bathroom/attic wall insulation

[mmg]

I posted this in a different forum a few days ago, but I think I should have posted here in the first place. It seems to have a lot more related topics.
Anyways, here's my situation:
I have a 3 level side split, which means that behind one of my upstairs bathroom walls (the top half of the wall anyway) is the attic. The attic itself is below par as far as insulation goes, but I'm having another 6 - 8 inches blown in. But that doesn't take care of the wall.
So, I need to know the best/proper way to insulate the upper half of the bathroom wall. As it is now, it sweats very heavily even after I installed an exhaust fan. The wall just gets too cold and the steam from the shower just drips down it. It's a plaster wall if that matters.
Do I just put the batts up on the attic side and that's it?...or do I treat it like an outside wall and use a vapor barrier on the attic side?...or none of the above?
Please advise.
Thanks.
MMG

[Phelps]

I can envision what you are describing. Yes, plaster walls do make a difference. They much more easily condense. It doesn't breathe as well. If you put your hand on a cold plaster wall and a cold sheetrock wall, you will notice how much colder the plaster will feel.

You don't want to put vapor barrier on the attic side of the studs in the attic. If this were me, I don't think I would even bother with trying to vapor barrier it in some kidn of manor, like trying to weave it around the attic studs, up against the bathroom wall. I'd just friction fit plain fiberglass (no kraft paper!) insulation between the attic studs behind the bathroom. Then I would paint 2 coats of paint with vapor barrier shellac based Zinsser paint on at least that attic wall, in your bathroom.

[pushkins]

I agree with Phelps, since you are able to get the the rear of the bathroom wall in question, I would fit NON vapour barrier insulation inside these studs. Since some attics have doorway access in your situation if you have that advantage I'd add drywall over the rear of the bathroom area after you've added the insulation. (not sure if you have that much access though).

[mmg]

Thanks guys.
It just so happens that I finished painting today with Zinsser paint. Except it wasn't 2 coats, it was about 6, plus half a bucket of drywall compound (I've come to the conclusion that the previous owners were crazy!...but that's a different story). I'm not sure if it's the same kind you're referring to though Phelps. The paint I used is a self-priming sealer and is mold and mildew resistant. Is that the same stuff?
As for the drywalling the back, I think I could do it with two pieces cut lengthwise...but that means I have to mud doesn't it?
Regardless, I'll go with the fiberglass - NO vapour barrier. Thanks again.

[pushkins]

no you wouldn't need to mud it. It was just as an after thought of adding another R value (as small as it might be). I'm a neat freak...lol

[Phelps]

mmg,

The primer paint I mentioned, by Zinsser, actually says on the bottom of the front of the can that it is a vapor barrier paint and that it is shellac based. You can't beat it for sealing weeping knots in wood or to act to stop moisture migration problems. The shellac in it absolutely forms a barrier even superior to oil paints.

I have used this successfully, myself, on terrible black mildewing and blistering PLASTER bathrom ceilings and upper walls.

Hi pushkins.