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?
Any high humidity area like a bathroom should have a good vapor barrier under the sheetrock. This barrier should be taped at the seams and ceiling forming sort of an air tight cell. Usually a kraft backed insulation is not a very good vapor barrier and has lots of gaps. To be on the safe side and avoid any condensation between the 6 mil and kraft, you could simply remove the kraft backing or cut several slices in it top to bottom. Usually it will not cause a problem unless you live in a very wet humid climate or the plastic is not sealed properly. The 6 mil is a much better material for your purpose than kraft backed.