Insulation question (greenboard in bathroom, kraft face insulation, etc.)

So we're redoing our bathroom and it's out to the studs. It's 2x4 16"-on-center wood frame house. The bathroom is on an outside corner of my house, and so 2 of the 4 walls of the bathroom on the exterior and have insulation. These 2 walls happen to be back and side of the shower. So I'm reading around and seem to be getting different things from different sites on kraft faced insulation, greenboard, cement board, and drywall. (I know to use cement backer board for the shower tile substrate).
I have just hung kraft faced insulation, so my question is with greenboard....do I use it in combination with the kraft faced insulation (r13)? Some out there say to not use greenboard if you've already got kraft faced insulation up b/c then there's 2 vapor barriers and the moisture could get trapped in between the layers.
Second question - greenboard on the ceiling or no? Some say it sags.
FWIW - the drywall I took down was 30 yr old normal gypsum board, and the insulation behind it was in decent shape, although poorly installed. The painted walls appeared decent for a 30yr old bathroom, although there was some moisture/drip lines from shower steam.
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Yes, greenboard is OK. It is a so-so vapor barrier, but its basic important property is moisture resistance. Best used with setting type compounds and fiberglass tapes for best water resistance.

Definite NO according to one customer service query years ago at a major drywall maker. Maybe not as important with some of the newer types of drywall (mold resistant, etc.) now on the market.

Likely your project will turn out quite well. Good luck.
Joe
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Foamboard insulates better
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Around here (northern climate) only 6 mil poly film is considered legal vapor barrier, kraft faced insulation is useless except to hold the insulation in place.

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