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Unread 12-31-2021, 09:10 AM   #1
jeffnc
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Thinset for Kerdi products over wood/drywall

This is something I've wondered about and never really figured out. (I was actually going to ask when I made my last call to Schluter technical support about something else, but the person I got on the phone was so clueless about technical issues that I didn't bother.)

To me, the key to understanding what thinset to use has always been whether the thinset can dry out in a reasonable period of time or not. Tiling over Kerdi membrane - unmodified because the thinset is trapped between 2 waterproof (or nearly so) surfaces. Ditra over plywood - modified thinset because the thinset can dry through the open channels.

But there are 2 applications that I found confusing - Kerdi membrane over drywall, and Kerdi tray over plywood. In both cases they seem to like the idea of dampening the substrate with a sponge before installation. OK that makes sense - you don't want the moisture wicked out of the thinset too quickly. But to me, that is the point: moisture is definitely going drawn into the substrate. (Obviously they're not recommending dampening Kerdi membrane with a sponge first.) So why wouldn't they recommend a modified thinset here? It will obviously dry out before long.
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Unread 12-31-2021, 09:39 AM   #2
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That's thinset mortar you're discussing, I believe, Jeff. Thinset is a method of tile installation.

Lots and lots and lots of discussion on the forums. Type unmodified into the Advanced Search feature and limit the search to the Hangout and you'll find a bunch to get you started.

The problem, as Schluter describes it, is that thinset mortars that are modified with latex polymer require drying of the latex along with the hydration of the cement in the mix and that is inhibited by being sandwiched between two impervious surfaces. It can, in the case of large format porcelain tiles over an A118.10 membrane, indeed take a long time (weeks, months) for the mortar to actually dry all the way to the center of such tiles.

But not all modified mortars use a latex modifier. And other modifying polymers are not as slow drying as the latex types. And in many applications it's not really gonna be harmful to have the mortar in the center of the large format tile still drying for a long time. Indeed, you'll note that all other manufacturers of A118.10 membranes require the use of a modified thinset mortar with their products, as do all manufacturers of porcelain tiles.

Many thinset mortars these days do not use a latex as their modifying polymer and many don't even use Portland cement. Not really practical, in my view, to limit the use of all A118.4 mortars because a few may have problems drying between impervious surfaces. And for many applications, it won't really make any difference just how long the final drying process may take. Yes, might still be moist and soft in the center of a 12x24-inch wall tile for a long time, but it's not gonna fall off the wall and won't likely interfere significantly with the grout application.

But if you want the Schluter product warranty, you must use the bonding mortar they recommend. If you don't want to be limited to unmodified mortars, you'll use some other membrane. Simple as that.
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Unread 12-31-2021, 09:51 AM   #3
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It doesn't appear you understood my question.

Drywall and plywood are not "impervious" so I'm not talking about sandwiching anything where it can't dry. I'm not talking about tile installation at all. Thinset can "dry" into plywood and drywall, just like paint can.
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Unread 12-31-2021, 10:03 AM   #4
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Yep, you're right, I'm missing your point.
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Unread 12-31-2021, 03:09 PM   #5
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Unread 01-01-2022, 10:04 AM   #6
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Well I didn't get that last post either.

The point is pretty simple I thought. When you are applying Kerdi membrane to drywall, you are using thinset. The Kerdi is impervious to water, but the drywall is not. Drywall sucks up water like a sponge. And thinset can dry through the drywall. So why do you need unmodified thinset here?

Same concept applies to installing a Kerdi shower tray over plywood.
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Unread 01-01-2022, 10:24 AM   #7
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When applying Kerdi to drywall, you are using the thinset method by applying thinset mortar. Yes, the drywall surface will absorb some of the water from the mortar, which is not necessarily a good thing, but so long as it leaves enough for the required hydration of the cement in the mortar, all will be well. Once again, there are thinset mortars modified with latex polymers that require drying along with the hydration of the cement. That could take a long time when sandwiched between the impermeable membrane and the drywall, but it will eventually dry. Schluter's position is that use of an unmodified mortar eliminates that "problem."

When bonding the foam shower tray to a plywood subfloor, a modified mortar meeting the requirements of ANSI A118.11 is technically required and even Schluter recognizes that. The only reason they allow - not require, but allow - the use of an unmodified mortar in that application is to simplify the use of their products and not require the installer to have both unmodified and modified thinset mortar on site to build a Kerdi shower. It's not sanctioned by the ceramic tile industry, but so long as the manufacturer recommends it, it's an acceptable practice.

Yes, it's all quite simple.
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Unread 01-01-2022, 10:41 AM   #8
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Your comment about "thinset mortar" vs. "thinset method" notwithdstanding (virtually everyone calls it thinset), it still doesn't really answer the question satifisfyingly.

I'll try to avoid using the term "required" vs. ""recommended", I'll just say that if it is written in Schluter's instructions, then they "instruct" you to do it.

They don't go too far to help you avoid using 2 different thinsets because when installing Ditra over plywood, they instruct you to use modified thinset for the Ditra installation, then unmodified thinset for the tile installation.

I don't think the slightly longer drying time for modified thinset is any more of a "problem" than the too-fast drying of unmodified thinset into plywood or drywall. After all, modified thinset is the standard in the tile industry (with Schluter products basically being a special case), and modified thinset is used constantly to install ceramic and most notably porcelain tile over plywood, cement board, and cement, so the drying of it can't be very much of a problem.
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Unread 01-02-2022, 11:53 AM   #9
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I don't understand why modified isn't required for their shower pans but Schluter has sort of "fixed" this by coming out with their own modified mortar, All-Set, which can be used basically everywhere.

So, you can still but just one product and not have to worry about switching. Plus, I've heard it's pretty good pookie.
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Unread 01-02-2022, 12:06 PM   #10
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All Set is allowed for everything Schluter, but it's quite expensive. If I didn't have a contractor discount I definitely wouldn't buy it. It is good though, but not every application needs something that good. I understand where Schluter is coming from - they can't be responsible for every other manufacturer's formula. But like I said, that's not really the issue I'm curious about.
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