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This is related to my other question about moisture barriers.
There is an attic above the bedroom and there is insulation with paper backing in place. The attic area seems far more likely to get moisture as the bathrooms both vent directly in to the attic and not out through a vent. I'll address that issue eventually.
So my question is, should I install a 4mil moisture barrier on the ceiling, or omit that from the ceiling?
I completely disagree with BMitch here. If you live in an area with sustained freezing temperatures, you should have a plastic vapor barrier between the drywall and the studs, wherever "warm meets cold" (exterior walls, top floor ceiling). You want this vapor barrier inside your insulation so the barrier is toward the "warm" side of the thermal break provided by the barrier, thus reducing the amount of condensation that forms on it.
Paper, like the backing of your insulation rolls, is NOT a vapor barrier. Neither is the insulation itself, even though its purpose is to "trap" a bubble of air and thus provide a buffer between warm and cold. Neither of these are continuous; the insulation is cut to fit inside the studs, and so air and thus moisture can still get around the paper. Moisture also soaks into paper. 6-mil plastic vapor barrier, installed properly, has no way through it; it's sealed against the studs so the drywall screws don't break the barrier, and all J-boxes are sealed with Tuck Tape or similar vapor-proof adhesion.
As for mold concerns, there shouldn't be any, even on the back side of the wall. Because the area behind the vapor barrier can still breathe, even with the insulation, there is little chance of there being enough water in the wall to damage the studs or cause mold to form on them. The big concern is the drywall, which is a sponge for any condensation that may form on it. It will trap this water and hold it for a long time, which along with the processed wood product (paper) on its skin, is a definite attraction for mold.
Vapor Barriers are one of the more controversial subjects in the residential building worlds. Some don't believe it in (especially in older houses, where nothing is usually airtight) but there typically is going to be local building codes that will dictate if you need it (usually) and where it should be.
Typically vapor barriers are on the warm side of the wall as keith states. In colder climates (Northern US, for example) we (traditionally) have more cold days than warm, so we place the vapor barrier on the inside (behind the sheetrock).
In the south, where it's normally AC weather year-round, one would put it nearer the outside of the wall.
The idea is as much of the interior of the wall structure dry as can be.
Some feel that paint can act as a proper vapor barrier. But that's tricky, as most walls will have holes (outlets, switches, nails, etc.)
The one thing everyone agrees on is that you never want to have more than one vapor barrier, as that will only trap moisture within the two of them.
I wouldn't do anything that could trap moisture in the insulation, since that would result in mold. The paper backing should be enough of a moisture barrier and, as with the walls, should be installed facing the heated/cooled portion of the home. E.g. in the attic, it should face down, above a garage with a room over it, the paper should face up, and on exterior walls, the paper should be on the interior side.
Edit: I'm not sure where others are getting their information from, but I'll provide a few pointers to relevant sites. First, the paper on insulation is a vapor barrier, and it's there by design. In cold climates, it's essential for keeping warm moist air in your home from going through the walls and freezing inside of the insulation, which will result in mold and mildew issues. The proper install method is to extend the paper tabs on both sides and pull back the insulation on the top and bottom so that the attached drywall seals the paper to the studs. See this site for more details.
And regarding house wraps, they are to prevent weather from getting into the home. Water that penetrates the siding cannot get through this, and it reduces drafts. However, like Gortex, Tyvek can breath, and it allows moisture in the walls to escape. Here's another site with more details on Tyvek.
Vapor barriers in ceilings seem to have caused more problems than they help. Your discussion and conclusions fail to address blown in insulation, nor the results, in more than a few instances.
Since I am regularly do home visits, I count as one who gets to see the results, such as mildew and mold on the inside of the home; in every instance where I've seen mold on the upper part on the inside of the drywall, I assume a vapor barrier above the ceiling and feel for it in the attic and yes, there it is! The vapor barriers in the walls and ceilings are preventing moisture escape. This is typical of a house that is too air tight; with all the new energy efficient remodeling products on the market, this has been a growing problem. In my opinion, forget the vapor barrier in the ceiling and do a proper installation of blown-in fiberglass and vent the attic with intake air and exhaust capability to more than a 1:150 ratio.
Ok, vapor barriers on ceilings were a problem 20-30 years ago because of poor insulation standards in the attic space. Most homes I've worked on that were built in the 80s have mold around the outer edges of the ceiling. Heat and air conditioning from the inside makes its way through the drywall and cndenses when it meets the exterior air. Yes, today's homes are more air tight but they also treat the air much better with improved HVAC systems that keep the inside air at optimal humidity levels.
