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Question:

do i need to put vapor barrier behind cement backerboard in shower?

i'm tiling my shower and i know i have to put up cement backboard, but do i need a vapor barrier behind it?do i cut the backerboard so the bottom of it touches the top of the tub or leave a little space between them? do i need to tape in between the boards? any help would be appreciated.

Answer:

The International Residential Code, as well as almost any of the other Model Codes, requires a vapor barrier to be installed on the warm in winter side of the insulation on any exterior wall. Any approved vapor retarder material will work. On drywall, two coats of most latex paints meet the requirement, also. The backerboard manufacturer will tell you what method of taping is required for their product; generally, it will be with a mesh-type tape embedded in whatever you are using to install the board. (This doesn't have to be thin-set cement, you can also use regular latex based tile adhesive. Just be sure to apply it according to the manufacturer's instructions.) Most tubs and shower pans have a lip on the front, back, and side. The board can sit right on this lip. Pack some of the thin-set or adhesive into this joint, if you need to build it up level with the surface of the backer. This is most important if you are not starting with a full tile on the bottom; the tiles should have at least 80% contact with most adhesives or cements.
May 11, 2017
u can put up a tar paper vapor barrier behind of the cement board if u want...anything is helpful so water does not get to the wood framing, but not really needed ....the cement board should not touch the top of the tub ( stay above the tubs lip) u do not want any wicking action....the finish tile should be an 1/8 of an inch or so off the top of the tub.....caulk between tub and tile. before u tile the corners and joints of the cement board should be meshed taped and covered with thinset. if u hage the board over the lip of the tub the your tile will not line up or you'll have a thick grout line towards the bottom corners because going over the lip will cause the bottom to flar out about an 1/8 of an inch or more.let the tile overlap the lip
May 11, 2017
This Site Might Help You. RE: do i need to put vapor barrier behind cement backerboard in shower? i'm tiling my shower and i know i have to put up cement backboard, but do i need a vapor barrier behind it? do i cut the backerboard so the bottom of it touches the top of the tub or leave a little space between them? do i need to tape in between the boards? any help would be appreciated.
May 11, 2017
Without just reading other answers, which may offer valid suggestions, my suggestion is OVERKILL can't hurt. I tile every day of my life and have done Bath Rooms more than much else. Certainly sealing the seams/butt joints will help. Since you have a TUB, I'll assume it has an UP LIP that butts to the wall. Your backer and tile can extend to the tub, below that lip, and after the fact be caulked. That would also apply to a plastic tub/shower surround. The enginerring in the LIP is to avoid water getting behind the TUB, and.or wall finishes to the studs, insulation, flooring, etc. A simple SHIM could be a piece of lumber the thickness of a grout line, on the tub, horizontal lip. Steven Wolf Just my two sense
May 11, 2017
Vapor barrier a good idea, 6 mil poly. Yes tape joints with proper materials, not standard tape & spackle. Overlap flange, but leave space between top of base and backer. Once you have installed backer board, stand on shower base and check for squeaks/rubbing at base/backer board. Cut backer if you hear anything....
May 11, 2017

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