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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
  4. sheeting** sealed between joists (no tar)
  5. old air barrier (puncture, torn, not sealed)
  6. wood furring
  7. gypsum board

What do you think? Is this possible?

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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

1 Answer 1

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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.

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