, and subfloor in the event of leakage from caulked areas around the perimeter as well as the shower head and controls. Your opinion? Thank you

      1. baxt1412 | | #3

        All about how much you want to spend and what will help you sleep at night. Nothing wrong persay about doing it

      2. Expert Member
        MALCOLM TAYLOR | | #6


        The best defense against moisture problems in the floor or walls is to make sure they are designed with adequate drying capacity, and good air-sealing, so that any incidental moisture that does get in can not do any damage. That's really no different form any of the rest of the assemblies in your new build.

        1. JennL | | #8

          One of the walls will be an exterior wall. The wall assembly will consist of vinyl siding, Tyvek wrap, plywood, and Kraft faced FG batts. If we include some moisture proofing such as tile backboard or Wedi membrane, will this affect the ability of the wall to dry? If so, what is the solution?

          1. Expert Member
            MALCOLM TAYLOR | | #10


            That exterior wall is designed to dry to the outside. it should be fine.

            As Akos said, the ability to dry needs to be combined with a good interior air-barrier. With those in place your walls are as safe as any of the others ones in the house.

    2. Deleted | | #9


  2. GBA Editor
    Kiley Jacques | | #4

    This video from builder Josh Oduin might be useful to you:
    Shower Waterproofing.

  3. user-6623302 | | #5

    Youtube is full of ideas.

  4. Expert Member
    Akos | | #7

    The big issue with a lot of tub installs on outside walls is there is the lack of air barrier behind the tub itself. Besides making the shower liquid tight, this is the major issue that you need to fix if you want mold free bath.

    Make sure the drywall in the tub area goes all the way to the floor, it is sealed with caulk against the top and bottom plate and the drywall is taped with a quality tape (ie 3m 8067) where it is notched for the lip of the tub to mount to the studs.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |