vapor barrier in bathroom?

Hi:
I'm refurbishing our bathroom. I had to tear out the old plaster/drywall because of the mastic left when I removed the old ceramic tiles on the wall. I'm ready to install drywall and plan to install the moisture-resistant kind.
Should I install some sort of vapor barrier, like poly sheet or whatever, before I put the drywall up? There was nothing like that there originally. Or is this only for outside walls?
Is there any sort of insulation--foam sheets or whatever--I can put in the wall that will help reduce sound? Would I want to put regular thermal insulation in there for this purpose, or would I need to get special sound insulation for this (none of the bathroom walls are outside walls, so there's no need to do this for thermal insulation). Should I even worry about sound dampening--the drywall seems rather thick?
Also, I'm a bit confused as to how to cut out portions of the drywall for plumbing etc. Do I want to try to cut all the necessary openings in the drywall before I nail it up or do you typically make the holes for the plumbing, electrical outlets, etc. after it's been attached to the wall? I can see advantages and disadvantages either way. Or some combination of both?
Thanks.
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Three coats of latex semigloss will give a perm rating of better than .99, which is very adequate as a vapor barrier. After all, it's a plastic film.

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lou wrote:

It should not be needed on an inside wall.

How much I would worry about it would depend on what was on the other side.
http://www.soundproofing.org/ has a lot of good information.
Don't count on standard insulation to help much. The sound type insulation is a lot better, but even that is not really great. An extra layer of drywall would make a bigger difference.

I usually put the plumbing and electrical in first. Then you cut out the needed holes as you go before putting it up on the wall. Most of those things are already proud of the wall and you can't just put drywall over them without making the cut outs.
--
Joseph E. Meehan

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<< I'm ready to install drywall and plan to install the moisture-resistant kind. >>
Check out the new moisture and MOLD resistant types now coming on the market. Cost difference is around a buck or so a sheet. HTH
Joe
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The green board *is* the vapor barrier, so you don't need another. You could use non-faced fiberglass insulation to muffle the sound. Using long metal ruler, calculate and cut the holes before nailing the greenboard onto the studs. Occasionally you can cut a hole afterward, but it's hard to get a saw into the board if there's an electrical box or a pipe or whatever immediately behind it. -B

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