Insulation

I live in central texas and have a finished cathedral ceiling bathroom over a two car gargae.  In remodeling the room I am thinking about placing foam insulation on the roof decking then placing drywall over it.  I want to do this also on the walls of the bathroom and under the subfloor, via access thru the garage, to create an envelope to reduce heat/cold from comming into the bathroom.  I have heard that placing the foam insulation directly onto the roof decking and/or exterior walls will prevent any condensation from drying and thus lead to deterioration of the roof decking.  Is there any truth to this and if so is there a way to prevent this.  Also, what type of foam insulation would you use and how deep should it be.  I have 2x8 rafters, 2x10 floor joists with standard 2x4 walls.  I appreciate any advice you can give.