sWe

I think you might find this post informative. It is about some missconceptions about waterproofing, epoxy, and tiles.

What the manufacturers call "waterproof" cementious adhesives, may have been modified with additives which make them absorb less moisture. Still, they're not waterproof per se. I haven't run into any such adhesives, atleast.
That, or they've simply confused waterproof with water resistant, and I think is the most common explanation.

There are waterproofing compounds which are cement based, but they aren't vapor proof, which means moisture will still be able to travel through it. Those products are normally used in external applications, such as outdoor pools.

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