Okay, it took me awhile to get this, but by permission, I'm quoting an e-mail I got from the TCNA rep. Basically, they no longer recommend gypsum board underneath a waterproof membrane in a shower construction because not ALL membrane or board manufactuers do. But, they DO allow it IF both the backer board and membrane manufacturers are in agreement. In the case of Schluter and Kerdi, Schluter has paid an independent testing agency to verify their claims that it works, and done properly, they will warranty the work. Since the TCNA has no general agreement from manufactuers of other products with the same general characteristics, they do not make a generic recommendation, but reference it to basically, follow the manufacturers instructions. Now, anything can fail if you don't have good workmanship. Here's the quoted e-mail info:
"Hello Jim,
While some membrane manufacturers promote use of their
membrane in a B422 installation with gypsum board (W243) as the solid backing,
other membrane manufacturers do not. At the same time, not all manufacturers of
gypsum board recommend their product be used in a B422 installation. Thus, the
Handbook leaves the determination of a suitable membrane/backer board
combination up to the membrane and backer board manufacturers to declare and up
to the design professional to select/specify. Ideally, one should find
recommendations from both materials manufacturers that agree. However, as you
are aware, manufacturers do put forth proprietary recommendations, which the
Handbook does not address, such as in the case you are asking about.
Many in the industry feel that building code prohibits the use of gypsum
board in wet areas, even when a waterproof membrane is applied to the surface.
However this is a matter of code interpretation, with the actual code language
somewhat ambiguous, and with some feeling the code only prohibits direct bond of
tile to gypsum board in wet areas.
At the Handbook meeting last month,
the Handbook Committee voted not to include a shower method where a mortar bed
(cleavage membrane, metal lath, and wall mud) is installed over gypsum board.
This is different from the method/materials you are asking about, but I am
mentioning it to provide some perspective on recent consensus out of the
Handbook Committee regarding use of gypsum board in wet areas. Again, the
methods are different; in no way do I mean to imply the Committee feels the same
about gypsum in a shower if a waterproof membrane were applied. What I can say
is, the amount of water/steam exposure generally is raised as a factor for
determining suitability in discussions of this topic.
For further
information, you may also want to contact the Gypsum Association.
Best
regards,
Stephanie Samulski"