Schluter's Kerdi membrane is acceptable for this steam shower if your tile selection allows for un-modifed thinset. If not then the sheet membranes from Dural, Wedi, Dal Tile and Noble are all good options. If you prefer working with Kerdi and need a modified thin set (most glass tiles and large porclean tile require it) you can use certain thin-sets from Ardex and qualify of an extended Kerdi Warranty. Contact Bob Russell from Ardex on the details. Robert Russell Robert.Russell@ArdexAmericas.com
Personally I prefer Noble Companies Noble Seal TS - it is best in class when it comes to Perm ratings and has a perm rating of 0.05 Perms. With any of the sheet membranes other than Kerdi you can use any modified thin set. Modified thinsets are required in Canada (as per the TTMAC guidelines) but not in the USA if you follow the TCNA guidelines.
The picture above is from the Noble Company website. Noble Seal TS can be ordered direct from the Noble Company online store and ships at a flat shipping rate of $5.00 in the US I believe.
Insure that all the penetrations in your steam shower are properly flashed and that the edges are sealed with the Noble Sealant 150. Sheet membranes wick moisture and this sealing of the edges will prevent water from wicking straight to the edge.
Has your shower already passed a flood test? Have your water lines been pressure tested? Was your steam line insulated?
Here is a peak at some Noble Seal TS being installed. Durock like many backer boards are thirsty so installing the sheets it's important that the thin set does not dry out to fast. We use a modified setting material and first wipe down the backer board to remove any dust or bond breakers. Then we load up the thin set a little heavy and let it dwell until all the surface has been burned or scratch in. Once we are ready to set the membrane we trowel it and install full sheets (5' wide). It is easier to do with two men and easier still if you work with these sheet membranes on a regular basis. I like to recommend to people not experienced in sheet membrane to work in smaller sections and work with a helper. You could do the entire surround all three walls in one go but this is too much. To give yourself more working time consider scratch coating the walls first with a modifed thin set and let this dry overnight. The next day you will have a much longer working time.