QUESTIONMy shower stall has two sides each with 1/2″ USG ultralight foam board installed, which is waterproof. I plan on installing 2 rows of 12″ x 12″onyx mosaic tile in center of each wall with 12" x 24" porcelain on each side of the mosaic tiles. The mosaic tiles have a mesh backing and can be flexed open in either direction, and look like this:
I plan on installing 12" wide Schulter Ditra membrane on top of the foam board using Laticrete 254 Platinum thinset and installing the onyx on top of the Ditra also with the 254 Platinum. The reason for this is so the onyx will be flush with the porcelain on each side. The 254 is made for use in swimming pools.
Someone told me that because there is no room between the rows to put the grout that mold would form between/behind the onyx even if the onyx is sealed because sealers are not a miracle product, even with the 511 impregnator sealer I said I would be using. He also said onyx is sensitive and will be hard to clean, darken/etch/stain over time when exposed to water.
I am asking you to get a 2nd opinion - will the problems mentioned above happen to the onyx even if using a sealer such as 511 impregnator? If so, is there another sealer that would prevent those problems?
ANSWERANSWER - First of all what happen to the the 3rd wall in the shower? I will assume it has a different substrate or you aren't tiling it.
Not sure why you are using Schluter Ditra on a wall. It is a uncoupling membrane that is primarily used on floors. Plus you already have a waterproof foam backer board if you install it to be waterproof. If you are trying to adjust the level of the wall surface because the thickness of the mosaics is less than the porcelain tile by using it, it would be more practical and less prone to problems if you install the porcelain tile leaving out the mosaics, but leaving space for them. Then skim coat that area with thin-set, which should not be more than 1/4" thick. Let it cure and the adhere the mosaics over it.
The stone and tile industry standards always say never butt tiles together due to potential expansion problems. Although some imported mosaics come that way. For an accent strip it might not be a problem. Sealing the onyx will help make it easier to clean, but it won't keep debris or soap scum to get into the tight joints. When you have organic material and moisture there is always the potential to get microbial growth if it isn't kept clean. I don't see it as a problem as long as you keep it clean and maintain it.