So I keep seeing conflicting answers on this. I'm renovating a bathroom that was torn down to the studs. On the exterior wall I have siding-tyvek-plywood sheathing-fg insulation inside studs.

My plan is for an alcove tub with cement board backer above waterproofed with redgard prior to tiling that sits above the flange of the tub. Do I put plastic all the way up to the ceiling behind the backer board? Some say that will trap moisture in the cement board. If not do I just put the vapor barrier on the bottom half up past the tub? What about the two interior walls?

  • How many perms is your Tyvek rated for? Jul 3, 2020 at 3:10
  • 56 perms I believe
    – redlude97
    Jul 3, 2020 at 3:22

1 Answer 1


The liquid membrane acts as the vapor barrier so you don't need plastic behind the cement board. Redgard data sheet says 0.35 perms at 30 mils so you can get away without an additional vapor barrier if you put on enough of the redgard. Do the same for the interior walls as well. Use a good quality primer and paint for the ceiling above if that gets drywall. Both Benjamin Moore and Sherwin-Williams make vapor barrier primers you can use.

When looking into this before, I went with Mapei AquaDefense which is a similar product but with better specs and a less "redrum" appearance. I think RedGard may have updated their specs since then. Leave a narrow gap between the cement board and tub, make sure to paint the redgard over the bottom edge of the cement board, and caulk the gap.

  • Thanks. What about at the bottom behind the tub do I just leave that part exposed or install plastic sheeting there? I'm ttying to instant how to get a continuous vapor barrier from the subfloor to the ceiling
    – redlude97
    Jul 3, 2020 at 3:25
  • @redlude97 You could probably put a vapor barrier across the studs behind the tub and attach it to the ledger board that the back of the tub sits on. Something I neglected to do: my vapor barrier runs along the entire interior of the bathroom, it's mainly there to keep steam from showers from escaping into the walls. The space behind my tub isn't vapor-barred but the whole house isn't that tight.
    – user116817
    Jul 3, 2020 at 14:01
  • Redlude97 thanks are nice but upvote Or accept a helpful answer. This answer would be similar to what I would give so I upvoted. When you accept a green check mark others can find the helpful information.
    – Ed Beal
    Jul 3, 2020 at 16:54
  • Ed Beal I did not know that. Thanks for letting me know how the forum works. I upvoted but did not have enough rep to increase the rating yet I have to be at 15
    – redlude97
    Jul 6, 2020 at 14:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.