Vapor Barrier? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-24-2008, 09:49 AM
Is a vapor barrier (either poly or #15 felt) absolutely necessary behind cement backerboard. I already have the backerboard up but can easily remove and put up a barrier. From what I have read it seems I need to do this but I was not sure. I have done a lot of researching on this site in building my shower stall properly prior to tiling and the info has been great. Somehow I blanked on the vapor barrier issue.

FYI - I have 1/8 and 1/2 inch backerboard up on top of each other to equal the wall thickness of the drywall it meets - so, if the vapor barrier is necessary, does it go between the cement boards or behind, stapled to the studs.

Thanks for the info- Jimmy.

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Brian in San Diego
02-24-2008, 09:56 AM

Welcome to the forum! Yes, you definitely need a vapor barrier of some type. If I were in your shoes I would get a paintable/trowelable surface membrane like Redguard, Laticrete Hyrdo-Ban and apply it to the surface of your CBU. The stuff is a little spendy but I would opt for that over taking all the CBU down, putting my vapor barrier up and then re-installing the CBU. The vapor barrier goes directly on the studs and it has to go inside the liner. It also needs to start at the bottom and each layer has to overlap the previous one. I think a surface applied and a roller would be much easier.

If you have any questions, please continue to ask them here. (Don't start new threads for the same project.)


02-24-2008, 10:00 AM
Hi Jimmy, welcome. A vapor barrier is needed in a steam shower. A water (moisture) barrier is needed for a regular shower or tubsurround. Depending on what you have, the felt paper or poly is fine for a regular shower but neither one makes for a good vapor barrier. Kerdi or Nobleseal TS would make good vapor membranes for steam showers.

For a regular shower that already has the CBU nailed to the walls, some folks just apply Redgard over the CBU. It will shed any moisture that gets back there. :)

Looks like Brian beat me to the punch. :)

02-24-2008, 10:52 AM
Brian and Davy- thanks guys. Done deal - it is a regular shower by the way. I figured as much but a little verification always helps.