Replies

1. GBA Editor
   Martin Holladay | Jan 31, 2018 04:47pm | #1

   RB,
   The nail holes don't really hurt the performance of the polyethylene as a vapor barrier -- but they do make the polyethylene close to useless as an air barrier, which is far more important.

   The usual solution is to install 1/2-inch drywall first, and tape the seams with mud and paper tape. Then install your pine boards. The drywall is your air barrier, and the fasteners through the drywall don't leak air.

2. 
   RB88 | Feb 01, 2018 05:37am | #2

   Thanks for the quick reply. I just read your article: "Calculating-minimum-thickness-rigid-foam-sheathing", which seems to indicate I should forego the poly altogether and also go with thicker exterior foam. Or can I use a drainage wrap behind the foam to provide adequate airflow?

3. GBA Editor
   Martin Holladay | Feb 01, 2018 05:49am | #3

   RB,
   Your original question raised more than one issue, and I should have addressed them all. I just addressed the problem of the lack of a good interior air barrier, and I didn't address the problem of the too-thin exterior foam.

   Your observation in Comment #2 is correct: In Climate Zone 6, a 2x6 wall with a continuous layer of exterior rigid foam needs to have foam with a minimum R-value of R-11.25. (Here is the link to the article which explains the underlying principles behind this recommendation: Calculating the Minimum Thickness of Rigid Foam Sheathing.)

   You can achieve the required R-11.25 with 3 inches of EPS or 2.5 inches of polyiso. (Green builders avoid the use of XPS because XPS is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see Choosing Rigid Foam.)

   You want an interior air barrier, but you don't want an interior vapor barrier. So you definitely don't want interior polyethylene with this approach.

   I don't recommend the idea of using foam that is too thin along with drainage wrap. These two details will both have the effect of lowering the thermal performance of your wall (while still putting the sheathing at risk of moisture accumulation).

4. 
   RB88 | Feb 01, 2018 12:17pm | #4

   Thank you. I have a similar scenario for my vented cathedral ceiling. 2x10 rafters. From top down: 1.25" vent space, 2" XPS sealed along edges, 6" Roxul, which is even with the bottom of the rafters. I'd like to add 1" of rigid insulation to the bottom of the rafters, then cover with pine boards. So I'd end up with a sandwich of rigid insulation with Roxul between. Is the permeability of the XPS enough to keep me out of trouble with moisture?

5. GBA Editor
   Martin Holladay | Feb 01, 2018 01:01pm | #5

   RB,
   You need a durable air barrier at the ceiling level. Some builders have used taped rigid foam as their interior air barrier, but I'm skeptical of the durability of this approach. For one thing, rigid foam can shrink as it ages, pulling away from the tape used to seal the foam seams.

   Second, we don't really know as much as we'd like to know about the durability of the adhesive bonds on tapes.

   My own advice is to install drywall with taped seams (as an air barrier) on your ceiling before installing the pine boards. If you follow this advice, you can install a continuous layer of rigid foam between the underside of the rafters and the drywall if you want.

   If you ignore my advice, remember that foil-faced polyiso is the easiest type of rigid foam to tape.

6. 
   RB88 | Feb 01, 2018 01:12pm | #6

   Thank you...I considered using polyiso but didn't know if its low vapor permeability would be an issue. If it's not an issue, I'll lean that direction.

7. GBA Editor
   Martin Holladay | Feb 01, 2018 01:22pm | #7

   RB,
   In cold-climate locations, it's traditional to have a low-permeance layer on the interior side of an insulated assembly, so polyiso won't cause any moisture problems.

   The only issue, as I tried to point out, is possible air leakage due to tape failure or foam shrinkage.