Hello All,
I’m working through a basement bathroom remodel.  It’s a small floor plan which forced me to put the shower into an exterior corner.  Thus two of the three walls of the shower surround are up against the 8″ poured concrete foundation.   

I got through the framing stage with what I believe is correct construction for my climate (zone 3A – Atlanta, GA) by putting a layer of 1″ XPS foam sheathing for a continuous R-5 to meet code on the wall I had to reframe.  This wall contains the supply plumbing for the shower mixing valve and showerheads.  The second wall didn’t need to be reconstructed as it already was well built and had a 1″ gap between the concrete and the back of the studs.  I’ve applied a continuous 1″ layer of closed-cell spray foam to that wall including between the studs and the concrete.  The plan is to install R-15 mineral wool batts between the studs and then hang 1/2″ cement board to the studs and then apply an edge to edge layer of liquid applied waterproofing (like RedGard) as my water control layer in the shower.  

My question is two-fold:

1.) With the layer of foam against the concrete and the liquid applied membrane on the interior side of the cement board, it seems like I’m at risk of creating a moisture trap, should any moisture get into the stud cavity, it can’t dry out in either direction.  On the other hand, there shouldn’t (in theory) be that much opportunity for moisture to enter into that space.  Should I be concerned?

2.) For my climate area, with most of the poured concrete walls being below ground, I figure that the approximate surface temperature of the foam insulation would be about 53 F with the mineral wool batts installed. That gets awfully close to my assumed dew point temperature (70 F dry bulb @ 55% rel hum = 53 F dew point).  I do have a dehumidifier installed for the whole basement so I could turn down the set point but don’t want the thing to run all the time.  I am in the humid South.  It is a bathroom that will be used by teenage boys who are not known for necessarily cold nor short showers.  To that end I do have a 110CFM exhaust van installed directly over the shower with a fancy humidity sensor for auto on/off.  Am I better off not installing the mineral wool batts so that the surface of the foam sheathing is that much warmer and therefore well above dew point?

Thanks all!

Matt