It won't get out. I've been researching redoing my tub surround and it appears that most people use 15 pound tar paper. I think the tar paper will eventually let the moisture through if any does get in, and it will be at a slow enough rate that it doesn't hurt anything. The only place they seem to use poly is if it is a outside wall and the concern there is you don't want moisture to penetrate from the outside. I'm no expert, but that's the way I read things right now.

If you fall under IRC, IRC requires most framed walls to include a Class I or Class II vapor retarder on the warm side of the wall during the heating season. For NJ, that would be the interior side. Vapor retarders are separated into three classes: Class I: Less than or equal to 0.1 perm (6-mil polyethylene); Class II: Greater than 0.1 perm but less than or equal to 1.0 perm (Kraft facing); Class III: Greater than 1.0 perm but less than or equal to 10 perm (interior latex paint on drywall). There are exceptions: if the wall has an adequate thickness of exterior foam sheathing OR if the wall is vented, ie, it has an exterior rain screen between the sheathing and the siding, then you can use a Class III retarder, which can be simple latex paint on the interior walls. If you use kraft faced insulation, then staple the paper to the 1-1/2" wide edge of the stud. Do not staple it to the side of the stud inside the framing bay. As to moisture movement, yes, 6-mil poly will stop diffusion and drying whereas kraft paper allows some albeit at a slow rate. If you tend to have exterior siding (like vinyl siding) that easily passes water, and if you don't have any housewrap under that vinyl siding to shed moisture that gets behind the vinyl siding, then Kraft paper may be the better choice. Walls can get wet, then when the sun hits the wall, moisture can be driven into the wall assembly from exterior to interior. If moisture gets into the wall, you want it to be able to dry. 6-mil poly really only allows drying back out to the exterior, reversing the path that allowed it in. Kraft paper, with it's higher perm rating, allows allows a bit of drying to the interior. So you are correct, 6-mil poly is indeed a better vapor retarder than kraft-faced insulation with regards to stopping vapor diffusion. But you might not want "the best" vapor barrier in your wall assembly if you have lousy walls. The whole vapor barrier/retarder business can be quite confusing. Most inspectors don't even understand it. The only time I usually recommend slashing the kraft facing on FG batts is when someone is adding more kraft-faced insulation over existing kraft faced insulation, or if they are adding 6-mil poly over the entire wall after kraft-faced batts were already installed. Remember, paint is quite permeable. Grout is permeable. So you would not want to slash the kraft facing behind painted or tiled drywall.

Here is my response to a similar question on one of the other boards. I reasearched this a lot, and though I'm not in a cold climate, most of the recommendations would hold for your situation: I had a similar situation. Some genius had put 3 inches of blown fiberglass in the attic (over the knob and tube wiring!) and that's it. The room was hot in the summer and cold in the winter. Here is what I did: - r13 on the floor between the soffits and the back of the kneewall - this was over conditioned space down below (I didn't cover it, but I could have if needed for storage)- rafter vents at the soffit-end of the insulation keep the soffits open - r13 kraft faced insulation on the back of the kneewalls (2x4 wall construction) - 2" isocyanate foam insulation (r14)with reflective surface on the angled portion of the ceiling, leaving 1.5" air space up to the unfinished part of the attic. My rafters were true 2"x6", so I didn't need the fancy channeled insulation. This insulation is R7 per inch, so I got R14 on the slanted portion. - R38 faced insulation in ceiling of the finished space, with eyebrow roof vents to keep air moving through there. - seal up all penetrations (electrical boxes, pipes, etc) with caulk or spray foam) The principles you need to follow are that you should insulate any of the unconditioned space where air from the outside is flowing. Any vapor barrier (e.g. the kraft facing)should face the INTERIOR - don't put insulation then vapor barrier, because then any moisture from the interior will reach the cooler air of the unconditioned space and condense, leading to mold. If you use the foam insulation, you have to cover it with a fire rated material (e.g. sheetrock). You could also consider using the new reflective sheets on the bottom of the roof sheathing - I didn't do this, but apparently it has a huge impact on heat gain. The impact - the upstairs is comfortable all year. It has no dedicated heat or AC, but never gets below 65 during the winter or above 72-73 in the summer. If it gets stuffy on the hottest days, I open a couple of windows to get a cross breeze. Hope that helps.

How many square feet of wall? How busy are the walls with regards to utilities?