Home>Discussions>BATHROOMS>Cement board: Vapor barrier under? Membrane over? Both?
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mvbellamy
Cement board: Vapor barrier under? Membrane over? Both?

I've been reading lots of conflicting information. I am starting a bathroom remodel that will include a tiled shower along an exterior wall.

It's become clear that I need to use cement board in the shower, but I am unclear about waterproofing it. I was planning on putting a vapor barrier over the new insulation before putting up the cement board, but I've read that if you do that, you should NOT put a waterproofing membrane over the cement board. If that's the case, do you treat the cement boar at all or just tile directly onto it?

I live in Interior Alaska, so insulation and efficiency are very important. The shower will be along one exterior wall which also has a window. The window is being replace with a vinyl window and I will tile the frame as well.

HoustonRemodeler
Re: Cement board: Vapor barrier under? Membrane over? Both?

You are correct. You want one waterproofing method OR the other. Surface waterproofing has many advantages, such as far easier waterproofing for niches and windows. The Surface waterproofing acts as the same vapor retardant that the sheet plastic would. The surface waterproofing gets applied to the window frame so the waterproofing is continuous.

IF you elect to use sheet plastic, keep it at 4 mil or greater, one single sheet, and overlap the bottom onto the tub flange or shower pan. Adhere the sheet plastic to the tub flange if you have a tub. Overlap the vinyl liner a few inches if you have a shower pan. Then tile directly over the CBU.

My preference is for surface applied membranes such as Kerdi, Hydroban, Hydro Barrier, Dalseal or Noble seal.

For all things tile check out the John Bridge tile forum. They have an extensive library, pictures, tutorials, and speedy answers.

mvbellamy
Re: Cement board: Vapor barrier under? Membrane over? Both?

Thanks for the quick reply. I've been checking out that forum and will probably start a thread there seeking advice on this project.

jkirk
Re: Cement board: Vapor barrier under? Membrane over? Both?

based on your climate the vapor barrier should be on the warm side of teh wall though beneath the concrete board. from there use kerdi membrane on top of the concrete board to water proof the shower assembly after that its just a matter of the tile going on

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