Mtcummins you seem to be a little slow understanding the TTMAC and ANSI Specifications.
Neither of the industries top organizations include drywall in shower construction. When drywall is referenced it makes further notation to include the words "Dry Area Only".
In the shower receptor drawing details as well dry wall is not listed.
So to repeat.
You can use water poofing over drywall, Kerdi, Hydro Ban and Aqua Defence etc. - just not in showers. Schluter does have men working in these orgaazations and if drywall was accepted in a shower they would have included it in the manuals. But they didn't because it is not accepted.
Your in Pittsburg, PA - in Pittsburgh your local Bureau of Building Inspection - (412) 255-2175 can offer you more guidance on proper shower construction procedures. I spoke with them this morning and Pittsburgh follows the International Building Code. The International Building Code removed drywall from acceptable backer boards in showers a few years back - I guess you missed the memo. If you order that code book you will not find drywall listed under shower backer boards. Maybe try emailing the building department and asking them if it is OK. BBI_DPWRequests@city.pittsburgh.pa.us
I got a hold of Eric in the Inspection Burea and we discussed this. I was told that in Pittsburgh a single family home would fall under the IRC. Under showers and tubs it mentions something like a non-absorbat backer board material is to be used.
Dry Wall is not non-absorbant. We discussed vapour barriers and his take on them and at the end of the call I asked him what he would use in his own shower if building new. Eric said cement board or wonder board.
I would think this very loose text in the building code would open the doors up to many crazy designs.
If you read this Document from Gypsum Nexus you might fell a bit better about not understanding all the new code changes. At the bottom of the first page on the right it makes mention that Green Board (water resistant) drywall is permitted in a bathroom renovation but not in the "Wet Zone".
So you have the people that make drywall say No - drywall is not allowed in wetzones of showers.
You have the International Building Codes change to exclude drywall from the Wet Zones
You have TTMAC and ANSI repeating the fact that drywall is not to be used in wet zones
You have the IRC stating a non-absorbat material is to be used.
And you have Schluter (and others) saying drywall is an approved substrate. Of course they say that because sometimes people like me like to waterpoof more than just the wetzone and we bring our membranes out past the backer boards.
I hope this helps.