Will moisture become trapped between a double air barrier in the ceiling beneath the attic

attic

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?

Best Answer

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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