worthy -- thanks for replying... unfortunately, I can not slide in the XPS behind the studs. By local code, we had to install fire blocking on the end of the wall. So now it is closed off. And, I can't install XPS in between the studs becuase I already have electrical and piping running through the studs. The gap between the wall and the stud is only a little more than 1/2". So even if I could slide in the XPS behind the studs, I would only get about r 3 value.... Thats why I figured it is almost better to put no insulation. The pipes arent gonna freeze; Plus, if I do get moisture back there, then there will be no fiberglass insulation to get wet. With no insulation, the wall will just dry out. (with the help of my dehumidifers and the bathroom vent fan, of course) If I get a chance, I will post a picture tonight of the studded wall with the roughed-in plumbing and electrical. thanks again. mm I'll try to post a picture later....

I've often written about foam being better than batts. In a concrete basement below grade there will be no air pressure differential to cause air movement from the outside in. Batts are OK here. The bathroom is not going to be sealed from the rest of the house, so whether the batts are faced or not is not a big deal. That is my view. Others may be more precise about reasons for one or the other. Definitely do insulate. It's a more comfortable room afterwards. The temperature gradient is lessened. Batts are OK. Any kind of batt. If you want to do somthing better, you could use spray foam but that is a whole new learning curve. Batts are easy. Methinks a little knowledge can be a dangerous thing.

For interior walls where there will be no poly, no need to slice the kraft. These are the walls where you're using it for sound deadening. On the wet shower walls, or any exterior walls where you'll also have the 6-mil poly, yes, take a utility knife and slash the kraft facing. You only want one vapor barrier in a wall assembly. If kraft, then just kraft. If poly, then just poly. If kraft and poly, go Anthony Perkins and slash away at the kraft. Mongo

Thank you both for your replies. I am in Michigan. John Manville makes a 2 inch foiled faced rigid foam board. I am concerned about the warmth being/staying in the bathroom not the garage. So perhaps because of my climate I could just use plain 2 inch XPS. But I was told to use the foil faced because it is fire proof. Mike and energy rater the garage temperature is the same. I wanted to use the foam board to reflect back and keep the bathroom warmer. The insulation company told me to place the foil face into the wall, that way any heat in the bathroom will reflect back into the bathroom. Boy this is so confusing to me. There is a heat vent in the bathroom under the vanity. I realize what you are saying about the supply problem in the bathroom, thank you for that. Interesting that I wouldn't see a difference in temperature in the bathroom with putting foam board on the garage side of the wall. I just had blown in cellulose in the attic and that is what the insulation guys told me,...to add the foam board to the garage wall that shares with it the bathroom wall. The other issue I have is the bathroom fan. The insulation company added the cellulose up to R49 and air sealed the attic...it is warmer on my second floor. BUT here is my question...The bathroom fan a new Panasonic when I first turn it on there is a whoosh of cold air that first comes out of it. I realize that there could be cold air sitting there at first on the attic side. BUT I felt it last night with the fan turned ON and there is a bit of cold air that circulates at the top of the fan. I stuck a tissue up there and it definitely sucks the air out of the room but it is weird. Needless to say it is it is chilly in the bathroom when taking a shower until the bathroom heats up from the steam of warm water from the shower. . Does anyone have any thoughts on that?? The duct from the bathroom fan is vented out to the roof and is an insulated duct. The only thought I may have is that it is not properly air sealed on the attic side. Any thoughts or ideas are most greatly appreciated. I am a new homeowner and am new at learning all of this. Your patience is most greatly appreciated.