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Old 02-07-2018, 04:04 PM   #1
BoneDry
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Tub/shower waterproofing

I'm doing an alcove tub/shower for the first time, with an exterior wall on the long side of the tub. The walls had been filled with blown in insulation, so when I took the plaster down I kept that insulation in there and put up a plastic vapor barrier as I had done in other places, going behind the tub. I then put up 1/2-inch cement board and plan to waterproof it with Redguard. It then occurred to me that the cement board would be in between the plastic and the Redguard, which I'm concerned could cause mold, although in theory no moisture should get past the Redguard, and if some minuscule amount did, it seems like it would run down the plastic or evaporate before the next shower. I haven't taped and mudded the joints yet so I could still take them down and take the plastic out leaving the insulation exposed to the cement board if you think that's necessary and/or better. Should I do that, or leave it as it is?

Thanks for your help.
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Old 02-07-2018, 04:39 PM   #2
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Mike,

Welcome to the forum.

You already know the answer; take the boards down and put a few vertical slits in the sheet plastic before restoring the boards.
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Old 02-08-2018, 12:06 AM   #3
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Thanks, that's what I'll do.
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Old 02-08-2018, 12:58 PM   #4
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dont remove board

If this was a steam shower it would be all set to go for tile. Any small amount of moisture that gets thru to the plastic if at all will be minor. do not remove the cbu to put slits in the plastic

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Old 02-08-2018, 05:16 PM   #5
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I'm getting some mixed messages here. It's a normal shower.
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Old 02-08-2018, 06:15 PM   #6
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Mold doesn't need a lot of moisture to grow it just need some
here is a good article about mold
https://buildingscience.com/document...bout-mold/view
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Old 02-08-2018, 10:01 PM   #7
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Newb here... so the following if really just my thoughts and not from experience:

Paul's advice sounds like the safest option, and from what I've read over the last couple months researching tile it seems to be considered/accepted as "best practice".

That said, Mike, I can't fault your logic... And, while I've seen multiple posts on various forums stating "don't create a moisture sandwich"... I don't think I've come across an industry spec or manufacture's instillation instructions which prohibit the practice. (of course, I'm new to all this, so that doesn't mean it doesn't exist).

I suspect it all comes down to the rate of moisture transmission through the redgard vs. the rate of moisture leaving the CBU. The Regard has a higher permeability rate than the plastic sheeting, but depending on how thoroughly the CBU is covered by the plastic sheet it could still dry out faster than the moisture is entering. For instance, if the edges of the CBU are not covered by plastic I suspect that may be enough to keep the CBU dry enough (just a guess on my part). Additionally, for a tub/shower situation (vs a steam room) it should be less of an issue. And maybe factor in how frequently the shower will be used... once a day probably not an issue, 3 showers a day and I might err on the side of caution and cut some slits.

I've read up on this because I'm thinking about a similar situation for my bathroom... I'm planning on using foil faced foam insulation between the framing, and I'm also planning on waterproofing my CBU (with hydroban). Now, in my case the wood studs wouldn't be "sealed" so it should still "breath" better than putting plastic over them, but still, I'm sealing things up a little bit.
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Old 02-09-2018, 12:37 AM   #8
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Today I took the cement board down and took out the plastic, cement board is now back up.
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