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Unread 06-17-2004, 04:22 PM   #1
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Water vapor question

I am tiling a tub surround in a 3 wall alcove. The back wall is an outside wall. On the two inside walls what should I use for a vapor barrier? What should I use on the outside wall? One other question. If the water from the shower goes through the grout, through the backerboard to the vapor barrier and then runs down the vapor barrier and the vapor barrier is attached to the tub with waterproof caulk and there is caulk between the bottom of the backerboard and the vapor barrier/ tub flange, how will the water escape? Won't you end up with a water sandwich which will eventually breakdown the backerboard/grout, or break through the vapor barrier? Just wondering how this is supposed to work exactly. Thanks.

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Unread 06-17-2004, 05:18 PM   #2
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I had related questions, see my thread "options...". My resolution was to go with Kerdi, which will provide a water-tight barrier behind the tiles so moisture never reaches DW. On outside walls I will still have vapor barrier to protect the studs from water vapor. Hmm, I guess there's still a risk of double VB between VB and Kerdi. Too risky to omit that VB though; I get the impression that others put VB behind Kerdi.... What are chances of mold issues down the road?
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Unread 06-17-2004, 05:57 PM   #3
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Yeah, isn't it a bit confusing? a bead of chulk on the back of the bottom of CBU to prevent water from wicking up, but moisture comes down from the top and no way to go.
Can we assume the water will not wick up to 6" over the liner and skip the chulking?
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Unread 06-17-2004, 07:45 PM   #4
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I like using #15 felt paper, some like poly. I don't think all that much water gets behind the CBU, especially over a tub. Not enough anyway to run down and get trapped.

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Unread 06-19-2004, 01:55 PM   #5
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Yep, it's a bit controversial. I use a poly barrier behind the mud (or CBU), lap it over the lip of the tub, and grout that joint. The grout will crack at the tub joint. I let it do that.

I'm fortunately too far away for anybody to chastise me effectively just now, so that's my story and I'm sticking with it.

And I ain't askeered of no steenkin' Tile Ranger.

My opinion; worth price charged.

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