He might be referring to "hydraulic cement". Hydraulic cement expands a bit as it cures, it's often used to seal foundation punch-throughs; holes though foundations for plumbing, electrical, septic, and cracks in foundations, etc. "Would you use Wedi, then Kerdi membrane over it (for the steam)? Or is hydroban also ok in a steam shower? " As a stand-alone product in a steam shower, again, Wedi is not appropriate when used with typical tile setting materials (portland cement based thinset and grout). To make Wedi vaporproof you'll need a vapor barrier. That's why Wedi's instructions specify using epoxy adhesive to set the tile and epoxy grout when using Wedi in a steam shower. The epoxy will be your barrier. If you don't use epoxy then you'll need some other sort of vapor barrier within the wall system. Wedi ($$$$$) plus epoxy adhesive ($$$) plus epoxy grout ($$$) is an expensive way to go in a steam shower. Kerdi membrane is a vapor barrier by itself. But Kerdi membrane over Wedi board is not an efficient use of your monetary resources. Anyone offering that idea? They're not looking out for your best interests. Regular old 1/2" Durock (or equivalent) cement board covered with Kerdi membrane will give you a water and vapor proof system at a fraction of the price (roughly 1/3rd the price) of Kerdi over Wedi. And you can use portland cement based thinsets and grouts. The same with Hydroban. Hydroban by itself is not a vapor barrier. If you use Hydroban in a steam shower you'll need a vapor barrier somewhere else in the wall structure. Hydroban, however, is a fine waterproof membrane for non-steam showers. So for the products you mentioned; Wedi, Kerdi, and Hydroban...for the "best performing" structure with most efficient use of resources, for a steam shower I'd use Kerdi over cement board with portland cement based thinsets and grouts. For a non steam shower I'd use Hydroban over cement board with portland cement based thinsets and grouts. If you really want to use a Kerdi "Board" or Wedi "Board" for the sake of using one of the fancy and expensive "boards", then for the steam shower use Kerdi Board which is water and vapor proof. For the regular shower use Kerdi Board, or use Wedi Board which is only waterproof. If you really want to use epoxies, then have those as well. Epoxies are generally overkill in residential construction. My opinion. In a commercial or in a very often used residential shower or steam shower, they may have benefits. But don't let them up-sell you on those products just because they are the "latest and greatest". They are fine products. They may have their advantages for certain design requirements. But they can also be inefficient, and even a liability, for other design requirements. Realize that the vapor-proof and water-proof comments I made regarding the specific products mentioned are fact. But my recommendations are simply my opinion. I have no aversion to spending more money if required for better construction. But I'm not one to waste money.

The trim is what completes the installation and gives the wet location listed fixture housing a "shower" rating. Shower trims have gaskets. You can look up brands and model numbers on most manufacturer websites. There is no differentiation in the NEC between shower and steam shower. Off on a lighting tangent... The NEC does recognise a difference between bathtub and hydro therapy bathtub because of the steam produced from a jacuzzi and requires wet location fixture above a jacuzzi but not a regular tub (non shower type).

@avaaustin The process of attaching fiberglass mesh to ceramic/porcelain tile differs from natural stone processing. Mesh on the back of ceramic mosaic usually doesn't have a layer of glue between the mesh "cells". At same time, the main reason of "resin backing" attached to natural stone is to reinforce it and fiberglass mesh is attached using a "glue" that is mostly polyester. It, first, adds an impervious layer to the back of stone and can significantly reduce the evaporation. Secondly, there could be problems with how well the bond is between the mesh and the substrate since dry set or polymer-modified mortars don't adhere to it very well. True porcelain tiles, whether sealed or unsealed, have >0.5% water absorption. At same time, marble is a translucent stone that will still absorb moisture as vapor even after an impregnating sealer is applied. I can't tell you whether Bulletproof sealer will help you in your situation. With properly installed water-in, water-out system and stone with no "resin backing" it is best to avoid any sealer. That's my strong opinion. Sealing the stone but not sealing the grout will not make any difference. Impregnating sealers are all considered permeable but if there is trapped moisture under the stone unsealed grout won't help.

That is a warrantyless frankensystem. Big red flag. Schluter DS is the only membrane with a low enough perm rating for a steamer. Not standard Kerdi. Sounds like a seat of his pants, not really knowledgeable installer.