I am remodeling a bathroom and I want the shower to be flush with the drywall so a chair rail can run all the way through it. A tile guy wants to do a mud set right on the studs. Is this possible in the 1/2" thickness of the drywall so it can be flush? He would put tar paper then the lath, scratch coat and mud. It would have a vinyl pan (he says that is easier to do with mud walls vs hardibacker) and he said he would put Redguard on the walls up to about 3 ft off the shower floor for extra waterproofing protection. Would that create a "moisture sandwich" with the tar paper as I have read about, or is it OK if the redguard doesn't go all the way up the walls? I find the whole distinction between vapor barrier/moisture barrier/waterproof confusing.
Also, are mud set walls worse for efflorescence stains on the grout than hardibacker with redguard on top?
Thanks!