Schluter-Kerdi shower system-moisture barrier question. [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-28-2008, 01:07 AM

I've been finding this site very useful. I'm finishing my walkout basement and at the stage of installing the bathroom. I've decided to go with a steam shower and I am in the process of installing the Schluter shower system.

My question is regarding moisture barriers.

Should I use a moisture barrier behind the backer board even when using the Kerdi membrane? My concern this would create a moisture sandwich.

The shower is 60"x32", and the 60" side is along a 2x6 outside wall thats insulated and covered with a poly. moisture barrier.



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02-28-2008, 01:14 AM
Should I use a moisture barrier behind the backer board even when using the Kerdi membrane?no you don't need one behind the Kerdi.

02-28-2008, 11:18 AM
Thanks for the reply. I'll do without the moisture barrier then, just use green drywall and Kerdi the shower from top to bottom including the ceiling.


02-28-2008, 03:16 PM

Brian in San Diego
02-28-2008, 03:43 PM
You don't have to spend the extra few bucks for green isn't necessary. If you do use green drywall for the ceiling I want to say the ceiling joists need to be 12" rather than 16". You can kerdi all the walls and ceiling but it isn't necessary to do so. You can stop the kerdi at about an inch above where the shower arm comes out from the wall.


02-28-2008, 05:10 PM
Hi Chuck..

Hold on a sec, Brian (a very knowledgeable guy) probably over-sited you saying "steam shower" and therefore said you only need to run the Kerdi up so far. Not! if it's a steamer the entire inside needs to be covered. I suggest using regular 5/8" drywall and be sure to use plenty of screws.

- pete

02-28-2008, 08:12 PM
Great, appreciate the feedback and tips. I'll go with the 5/8" drywall. Much nicer option than CBU to work with -


Brian in San Diego
02-28-2008, 09:03 PM

Good catch! I did miss the "steamer" part. Forget everything I said 'cept the part about the greenboard. You know you need to slope the ceiling, right? Otherwise you're gonna have water dripping on yer head.


02-28-2008, 09:28 PM
why would you use green drywall? l understand that the kerdi should protect it but why take that chance? why use anything that could create mold in the future? why wouldn't u just use cement board? l am new to tiling so if l am way off base here someone please let me know the reasons why so l can learn as much as l can.


02-28-2008, 09:36 PM
you can use regular drywall because no water should ever get to it if the Kerdi is done correctly.

02-28-2008, 09:43 PM
Welcome, Robert. :)

Don't wanna get into a long discussion (or even a short one) on someone else's project thread, but the Kerdi membrane is part of the Schluter Shower System, which is approved for use over plain sheetrock walls. You might wanna do a little research on it and then perhaps search in the Professionals' Hangout for some of the many discussions we've had there. Start here ( for the basics of the system.

I personally wouldn't do a steam shower using sheetrock, but I do regular showers that way and never look back. Izza good product. :)

My opinion; worth price charged.

02-28-2008, 10:31 PM
Hi Brian,

in the process of framing the ceiling, going with an A-frame lid with a good pitch.

I've got myself thinking about drywall again, since it's not a very large area I might go with something like durock, a little more $, but may keep the city inspector happier at the end. Not that I dispute drywall will work provided I do a good job with the Keri.

I assume you would tape the seams as normal on the drywall or backer board, then apply the non modified thinset.


02-28-2008, 10:42 PM
Alkali Resistant Mesh tape

Brian in San Diego
02-29-2008, 12:35 AM

I was of the opinion CBU should be used in a steam shower under kerdi, but the Schluter folks do not make a distinction in a residential shower application. It won't hurt to use a CBU. Just be sure you get the CBU damp prior to putting up the kerdi. There's another thread going on right now where a member in Hawaii was having trouble getting the kerdi to adhere to the CBU. He did some searching and found the recommendation to apply SLC primer to the CBU first. He did so and he said the kerdi went up much easier. it may be worth a try if you decide to use CBU. Like my namesake said, use alkalai resistant mesh tape and thinset for the seams. If you use drywall, I would not tape and mud the seams. Fill the seams with thinset when you put the kerdi up.


02-29-2008, 11:44 AM
If you're going to use Kerdi, Herr Schluter says taping is not necessary, but it won't hurt anything. With drywall, Schluter recommends NOT taping with joint compound.

Another tip for Kerdi and drywall, fill the tape beds with thinset, allow to dry, then Kerdi. :)