Replies

1. GBA Editor
   Martin Holladay | Jul 09, 2015 05:05am | #1

   Eric,
   You didn't state your climate zone.

   Here is a link to an article that tells you what you need to know: How to Build an Insulated Cathedral Ceiling.

   If you live in Climate Zone 7 or anywhere warmer, the R-30 nailbase layer meets code requirements for a sandwich assembly consisting of rigid foam above the roof sheathing and air-permeable insulation (for example, cellulose or fiberglass) under the roof sheathing. If you go this route, your roof sheathing will be able to dry inward if it ever gets damp. I would advise adding enough cellulose between your rafters to hit R-49 for the total assembly.

   If you live in Climate Zone 8, your nailbase layer isn't thick enough to go this route.

   If you want to install spray foam under your roof sheathing, use open-cell spray foam, not closed-cell spray foam. The open-cell spray foam will allow the roof sheathing to dry inward. Moreover, open-cell spray foam is more environmentally friendly than closed-cell spray foam.

2. Expert Member
   Dana Dorsett | Jul 09, 2015 08:11am | #2

   GAF nailbase is polyiso, and with that material it's imprudent to use the labeled R value when looking at minimum requirements for dew point control for your climate, since it's performance falls substantially at cold/very-cold temps. In US climate zone 3 or lower it's fine, but in zone 5 or higher it has to be derated (by how much is dependent upon the stackup) for this effect.

   With location/climate it would be possible to be more precise.

3. 
   n2dirt | Jul 09, 2015 09:45am | #3

   Thanks for the help I'm in zone 3.

4. Expert Member
   Dana Dorsett | Jul 09, 2015 03:42pm | #4

   In US climate zone 3 you can count on it being pretty close to the labeled R28-R30 or whatever they say it is.

   IRC 2012 only calls for R38 in zone 3:

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

   With a calculated U-factor you'd only need to be under U0.030 (see Table N1102.1.3 ), which would be the performance of a continuous R33, so you're really almost there without any additional R on the interior.

   From a dew point control point of view you can go with 5" of open cell or even 5" rock wool or fiberglass without any moisture problems at the roof deck whatsoever. If it's a plywood or OSB roof deck you can tape the seams and caulk the rafters to the roof deck and insulate with cheap fiber. Alternatively, you could air seal the exterior with peel'n'stick membrane WRB products. If it is 2x6 rafters at a standard spacing, you might as well go with R23 rock wool or R21 fiberglass batts which would usually be cheaper than an open cell foam or blown fiber solution. With over half the R on the exterior, the temp at the roof deck would almost always be above the ~40F wintertime dew point of your interior air, and even without latex paint on gypsum or other interior side vapor retarders it would be just fine.

   If going with a spray foam solution, at any given foam-R you'll be better off with open cell foam than close cell, since the path through the thermally bridging framing is nearly twice as long (= 2x the R). Putting high R/inch foam between framing is usually a waste of good foam, unless it's necessary for it's lower vapor permeance. In your case you have gobs of dew point margin from the R30-ish nailbase panels. The high global warming potential of the closed cell foam's blowing agent (usually HFC245fa) is reason enough not to use it, except when necessary, but in this case R20-ish open cell foam will substantially outperform R20-ish closed cell foam due to the thermal bridging issue.

5. 
   n2dirt | Jul 09, 2015 11:16pm | #5

   Thanks so much. Should first layer of OSB have an underlayment? My original plan from outside to the in was metal roof, tar paper, nail base, zip panel and OC foam.

6. GBA Editor
   Martin Holladay | Jul 10, 2015 08:46am | #6

   Eric,
   If you are installing Zip sheathing as your roof sheathing, the Zip sheathing doesn't need to be protected by a layer of roofing underlayment. You can install your nailbase right on top of the taped Zip sheathing.