Steam Shower help needed [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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10-05-2006, 01:56 PM
I have been reading this site for the past few days and it is great! I am not a professional tile installer just a DIY'er. I am installing a steam shower in a master bath addition to my home. My municipality does not allow sheetrock to be used as a backer for any tiled surfaces and I will be using 1/2" wonderboard on the walls and ceiling with a glass door/transom combo. My question relates to vapor and waterproofing.

I planned on using Kerdi but it is quite expensive. Also given that I am using Wonderboard I don't know if Kerdi is overkill/unnecessary. The Laticrete website claims it is suitable for steam showers when using a vapor barrier behind the CBU. Has anyone done this and, if so, how did it work out? I don't want a lot of steam trapped in the walls. I was planning on using poly over the wall studs, then the wonderboard, then the Laticrete waterproofer. I am using a solid surface shower pan with a built in tile ledge so I can run the poly sheet down into the pan and cut it off.

My other question relates to insulation. The steam shower is on the second floor and its ceiling will have to be insulated. What about the other three walls? Should I insulate those and, if so, with or without the paper vapor backing on?

Thanks for any help/insights anyone can provide.

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10-05-2006, 02:33 PM
Do you have a first name for us? We all like to be freinds and don't want to refer to you as Fool.
I would insulate all the walls and use the poly vapor barrier. I have used the Latacrete 9??? stuff often but never for a steam shower. I'm sure that it will be fine. I do think though that when you look at the price of a five gallon pail of the stuff and the fabric the price might be more than a Kerdi Kit.
Keep all of your posts here and we will look forward to following you project.

10-05-2006, 02:49 PM
Thanks Jerry,

I'm Paul, and I think that I am overanalyzing this whole project to death! I first thought I would just use the paint on waterproofer - Redguard- but some posts on this site suggested that might not be a good idea with a steam unit and possible vapor transmission. Fine, I thought, I will just put some poly behind the concrete board - problem solved. However I then read concerns about sandwiched moisture....Sigh.

Here are my options:

1. Poly, CBU, RedGuard (in that order)

2. Poly, CBU, Laticrete 9235

3. No Poly, CBU, Kerdy

I think #1 is the cheapest, I'm not sure about #2 vs. #3. I did read that putting Kerdi over wonderboard or other cbu's was a little more difficult and if using cbu's (which I have to because of our local building code) then Kerdi wasn't the best option. Feedback on these options.. Please.

Many Thanks!!

10-05-2006, 03:07 PM
Welcome, Paul. :)

I'm one who takes serious issue with using something like Laticrete 9235 on the inside of a steam shower and a poly barrier behind it. Not sure exactly how their reasoning works on that, but I think it faulty.

My reasoning is that if the 9235 is not a vapor barrier (if it were it would be approved for steam showers on its own) it will allow vapor to penetrate into the wallboard when the steam shower is in use, but is enough of a barrier not to let the vapor pass the other way when not in use and the temperature differential is much lower. But the vapor that has penetrated also cannot escape behind the wallboard because a properly installed poly sheet (no unsealed penetrations) is a vapor barrier. So you get a moisture sandwich. And I don't care what's in the middle, that ain't a good thing.

The RedGard is approved for use in steam showers, but it must be applied in the proper thickness all over and that is not an easy thing to accomplish.

The Kerdi apparently works in steam showers but nobody seems to be able to show me where it says it has a perm rating of less than one, which would make it a vapor barrier.

Noble Company has a membrane that is recommended for the walls of steam showers, too. Their site also has a nice diagram of the proper method of construction. It's more complicated than building a regular shower.

None of the materials, properly applied, is inexpensive and that really shouldn't be the deciding factor in your construction. The cost of the membrane will be a very small portion of the overall cost of the steam room.

Steam showers are a constant point of contention hereabouts, and there aren't many agreements to be had. Fortunately, residential steam showers are usually not presumed to get enough use for the various methods used to see a lot of failures. Or at least we don't hear about them.

On the insulation, yes, you want as much as you can get in all the walls and ceiling, especially if you have any exterior walls.

My opinion; worth price charged.


Schluter's website does now have the perm rating for Kerdi clearly listed on their website here. ( Says the perm is 0.75, clearly a vapor barrier.

John Bridge
10-05-2006, 04:42 PM
Hi Paul, :)

Since you're going with backer board, use the RedGard without a barrier behind the board. As CX mentioned, residential steam showers do not recieve heavy use.

After the shower has been used, wipe it down with a towel and leave the shower door open. I think you'll be fine.

BTW, Kerdi is approved for steam showers. :)

10-05-2006, 05:47 PM
Thanks for everyone's input - it is appreciated as I certainly don't address these issues on a daily basis.

I am going to review the redguard info one more time and then decide between it and the Kerdi. I did purchase and read John's book on Kerdi showers - very well written and explained. I am starting to wonder whether I should skip the wonderboard and use hardibacker instead (smoother surface) and then use the Kerdi. I went over the costs again and it really isn't that significant a difference.

I don't think a got a specific response yet to the insulation issue. I was planning on using the same R-19 batts that I am putting in the exterior walls (except the ceiling - that gets R-31). Those have the vapor barrier facing on them. Should I remove the facing before installing them in the shower walls?


10-05-2006, 05:49 PM
Hi Paul,

Don't bother removing the facing -- just install it and then cut a bunch of slits in the facing with a utility knife. That'll do the job.

10-05-2006, 07:01 PM
Thanks to our friend Jim Carlin (JJC) who sent me this link ( to the page on Schluter's web site where they now clearly indicate the perm rating of Kerdi is 0.75. That is, of course, below the 1.0 required to be a vapor barrier. Lest there be confusion, smaller number is better here.

In past the description was always, "...polyethelene waterproofing membrane with a very high steam-difusion density."

Nice to see that number in print. :)

My opinion; worth price charged.

10-05-2006, 07:54 PM
We have hung miles of Kerdi on Hardi. Make sure that you wipe it with a damp sponge prior to application of the mortar for hanging the Kerdi if you go that way.

10-05-2006, 08:00 PM
On the insulation, yes, you want as much as you can get in all the walls and ceiling, especially if you have any exterior walls.
I would buy unfaced insulation, Paul, but what Marcus suggested will work, too. But I pull the entire Kraft backing off the batts when that's all I have to work with at the time.

My opinion; worth price charged.

10-05-2006, 08:31 PM
Thanks again all. This board is great. It's the little details that help. Once I get underway I'll try to post some pics and follow up questions if any arise.