A continuous air barrier is more crucial, though in a high-humidty environment like a shower a vapor barrier is wise as well. The 6 mil poly can serve both functions as long as there are no unsealed penetrations and the poly is sealed at edges and seams.
Laticrete 9235 is a water barrier but not a vapor barrier (3 perms). You might use it on the shower base (if that is tile), but I would use a vapor barrier on the walls and make sure there is a quiet, efficient exhaust fan operated by a short-term timer.
What kind of double wall system are you using? Can you share a cross-section?
- Robert

--- On Thu, 12/18/08, Tim Yandow <[log in to unmask]> wrote:
>I am interested to know if anyone has an opinion about or experience with
the following situation:
I am currently constructing a 2000 sq ft double wall, dense pack cellulose
house. Due to a number of factors, an upstairs shower stall, 4' X 4'
up in an outside corner of the house (north west corner). I know this is
not a great spot for it, but there it is. The tiler would like me to place
a vapor barrier over the framing before the hardy backer and tile go on.
There is 12 inches of cellulose in the walls with a thermal break (2- 2x4
walls) behind the shower. This I presume is to keep moisture from the
shower away from the insulation. I am wondering if this is a good idea
given the porous nature of tile and hardy backer. Any input? Thanks.
Tim Yandow

I am not sure what makes the most sense here