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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?

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You should also really look into venting the bathrooms to the outside. –  auujay Aug 17 '11 at 21:34
    
fixing the bath vent should be the very first thing you do. –  DA01 Aug 17 '11 at 22:25

4 Answers 4

up vote 3 down vote accepted

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.

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The paper backing of kraft-faced insulation is a code-qualifying vapor barrier when properly overlapped and taped with normal yellow masking tape. It's equivalent to a 5 year tar paper... not a great one, mind you, but it is one. –  Karl Katzke Aug 18 '11 at 1:05
    
@Karl: my opinion: that's too hard to get right, and the wax paper is just not as good as 6-mil plastic. If you don't get it right, and you end up with mold in your insulation or walls (possibly years later), the repair cost easily trumps the small extra amount of money and time to just do it right in the first place (install 6mil plastic). –  gregmac Aug 18 '11 at 1:10
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@KeithS: I'm going to partially disagree with you on this (there's too much completely disagreeing going on around here). I stand by the paper on insulation as a vapor barrier (it's not just paper), but if anyone wants to add an additional barrier on the same side as the paper, then I've got nothing against overdoing anything. –  BMitch Aug 18 '11 at 1:50
    
I'm going to break the trend and agree with BMictch about it being-sufficient-but-extra-can-be-added. And it ain't just paper. –  Karl Katzke Aug 18 '11 at 2:15

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.

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Completely disagreed with you on "doing anything that could trap moisture in the insulation" -- in a normal situation, it's not the insulation you have to worry about, it's the drywall. The proper approach according to building code is to use housewrap on the outside of the house (which is sort of a moisture barrier) and a kraft paper, tar paper, or > 4 mil plastic barrier on the inside of the wall against the drywall. –  Karl Katzke Aug 18 '11 at 1:07
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@Karl, you're welcome to your own opinion, but I stand by mine. I strongly discourage anyone from doing anything that could trap moisture within the insulation. The house wrap is a weather barrier, not a vapor barrier, there's a difference. –  BMitch Aug 18 '11 at 1:39
    
Yep, it wouldn't be a good idea to do 4 mil plastic inside and out... but water will condense on the weather barrier. :-P It should be left open at the top and bottom to allow water to drain out. –  Karl Katzke Aug 18 '11 at 2:16

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.

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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.

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Interesting, though long, article explaining vapor barriers: buildingscience.com/documents/digests/… –  gregmac Aug 18 '11 at 1:15
    
Thank you gregmac! That is a fantastic article, and site! Since becoming a home owner i've been looking for a no non-sense website to explain some of the nitty gritty details. Thank you 100x! –  mattsn0w Aug 23 '11 at 15:26

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