I’m preparing to tile a Schluter shower system and have received different opinions on what to use regarding thinset. The wall and ceiling tiles will be 12″ x24″ travertine. Schluter recommends installing Kerdi with unmodified thinset, followed by unmodified thinset for the tiles too. The tile supplier recommended modified thinset which they use when installing, with good results. I understand modified requires more time to properly dry, which I have. I also don’t want any mould issues in the future. Can anyone offer some real world advice on these conflicting recommendatons?
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I'd follow the manufacturer's directions. I haven't installed Kerdi but was under the impression that the thinset under the kerdi was one way and under the tile the other. IOW if unmodified under the kerdi then modified above it. or is it the other way around. . . anyway read the directions again.
Following advice given on the John Bridge forum, I used Versabond which is a semi-modified thinset. There have been numerous tales of woe by people who have used heavily modified thinsets, so think twice before using it. For more advice read up on johnbridge.com.
Thanks for directing me to that site, isn't the internet great? I received 8 replies in a couple of days from guys who strictly do tile.The problem is voiding the Schluter warranty, but if people who do these systems all the time have success with that method, I trust real world advice.
Most of the thinset manufacturers have rapid set modifieds, which my Schluter rep tells me are fine with their product when you are setting stuff where modifieds are recommended, like glass and porcelean. The rapid set I use behaves just like their other thinsets, has similar open time, but when it goes it goes fast.
You CAN use Versabond which is "lightly" modified. It's not to spec by the company but even John Bridge'll tell you, you can use it. I've done it countless times with success.
You use unmodified when the surface behind Kerdi or Ditra isn't breathable...like between tile and Kerdi.
If it's between Kerdi and a CBU or plywood or concrete then you can use modified. The moisture needs a place to evaporate/dry next/in/to but between unbreathable spaces you need unmodified.
From what I gather in the specs, unmodified dries without needing air(breathing), whereas modified needs up to 28 days depending on conditions. Since Kerdi is watertight and doesn't breathe, if you put modified on the substrate/wall then cover it with Kerdi, how does it evaporate? Also aren't certain tiles and grout breathable,porous? Is it all engineering gobbledygook? I just wanna get 'er done.
Once you set tile over Kerdi or Ditra, It can only dry through the grout joints, which is why it can take so long, depending on the amount of joints, their size , permeability of the tile, etc. Setting Kerdi over a substrate like plywood or CBU, the moisture can migrate into the substrate and eventually dry out the back side.
The rapid sets have the benefits of being modified, but have some sort of chemical trigger to accelerate the whole drying thing.