Replies

1. GBA Editor
   Martin Holladay | Aug 12, 2018 08:20am | #1

   Kazio,
   Q. "While substandard, will the stack up be reasonably safe mositure-wise?"

   A. Yes. In your zone (Climate Zone 5), for this type of roof assembly, you need to have at least 41% of the total R-value of the roof assembly coming from the spray foam layer. Your stack-up passes the test. More information here: "Combining Exterior Rigid Foam With Fluffy Insulation."

2. 
   unclemat | Aug 13, 2018 04:52pm | #2

   Thanks, much appreciated!

   I also found this paper linked on your site, which was very helpful:

   https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf

3. Expert Member
   Dana Dorsett | Aug 13, 2018 06:34pm | #3

   Using an HFO-blown closed cell foam is more expensive than HFC blown foam and is ~30-40% more expensive, but it it's higher R/inch (giving you more dew point margin) and a LOT greener due to the very low impact HFO1234ze blowing agent compared to the industry standard HFCs, which have a global warming potential on the order of 1000x CO2.

   At 2" the vapor permeance of the closed cell foam layer is about 0.4-0.8 perms, depending on the product, which is an adequate drying path for the roof deck, but is also a Class-II vapor retarder sufficient to keep wintertime moisture accumulations well bounded. When it's time to re-roof you have the option of adding continuous exterior insulation to bring the thermal performance up. In eastern MA there are multiple vendors selling reclaimed roofing foam at 1/4-1/3 the cost of virgin stock foam. Since the assembly already drys toward the interior through a Class-II vapor retarder any amount of exterior foam is fine- it doesn't have to be the IRC prescriptive R20 for dew point control at the roof deck.

   To hit code-min performance on a U-factor basis, with ~R28 in the rafter bays usually only takes 2.5-3" of continuous roofing polyiso (R14- R17), even though that is well-shy of the R49 it would take to get there on an R-value basis. But ANY amount of thermal break over the exterior of the rafters delivers a disproportionate amount of performance relative to adding more R between the rafters where it's performance is undercut by the thermal bridging of the framing.

4. 
   unclemat | Aug 14, 2018 02:51pm | #4

   Thanks, Dana. I am aware of HFO-blown cc from your prior posts and I will certainly look into it.

   Will be another 30 years before a new roof is needed, so exterior insulation will have to wait.

5. 
   unclemat | Oct 24, 2018 04:33pm | #5

   Howdy. So 2" inch of closed cell got installed along with 4" of open cell. The rafter bays were so crooked and unevenly spaced that installing mineral wool batts would have been nightmare, so we went with open cell to fill the rest of the cavity.

   Does that change anything with respect to vapor management? Like would Membrain be good idea? I suppose the 2 inches of closed cell should take care of protecting the roof deck.

   Thanks!

6. GBA Editor
   Martin Holladay | Oct 24, 2018 04:39pm | #6

   Kazio,
   In Zone 5, at least 41% of the total R-value of the roof assembly needs to come from impermeable layer of rigid foam.

   In your case, you have a total R-value of about R-27.8,
   and 41% of 27.8 would be R-11.4,
   but in fact you have about R-13 of closed-cell spray foam, which is more than R-11.4,
   so you are good.

7. 
   unclemat | Oct 25, 2018 10:26am | #7

   Thank you!