Liquid waterproof membrane on backerboard with polyethylene sheeting
I'm redoing my shower/tub wall with tile. I've removed the old tile and wall board, and the plan was to use 6 mil poly over the framing and then add 1/2 backerboard (Durock) and then tile with thinset. We've decided that a couple niches would be nice in the shower but it seems you really have to use a waterproof membrane inside of the backerboard, rather than plastic behind the backerboard. Is this correct....that using a waterproof membrane like Redguard is the way to go if you want to add niches in the wall (rather than poly behind wall)? I wanted to use poly sheeting instead of a liquid membrane because I didn't want to fur out the walls. The new faucet valve (and stub for spout) have already been set for just backerboard and tile, the largest gap (between tub and wall) is less than 1/4, and it'll be easier matching cbu in with existing drywall if I don't fur out. The existing walls aren't perfectly flat and I trimmed-up a bowed stud already to create a flatter surface for the cbu. About half of the cast iron tub is tight against the studs. The front wall and one corner have a gap of less than 1/4. The bottom line is that I'd prefer to not mess with the fur strips since the gap is small. The front of the backerboard would still be in front of the flange; I just couldn't overlap the entire board over the flange. My cast iron tub flange is not very high either; it's more like a bump about 1/4 high.. not sure if this is common. So from what I've read it seems that I should use a membrane if I want to install some niches. Could I Redguard the backerboard but still add a 6 high plastic apron (kind of like flashing) at the bottom and behind the backerboard? This would add a little more protection in case water doesn't drip straight down the inside face of the backerboard. I realize that you should not use both systems (membrane in front and plastic behind backerboard) but I was curious if the 6 plastic at the bottom would be insignificant and not cause any problems. If anyone has any experience with this type of setup, please let me know. THANKS! Any vapor barrier needs to be behind the cbu. I understand you can't do that with the niches. Using redgard on all the seams is fine. Don't add any poly on top of the cbu, as the redgard will seal it completely. The bottom of your niches should be slightly tilted to allow for water to run out, but not too much for the soap to slip out. This may seem obvious but do not add niches in exterior walls. In most cases it does not leave room for insulation. We were eating dinner at a friends house several years ago and they wanted me to take a look at their crazy shampoo bottle niche in their newly remodeled bathroom. Many mornings in the winter the back of the niche was covered in frost ice. I didn't mention that all 3 walls are interior walls...we should hopefully avoid chilled shampoo. If I switch to the Redguard approach (to accommodate installation of niches), I would like to also add a 6 high strip of poly sheeting behind the cbu just above the tub flange to ensure water flows over the flange and into the tub. Do you see any problems with possibly sandwiching moisture at the bottom of the cbu (I'll have redguard on the tub side and the 6 strip of poly flashing behind the cbu)? You can use the 6 mil poly behind the cement board, and still redgard the niches. Cover the entire niche with cement board. Tape and mud the all seams with alkali reisistant mesh tape and thinset. Take care to get everything as smooth as possible. Apply several coats of redgard to the entire niche and 6 outside around the entire niche. You can dilute the first coat of redgard 50/50 with water, as cement board is thirsty and will such the moisture out of the first coat of redgard. Then apply at least 2 more coats (undiluted) to get the proper thickness per redgard instructions. No need to redgard the entire shower. If you decide to do so, do not use the poly behind the cement board. Johnny, if I understand you correctly you are saying I can continue to use the poly behind all of the cement board as the primary moisture barrier, and the redguard would simply be for sealing around the niche seams. Won't there be a problem with the niche conflicting with the poly? Cut the poly where the niche is. Redgard the entire niche, not just the inside and outside corners. Also redgard 6 outside the niche all the way around.
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