Replies

1. Expert Member
   Dana Dorsett | Jun 29, 2020 05:14pm | #1

   >" For insulation , 4” of closed cell foam under the sheathing and then 2-3.5” layers of mineral wool batts for the remainder of the bay which would provide about R-54. Thoughts? Also would it make sense to add 1.5” of rigid Insulation above the sheathing."

   Per the IRC's prescriptives in Chapter 8, for dew point control at the foam/fiber boundary at least 50% of the total R (R25 out of a presumptive R49) has to be on the exterior to the fiber insulation in a zone 6 location. (for reference see: https://up.codes/viewer/washington/irc-2015/chapter/8/roof-ceiling-construction#table_R806.5 )

   The 4" of closed cell foam is at BEST R28- that's optimistic for long term performance, even with HFO blown foam. For HFC blown foam assume no more than R23 @ 4" long-term, for HFO blown foam assume R25. With 7" of rock wool you're looking at R30, so it fails the IRC prescriptive. (R30 rock wool batts designed for 2x8 rafters are available which would be simpler and usually cheaper than 2 layers of R15 batts.)

   Closed cell foam is expensive. Unless the roof lines are all cut up with dormers hips & valleys (a generally bad idea in snow country), going with 6" (R34) of 2lb roofing polyiso ABOVE the roof deck should be substantially cheaper than 4" of HFO blown closed cell foam (roughly half, likely a bit more than half when you add in the cost of a nailer deck and fasteners above the foam), and at a 7% framing fraction would come close to meeting code on a U-factor basis WITHOUT the cavity insulation if there is also a layer of ceiling gypsum on the interior side of the rafters (for the 2 extra air films and the R-value of the gypsum itself.) That works because the insulation is not thermally bridged by the low-R rafters.

   Going with just 5" of roofing polyiso (R28.5) and dropping to R23 rock wool (5.5") would get you there on an R-value basis, and would outperform your proposed stackup due to the lower thermal bridging, and even derating the polyiso for climate extremes to R5/inch it would still have adequate dew point control on R23 fiber insulation.

   Alternatively, dropping back to 2x8 rafters (if that still meets loading requirements), with 5" of polyiso and 7.25" of cellulose (R27) also works. Even though the polyiso would underperform it's labeled R-value during the coldest hours of the winter, the substantial hygric buffering of the cellulose would be sufficiently protective of the roof deck.

1. 
   WalpoleJoe130 | Jun 29, 2020 08:00pm | #2

   Dana - I really appreciate your thorough response! I have a couple of follow up thoughts and questions.

   While perhaps not ideal in central NH, the roof does have quite a few valleys and 3 doghouse dormers. Your comment on that got me thinking that sheet insulation above the roof sheathing would be challenging with all of the nooks and crannies. So while the rigid foam above the sheathing helps with breaking thermal bridging, I'm thinking of avoiding it.

   I read Section R806.5 and it appears that if I did use 4" HFO foam directly under the sheathing (R-25 at R6.3/inch) and then install a air-permeable insulation below that equivalent to R-25 would meet the R-49 minimum requirement and the 50% rule. Do I have that correct?

   I don't see where it specifies that 1/2 the R value has to be impermeable. Would it be acceptable to use the 4" of ccfoam and 7.25" rock wool to get an R-value (between rafters) of R-55?

   Thanks again for sharing your knowledge!

2. Expert Member
   Akos | Jun 29, 2020 09:49pm | #3

   SPF is not something you design into a new build. It is expensive, not the most environmentally friendly and can create a big mess if improperly installed.

   There was a thread on here from somebody that went through a new build with a complicated roof and SPF. Not only wasn't the building as air tight as SPF people like to push, but also ended up with ice dams.

   First priority is figuring out how to simplify the roof without affecting the look of the house. Some houses look great with doghouse dormers, on some, it looks like a terrible afterthought.

   If you must have dormers, shed dormers are much easier to insulate well.

   The best roofs are always the simplest, my order is:
   -vented roof blown in insulation
   -vented roof with batts
   -unvented roof with rigid above and batts bellow
   -unvented with SPF

   In your case, the 2x12 rob most of the R value benefit of the SPF. You roof ends up something like R40 assembly, much less than the R54 center of cavity. The thermal bridging from the rafters can also create some striping and ice melt. Might be an issue if you are in heavy snow country.

   At minimum, build the roof with I-joists (either 9.5" or 11 7/8"), the thinner webs of the joist reduce the thermal bridging by a fair bit and give you close to an R48 roof.

   The best would be going with a roof with rigid insulation above the deck and fluffy between the rafters. With that type of assembly, you can meet code on U factor basis, which means an R38 assembly. You can exceed that with 4" polyiso above the roof and R19 batts in the rafters. This would be similar assembly R value as the SPF above without the ice dam potential. The overall cost would be similar to the flash and batt approach without any of the drawbacks of SPF.

3. 
   WalpoleJoe130 | Jun 30, 2020 09:29am | #4

   Appreciate the response, Akos. Regarding the roof style/setup, it can't/won't be simplified. The pitch is steep (12:12). Using I-joists for rafters make sense and can be incorporated. I will relook at venting with either blown or batt insulation.

1. Expert Member
   Akos | Jun 30, 2020 12:30pm | #5

   For venting to work, all rafter bays need to have air flow, this is hard to do with dormers. Sometimes, it is simpler to spray foam the roof above and bellow the dormer and go with vented every where else.

1. 
   WalpoleJoe130 | Jun 30, 2020 12:44pm | #6

   Good suggestion, Akos. thx