Replies

1. GBA Editor
   Martin Holladay | May 27, 2018 05:57am | #1

   Kelly,
   In Climate Zone 5, your roof assembly should have a minimum R-value of R-49. If you plan to combine air-impermeable insulation (that means rigid foam or spray foam, or both) with air-impermeable insulation (fluffy insulation like fiberglass batts, mineral wool, or cellulose), you have to follow certain ratio rules designed to prevent moisture problems.

   The basic rule for Climate Zone 5 is that a minimum of 41% of the total R-value of the roof assembly needs to come from the air-impermeable insulation (rigid foam or spray foam). So if you total insulation achieves R-49, the rigid foam + spray foam layers must have a minimum R-value of R-20. For more information on the principles behind this recommendation, and an explanation of the ratio rules, see Combining Exterior Rigid Foam With Fluffy Insulation.

   Your plan to use only 3 inches of air-impermeable insulation is on the light side, especially because the performance of polyiso needs to be de-rated in cold weather. (For more on this issue, see Cold-Weather Performance of Polyisocyanurate.)

   If we say that the polyiso will perform at about R-5 per inch in cold weather, here's my recommendation: The best approach would be to install 4 inches of polyiso on the exterior side of the roof sheathing, and to skip the spray foam. Exterior rigid foam does a much better job of keeping the sheathing dry, and limiting thermal bridging through the rafters, than spray foam installed on the underside of the sheathing.

   If you insist on installing spray foam, I would advise that you come up with a plan to use a total of 4 inches of foam (a combination of rigid foam and spray foam).

2. 
   kelly_coons | May 27, 2018 11:09am | #2

   Thanks Martin, my biggest concern has been with the exterior rigid insulation being enough to keep the dew point out of the roof sheathing, I understand in my climate zone on a wall anyway that I need 2" or R-10 to keep the dew point on the outside of the sheathing, but I have been unsure whether the spray foam on the underside will help with that with just the 1" ISO. Or contribute to trapping moisture at the roof sheathing. My main reason for using the spray foam is for an air barrier and my insulator tells me I should use 2" to meet vapor barrier also. But with 1" of rigid in my mind it makes me think the dew point would hit inside of my roof sheathing with just 1" of rigid and cause me issues down the road. I could still use 2" of rigid and not affect the design but probably not 4" as we would run into some issues with the fascia coming together in certain areas. Does the 2" rigid serve as an adequate air barrier and do I just spray the vertical wall of the truss from the plate line up to the top chord?
   We are using zip system wall sheathing for our air barrier on the walls.

   Thanks in advance for your insight.

3. GBA Editor
   Martin Holladay | May 27, 2018 03:55pm | #3

   Kelly,
   Re-read my answer.

   Q. "Does the 2-inch rigid serve as an adequate air barrier?"

   A. It might, but it's usually best to detail the sheathing as the air barrier (by taping the sheathing seams with high-quality tape). In your case, if you aren't installing 4 inches of rigid foam, and all you install is 2 inches of rigid foam, you'll need to install 2 inches of closed-cell spray foam on the interior of the roof sheathing -- not as an air barrier, really, but to fulfill the requirement for a minimum of R-20 of foam insulation.

4. GBA Editor
   Martin Holladay | May 28, 2018 05:39am | #4

   Kelly,
   Q. "I have been unsure whether the spray foam on the underside will help with that [the dew point issue] with just the 1 inch of polyiso. Or contribute to trapping moisture at the roof sheathing."

   A. Closed-cell spray foam is a vapor barrier. It will prevent diffusion of interior moisture into the sheathing. However, if the closed-cell spray foam is too thin, interior moisture can condense on the interior side of the cured spray foam. This happens because the cured spray foam gets cold in winter. That type of condensation is what you are trying to prevent by making sure that the rigid foam plus spray foam layer meets the R-20 minimum target.