Replies

1. Expert Member
   Dana Dorsett | Jul 26, 2019 04:07pm | #1

   What climate zone? (It matters if this a high snow load climate.)

   >"Our new home design will have a non-vented attic space, with the underside of the roof deck (between rafters) insulated with R-50 closed cell foam."

   Can I convince you to do something different? Closed cell spray foam is one of the least-green ways to insulate a roof, even if using HFO blown foam. If blown with industry standard HFC245fa it's the opposite of "green", due to the extreme global warming potential of the blowing agent (~1000x CO2 @ 100 years).

   Going with 6" of rigid polyiso foam board (which is blown with comparatively benign hydrocarbons) ABOVE the roof deck hits the same performance level despite the lower R-value, due to the absence of therally bridging rafter elements. Rigid polyiso above the roof deck is usually cheaper, and definitely greener & more resilient, since it keeps all of the structural wood inside the thermal boundary of the house. If that's going to make the roof to thick, splitting the difference, with enough insulation above the roof deck for dew point control for the local climate, and fiber insulation on the interior is still usually cheaper & better.

1. 
   charlie_sullivan | Jul 26, 2019 04:16pm | #2

   Dana's comment is right on the money.

2. GBA Editor
   Brian Pontolilo | Jul 26, 2019 04:28pm | #3

   Mr Dude,

   I also think Dana is making a wise suggestion and there are even more options that could minimize the amount of closed cell foam your are about to use. See the article below.

   http://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling

   However, to answer your question, if you do go ahead with the closed cell spray foam. There is no need to fill the eave space above the soffit. Install blocking to stop the foam above the top plate.

3. Expert Member
   Akos | Jul 26, 2019 05:42pm | #4

   If you are going to spray R50 cc SPF, make sure your rafters are TJI. Spraying between dimensional lumber is a waste (you get R37 roof with 16oc 2x rafters when you account for thermal bridging).

   In terms of spraying under the eaves. In snow country it doesn't hurt to get a couple of inches there (R15 is enough), prevents ice dams if you have big overhangs above the south facing walls.

4. 
   Mr_Dude | Jul 26, 2019 05:59pm | #5

   Thank you for the information so far. Some further details and questions:

   - Dana, we are north of Chicago in Illinois, therefore Zone 5 . . . we are ~15 miles south of the Wisconsin border which is Zone 6.
   - Dana, regarding environmental implications, I will definitely re-examine my entire system. In the last hour I have written for input on alternative blowing agents from my two bidding subcontractors. My understanding from reading about HFC-245fa is that it will be banned in 2020 except for application of closed cell foam in on-site applications . . . and indeed it has a GWP of 950 at 100 years.
   - Thank you [Brian] for the link to the 'cathedral ceiling' article . . . I recall reading this a couple of years back.
   - I do have concerns about a full redesign of our Great Room ceiling system. The Great Room has a span of 40' x and a depth of 36', symmetrically split along the 36' axis . . . roof pitch is 9:12. I believe the construction is a bit unusual. Supporting the roof deck are the two end gables and 3 intermediate structural timber trusses, all spaced at ~ 9'. Atop each gable and truss will be a 5/8" plywood strip. Atop these strips, 2 x 12 Douglas Fir purlins will run perpendicular to the end gables and intermediate trusses . . . 7/16" OSB decking will be mounted above this. The 2 x 12s give us 11-1/4" of depth for insulation and low profile insulated cans [with LED lamping]. 1/2" gypsum board will be mounted to the underside of the purlins, with each drywall end able to be 'tucked' into the 5/8" space above each truss . . . this will eliminate drywall taping against the trusses and enable efficient painting and 'clean' lines.
   - At one point I considered a SIPS panel system, however, the need to accurately spot locations for electricals ahead of factory fabrication proved too challenging.
   - Brian, regarding the soffit space, do I need to vent it at all if I terminate insulation above the top plate? On the one hand, I would expect it to be impossible to seal the boxed soffit, thereby seasonal air will make its way into the space . . . on the other hand, venting the space with a prevented soffit product such as LP SmartSide would invite dust and insects.