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Discussion Starter · #1 ·
I'm remodeling my walk in shower. I've removed everything down to the framing. The back shower wall is an exterior wall with fiberglass insulation and a vapor barrier. I'm using kerdi board (not the membrane). Can I install the kerdi board over the vapor barrier or do I have to remove it first?

I've heard about avoiding a double vapor barrier sandwich. Not sure if this applies when using kerdi board since it's essentially a vapor barrier.
 

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By vapor barrier, you are talking about plastic not the kraft paper? Remove it is your best option - in most cases you won't have any issues if you leave it, but you asked for best... As for kraft paper (it is a retarder not a true barrier), I would go with unfaced - if faced is installed & in perfect condition (yeah right) then leave it
 

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Not sure why you're referencing that video, since it doesn't seem to have anything to do with vapor barriers on insulation. Personally I'm not a fan of putting plastic in my walls anyway, regardless of showers, at least in my climate. I've seen more water get trapped and cause rot than I've seen water infiltration due to missing plastic barrier. I'm in the Southeast USA, you should check (maybe in a different forum) for Canada.

Anyway, with regard to the video, I've seen videos from him before and he's a bit goofy (he gets a little opinionated about things he doesn't fully understand), but mostly right. Here are some things he didn't get right:

  • It's not "cement", it's thinset, and they are different.
  • He implies Schluter won't warranty without KerdiFix, which isn't true. If using a caulk instead of Kerdi at some point, they just require a silane sealant if not using KerdiFix. (The stores probably don't carry KerdiFix because of how expensive it is. One caulk I use in place is NP1 Masterseal).
  • He says the reason water can't get past the Kerdi seams is because the gap between the Kerdi Band and Kerdi Board is so thin, which isn't true. The key to the Kerdi technology is the fleece on the surface, and the fibers block the passage of water. Strange, but it works. Thinset itself is not waterproof, and no matter how thin you make it if it was just thinset water could penetrate it just fine.

His points about not getting paranoid about waterproofing, at least in a tub surround scenario, are probably true. There is not standing water. The way he shows waterproofing with the caulk would probably work fine. Keep in mind that even Kerdi Fix or other approved caulk does NOT waterproof the same as Kerdi and thinset. (If you truly wanted to waterproof with the caulk, you would need to scrape OFF the fibers til you get to bare plastic, which is the opposite of what you want with the thinset.) So I would never do that in the bottom corners of my shower floor, or near the drain, for example. But high up on a shower wall I think it's fine.
 

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Discussion Starter · #6 ·
Not sure why you're referencing that video, since it doesn't seem to have anything to do with vapor barriers on insulation.
The back shower wall in the video is an exterior wall with fiberglass insulation with a poly vapor barrier. Same situation as mine. In the video at the 4:20 mark, he installs the Kerdi board over the poly.
 

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OK I missed the poly on first glance. You can google kerdi board over poly vapor barrier. Most recommend removing the poly. But I guess it's a bit more complicated than that if you're not going full to the ceiling.
 

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Out here , everything gets poly and inspected before anything goes in the shower, when the shower leaks it will find a way to get behind the poly, staples screws nails stop air but not water. 2 VB side by side have no ability to do damage with out air and moisture. This is much of a non issue.
 

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Discussion Starter · #9 ·
There is lots of info about removing poly when the Kerdi membrane is used over drywall or cement board. However my application is a little bit different using Kerdi board.

Neal, I think your comments makes sense. No issues with leaving poly behind kerdi board
 

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There is lots of info about removing poly when the Kerdi membrane is used over drywall or cement board. However my application is a little bit different using Kerdi board.

Neal, I think your comments makes sense. No issues with leaving poly behind kerdi board
What would happen if you put in a one piece plastic shower in front of poly, nothing it is done every day.
 

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What would happen if you put in a one piece plastic shower in front of poly, nothing it is done every day.
I think the issue would be if there is air movement back there, and humid air has a chance to contact colder materials. With the one piece shower, it's basically sealed off so it shouldn't be any problem. I think the worst case scenario would be if there were small air openings, humid air (such as from your bathroom after a shower) condensed in there, but then didn't have a very good way to dry out very well before the next shower. Not sure how likely that would be.
 

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You're talking about directly through the Kerdi Board, true. But you are not sealing it off like a steam room would be. So vapor might indeed get back there - through spaces not covered by Kerdi. Again, not sure how likely that would be.
True there would be a better chance for air movement around the plastic shower but what happens where the VP is against code.
I can not imagine air getting passed kerdi.
 

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Yeah code is code, and Canada is Canada. Here the problem will tend to be in humid summers, it is cooler inside than outside, so condensation could occur on the outside of the vapor barrier. In Canada the problem would tend to be winters, where it's colder outside so condensation could occur on the inside of the vapor barrier during a steamy shower, assuming humid air could get in that cavity. Maybe it couldn't.
 

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Yeah code is code, and Canada is Canada. Here the problem will tend to be in humid summers, it is cooler inside than outside, so condensation could occur on the outside of the vapor barrier. In Canada the problem would tend to be winters, where it's colder outside so condensation could occur on the inside of the vapor barrier during a steamy shower, assuming humid air could get in that cavity. Maybe it couldn't.
Or shouldn't if closed up right.
 

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so how did things turn out? I am in the same situation debating if I should leave or remove poly from outside wall in shower. Will be using densshield and Kerdi membrane. Since it is an old post maybe there is some new information and experiences.

Thanks team
 

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so how did things turn out? I am in the same situation debating if I should leave or remove poly from outside wall in shower. Will be using densshield and Kerdi membrane. Since it is an old post maybe there is some new information and experiences.

Thanks team
You might argue about installing it or not but taking it down would just be silly to me.
 
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