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Unread 02-20-2008, 10:58 AM   #1
Devin
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Join Date: Feb 2008
Location: Brooklyn, NY
Posts: 1
Vapor barrier and waterproofing membrane

Avid reader, but this is my first post.

I’m renovating my bathroom (no surprise here), and I’ve framed out the tub enclosure, with the tub now set. Various existing conditions leave me with the following: a deep window sill (12-13”) at one of the side walls (relatively close to the control wall, so that it will probably get some direct spray), a 5-6” continuous ledge along the same side wall, and a tiled soap alcove above the ledge and below the sill. All horizontals slope 1/8 – 1/4” per foot towards the tub.

My plan for the enclosure was: continuous plastic vapor barrier (don’t have the mil in front of me) on top of kraft faced insulated studs, then 1/2 cement board (2 deliveries, one of durock, one of permabase), then thinset & tile. The rest of the bathroom will be greenboard with paint and tile (on mastic here), no vapor barrier, though the kraft facing will act as a vapor retarder.

Obviously, I need to address the waterproofing at the sill and ledge. Plumber recommended chloraloy under the cbu (at least, that’s how I understood his recommendation), but that doesn’t make sense to me as I’ll be screwing through it for the backer (self-healing? I haven’t heard it mentioned) and I think it would need to be continuous between sill and tub flange. Roofing felt is another obvious choice (probably replacing the plastic vb), but with the same healing and continuity issues. Kerdi membrane on the face of the cbu seems to be a favorite of this board, and I assume that I’d eliminate the plastic vb in this case. Then there are the paintable waterproofing – redguard, hydroban & laticrete 9235 also at the face of the cbu. These raise the issue of trapping moisture between vapor barrier and membrane, though as I understand it, I would only waterproof horizontal surfaces and the vertical returns, so maybe that’s not enough surface area to be concerned about.

That’s a lot of options. Any recommendations? Any other red flags in what I describe? Anyone concerned about the vapor barrier on top of kraft facing (it seems questionable to eliminate the vb altogether).

Thanks for the advice, Devin
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Unread 02-20-2008, 12:58 PM   #2
bbcamp
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Join Date: Jul 2002
Posts: 30,274
It seems like you are on top of this thing. However, I recommend that you slit the kraft paper on the insulation everywhere you cover it with the plastic sheet. We don't want any moisture retaining "sandwiches" which could occur if through Murphy's help, yoiu do get moisture bwtween the plastic and kraft paper.

As far as you ledges go, I'd use a paintable membrane, since you have traveled down the backerboard route so far. I wouldn't worry about the moisture sandwich there, since it's a relatively small area.
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