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In the ceiling of my house, the air barrier is torn and its joints are not taped. It is aluminized paper with tar.

In the attic, under the glass wool insulation, I am thinking of adding a polyethylene sheet between the joist, sealing it with caulking. This approach would avoid redoing all the ceilings, but I fear that moisture may become trapped between the two air barriers.

This would be the arrangement of the resulting layers:

  1. attic insulation
  2. attic insulation + joist
  3. polyethylene sheeting sealed between joists (no tar)
  4. old air barrier (puncture, torn, not sealed)
  5. wood furring
  6. gypsum board

What do you think? Is this possible?

  • Where do you live and why do you want a moisture barrier in your attic? – DMoore Oct 9 '13 at 6:47
  • I live near Québec city, Canada. Cold in winter. I want a better air barrier to reduce air leakage. – Amorok Oct 9 '13 at 12:01
  • @Amorok, please accept an answer or respond to the existing ones as to why they're not acceptable. – isherwood Jul 11 '17 at 19:20
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The best solution for your climate is to have the poly sheet between drywall and joists but since you already have the drywall up what you propose seems like a good option. For your region the Building Science reports suggest the vapor barrier inside of the insulation.

I have actually seen this done two ways. Up and over every joists, back flat on drywall and back over next joist and taped everywhere. And then I have also seen the vapor barrier laid flat on the joists and taped.

There are pros and cons to each method. First going up and over every joist is a pain to install. It literally takes forever. You might spend a whole day doing this right.

By going flat on top of the joists it is easier. You also benefit from a "neutral zone" that brings basically free r-value. The flip side is if you do have a leak it will probably find itself into a tape seem (opposed to just sitting on the poly on the drywall). Also and the reason I don't install like this - makes it very hard to move electrical or run new electrical in the attic (why I also would never spray foam an attic).

So I am on board with your plan but probably minus the sheeting. If the sheeting is already there then I would attach the poly sheets right on top of the sheeting and make sure it is taped well. I mean you can put up as many vapor barriers as you want on top of each other. As long as the top one is installed right all should be fine.

  • "Free R-value"? You're heating that space. There is no r-value to that air space. To me, the poly sheet is a moisture barrier and should not be installed on the ceiling. A vapor barrier has a perm rating of 1 or less and should be installed on the ceiling. Poly sheeting does not allow the moisture back out when the season turns to summer and the transfer of VAPOR changes direction. The poly sheeting will leak vapor in the winter and become moisture at the dew point. (It could leak around light fixtures, etc.) in the 80's and 90's we installed poly sheeting, now we slice it up if we find it – Lee Sam Mar 1 '17 at 9:09
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    @LeeSam Ummm there is r value in air space - read up. – DMoore Mar 1 '17 at 15:04
  • Negligible... AND If the insulation had a radiant barrier face AND faces a 3⁄4” air space you would get an effective emittance of 0.82 and an R-value of 0.90. Also, the radiant barrier doubles the R-value of this air space, so normally (like in this case,) it would be one-half of the calculated R-0.90... ALSO, air spaces have an upper limit for thermal resistance regardless of their size. The largest you can get from any table maxes out at 3.5”. This is because the insulative quality of an air space does not get much better when it is bigger than 3.5"...(see next comment...) – Lee Sam Mar 2 '17 at 1:29
  • Then the air space starts becoming "conditioned" space. So if you ever needed to calculate the R-value for a 6” air space, you'd use the 3.5”. (See toollending.com. The thermal resistance is on page 25.4 of the ASHRE manual.) – Lee Sam Mar 2 '17 at 1:31
  • Oops..., I forgot to say, "technically" you're correct, there IS a value for air space...but I think negligible. – Lee Sam Mar 2 '17 at 1:39

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