I am redoing our master bath. We completely gutted the room, and I have two exterior walls with R13 kraft paper faced insulation. Should I add a 6 mil plastic vapor barrier to the walls? Would it trap in moisture, or help as an additional vapor barrier? I'm thinking it will cut down on any leaking drafts as well. I have two large headers for two windows that have no insulation behind them (no room) I'm thinking the plastic will cut out any drafts. I have the same kraft insulation in the ceiling also, and want to do that with plastic as well.
Also is R13 the maximum I can use in a typical 2 x 4 studded wall?