Replies

1. 
   JustHousing | Jun 05, 2018 09:45am | #1

   Have you considered using rigid mineral wool as a layer of continuous insulation against the interior of the rafters? Rigid mineral wool won't shrink and it can allow drying to the inside. A couple of manufacturers sell a 1.25" thick product, R-5.

2. GBA Editor
   Martin Holladay | Jun 05, 2018 09:54am | #2

   Ben,
   If you install 3.75 inches of closed-cell spray foam on the underside of your roof sheathing, you aren't going to get any drying to the interior, with or without rigid foam under the rafters.

   Your 3.75 inches of closed-cell spray foam is a vapor barrier.

   If you want your roof sheathing to have a downward drying path, and you still want to use closed-cell spray foam, you'll need to install ventilation baffles under your roof sheathing before your spray-foam contractor arrives.

3. 
   user-7079709 | Jun 05, 2018 10:00am | #3

   Rachel,

   I had not considered that, I will certainly look into it.

   Thanks!

4. 
   user-7079709 | Jun 05, 2018 10:02am | #4

   Martin,

   From the article I linked,

   "Install a layer of closed-cell spray foam against the underside of the roof sheathing, and fill the rest of the rafter cavity with an air-permeable insulation. Again, this type of assembly is designed to dry to the interior, so the assembly should never include an interior polyethylene vapor barrier."

   I assume that this is talking about the underside of the spray foam being what 'dries to the interior'?

   Thanks,

   Ben

5. GBA Editor
   Martin Holladay | Jun 05, 2018 10:09am | #5

   Ben,
   You're right that the sentences you quote are somewhat confusing. I'll do my best to edit the article to make it less confusing.

   Briefly, the closed-cell spray foam doesn't allow the sheathing to dry toward the interior if the sheathing ever gets damp. However, if the spray foam layer has thin spots that allow condensation to form on the interior surface of the cured spray foam during the winter, the moisture will be able to dry to the interior as long as there is no interior polyethylene.

   You are correct that adding rigid foam to the interior side of the assembly limits inward drying. However, it also limits the flow of interior moisture toward the cured spray foam during the winter, thereby reducing the chance of moisture accumulation in the first place.

   These types of complicated sandwiches are difficult to analyze. That said, I don't think that adding interior rigid foam in your case would be risky.

6. Expert Member
   Dana Dorsett | Jun 05, 2018 03:25pm | #6

   Using HFO blowing agents it's fine to install 4" or more of R7/inch closed cell foam per pass. It's somewhat more expensive than HFC blown foam at 2", but since there isn't a cooling off period and can be installed in one lift it may be cheaper to do HFO blown foam at 4".

   It's simply not correct that 4" of closed cell foam is a vapor barrier, or offers no drying. Most closed cell foam would be between 0.2-0.4 perms @ 4". That isn't a huge drying capacity, but it's way better than a foil-facers on rigid foam or 6 mil polyethylene vapor barriers (~ 0.05 perms). It's 2-3x the drying capacity than through an asphalt shingle + #30 felt layup (~0.1 perms).

   Both XPS and HFC blown closed cell foams are the opposite of "green", since the HFC blowing agents used are powerful greenhouse gases, 1000x CO2 (or more) @ 100 years. The HFO blowing agents are all under 5x CO2, and the pentane variant blowing agents used for EPS or polyisocyanurate are about 7x CO2.

   From a drying capacity point of view, a couple inches of unfaced Type-II (1.5lbs per cubic foot nominal density) is still over 1 perm, and does not present a drying problem. A single inch is over 2 perms. As the HFCs diffuse out of XPS over a few decades it's performance eventually drops to that of EPS anyway, whereas the performance of EPS doesn't depend on the blowing agent, and is stable over many decades.

   [edited to add]

   By comparison, an inch of XPS runs about 1-1.2 perms , 2" is about 0.5-0.6 perms. That is only half the drying capacity of Type-II EPS, but still more than twice that of 4" of closed cell spray foam.

7. 
   airfix | Jun 05, 2018 06:24pm | #7

   I have a similar roof assembly plan as the OP, un-vented cathedral, in climate zone 6a and worried about the thermal bridging through the rafters. I'm using 14" TJI rafters on a 12" O.C. spacing. For condensation control I'm planning to use 5.5" of ccSPF followed by 8.5" of netted and blown fiberglass. This gives me a cavity R of about R-68 and an R across the TJI of only about R-18.5.

   In the snow season am I just opening myself up to ice dams because of the thermal bridging?

   Am I wasting my money on the 5.5" of ccSPF if I don't deal with the thermal bridging of the rafters and what is the best way to deal with the rafter thermal bridging?

   Steve

8. GBA Editor
   Martin Holladay | Jun 06, 2018 06:35am | #8

   Steve,
   Q. "In the snow season am I just opening myself up to ice dams because of the thermal bridging?"

   A. No. The OSB webs of the TJIs are about 1/2-inch thick, which is quite narrow. Don't worry.

   Q. "Am I wasting my money on the 5.5 inches of ccSPF if I don't deal with the thermal bridging of the rafters?"

   A. No, although you'll probably get responses from GBA readers who point out that there are plenty of ways to insulate a cathedral ceiling without closed-cell spray foam, and that these other ways may be cheaper.

   Q. "What is the best way to deal with the rafter thermal bridging?"

   A. Answering this last question is easy: The best way to deal with rafter thermal bridging is to install an adequately thick layer of rigid foam on the exterior side of the roof sheathing.