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Unread 08-04-2020, 02:37 PM   #1
Join Date: Aug 2020
Location: Los Angeles
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Wall/floor & wall/wall connections for foam panels

First, John, thanks for an amazing resource. I have learned a lot here.

I am a DIYer gearing up for a home bathroom remodel. I am leaning hard towards foam panels, either Kerdi or WEDI. There's an alcove shower/tub with an (fiberglass) awning window, which is going to be tricky to keep watertight.

My question is about how to correctly connect panels and floor membrane at wall/wall and wall/floor seams. I am in southern California where future seismic activity is guaranteed. The home is a century old, and already settled a little bit in some places. I want keep the stud cavity dry of course, but I also want to allow for some flex when the walls move relative to each other and to the floor.

On the tile level, my understanding is that this is best accomplished with sanded caulk, tinted to match the grout one is using.

But what about the panel/floor and panel/panel connections? Kerdi and WEDI manuals both seem to advocate for butting panels tightly where walls meet. Kerdi suggests membrane tape with thinset. WEDI suggests a bead of their joint sealant. I am super skeptical of using thinset as a water barrier. I'm also concerned about some recent tests with Kerdi that show moisture migration along the fleece. It's painful to watch Youtube videos with seemingly well-built systems that go rotten in a few years due to a few little pinhole leaks. It's hard to find a perfect solution here, but I am leaning towards WEDI despite the high material costs.

There is also the question of how close the decoupling membrane on the floor should get to the walls. Should the membrane tuck under the wall panels? Or should the panels touch the subfloor? I have seen different recommendations for gap allowance.

Thanks in advance for any insights on this.
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Unread 08-04-2020, 03:58 PM   #2
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You can attach KerdiBoard panels using KerdiFix if you wish. That (probably) would have about the same properties as the Wedi sealant. KerdiBoard itself is waterproof...the outer layers are to help add stiffness and be a better interface to Kerdiband, or Kerdi membrane. YOu then would normally still use Kerdiband across the junction.

Once you understand how a Kerdi seam works, and you have prepped things properly, it's not hard to make a waterproof seam. Several keys to that. Using a quality thinset that is mixed properly, dampening the surface prior to spreading it so that the moisture content of the thinset doesn't get compromised making it dry out and not be able to flow into the fleece, or leaving too much thinset between the seams. Well, a pucker or gap would be an issue, too!

Kerdi obtains its water tightness by the interaction of the fleece to fleece proximity...it needs to be close with the thinset flowing around the fleece well. That requires a quality thinset that has uniform particles (small, too) that is mixed with the proper amount of water according to the instructions. And, you can't let things sit on the wall to dry out prior to trying to embed things. Schluter could have just used KerdiFix, but the cost would go way up...thinset is much cheaper, and works. Plus, it's a lot easier to clean up thinset than excess KerdiFix which sticks to nearly anything! I've watched Wedi being installed, but not done it myself, but it looked like their sealant was just as sticky.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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Unread 08-04-2020, 04:05 PM   #3
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Welcome, Westy.

1. The direct bonded waterproofing membranes that use thinset mortar to bond fleece covered sections work if they're done correctly. Yes, Isaac has some videos online that would appear to indicate that they don't work, but there's gonna be a flaw in the installation causing that. I've seen too many cardboard box "ice chests" lined with those membranes to believe they can't be made waterproof.

As for your movement issues, I don't think you can hope to accommodate an earthquake with either of the systems you mentioned. With the Kerdi, you would have the option of using their Kerdi-Flex at the joints. Would that be enough? I dunno. I've never used the stuff.

With the Wedi you depend upon a bead of sealant between the panels to provide both the waterproofing and any movement accommodation. I think much less of a chance for that to accommodate any serious movement.

Best way to accommodate earthquake movement in your shower construction would be to construct your showers someplace other than southern California, eh?

Don't understand your question about the "decoupling membrane." You are talking about an uncoupling membrane such as Schluter's Ditra? If so, are you envisioning that in your shower or elsewhere?

My opinion; worth price charged.

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Unread 08-04-2020, 04:09 PM   #4
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It's also important to realize that a flood test is meant to find leaks. If you have a properly installed membrane seam, it won't leak. Flooding it for a day or two won't hurt it. But even that is far above the amount of pressure that your seams will be subjected to in a residential shower.

As for absorbing movement, everything has it's limitations. I would lay my money on Kerdi band to hold up better than a sealant in the corner.

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