I’ve studied the available postings and articles concerning insulating cathedral ceilings and are still unsure of the answer to; can I use “breathable” un-foiled faced polyiso below rafters with no drywall and open cell spray foam in the rafter bays in zone 3? What are the issues?

If you can do soffit to ridge venting above the open cell foam it doesn't matter what the vapor permeance of a rigid foam layer on the interior would be. You'd have to put a somewhat rigid somewhat vapor permeable surface an inch away from the roof deck to do that. Cut'n'cobbled half-inch rigid XPS works fine, as does fiberboard, or 1" unfaced EPS. Cut some 1" EPS or polyiso strips and tack/glue them to the roof deck adjacent to the rafter element as stand-offs to guarantee the code minimum vent gap for whatever rigid stuff you use as the spraying surface.

For the interior side foam, you would have to use fire-rated Dow Thermax polyiso (which has vapor impermeable foil facers) to be able to skip the gypsum board and still meet fire safety codes.

Thank you Dana. The home has four hips merging into a 5' ridge and the hips themselves, both attic and cathedral ceilings, do not have access to the very short ridge.Therefore I am required to have an unvented attic/living space and I am concerned. I read where open cell foam has a vapor permeance of 11 perms at 5-1/2" and R 3.9. With 6" rafters we have R rafter bay of 21, still short of being in the 30's. Roofers here won't touch rigid insulation above roof deck so to obtain additional total R and lower framing factor I am looking at rigid foam board attached to the bottom of the rafters. If 2" of fiberglass faced rigid polyiso is used we are now at 32'ish R and 1.2 perms vapor permeance (for 1") which allows at least some vapor transfer back into the home if the open cell (11 perms) needs to give up some moisture. With foil-faced polyiso I am concerned any rafter bay moisture would be forever locked in. Therefore, leading to my original question about the need of drywall below the polyiso.

T hit R-30 ish or a bit more without moisture trapppng risk, first protect the roof deck with a flash-inch of CLOSED cell polyurethane (~R6). Then install 1.5" of polyiso only on the rafter edges, creating a total cavity depth of 6". Damp spray 6" of JM Spider at 1.8lbs density (~R25-R26), and cover it with wallboard, long-screwed through the foam strips.

The center cavity R will be in the low 30s, but with the ~R9 thermal break on the rafters it performs comparably to R38 between joists.

If it turns out to be cheaper than damp sprayed Spider, you could install 0.7lb density open cell foam under your 1" flash of closed cell foam, but you'd need 2" rafter edge strips for 6.5" of depth, the oc foam would have to be installed in two lifts with a cooling period between lifts, and it would be a bit more difficult to trim the spray foam flush with the rafter-edge strips.

Dana, with your last commentary I am more comfortable with insulating my unvented attic. Furr out my rafter edges with polyiso of whatever flavor and then cover with drywall. Researching Spyder now, though I don't believe the local trades are familiar with the product, so I will head towards plan B. My foam installer is ready to do CC or OC as I wish so no problem here. Also, my drywall installer does not have an issue with installing through polyiso. Thanks again.

With closed cell you don't need or want more than an inch, two at most, otherwise it creates a moisture trap at the roof deck between barely-permeable (and often wet) roofing layups, and barely permeable closed cell foam. At 1" you would be at about 1-perm on the closed cell foam, which is about the right compromise between drying capacity and limiting wintertime moisture uptake.

If you did it all with half-pound or 0.7lb foam there is a risk of summertime moisture cycling, where overnight the roof deck and exterior open cell foam would take up moisture, which gets driven off by the sun during the day, leading to oddly shifting relative humidity in those rooms. This doesn't happen in every house in zone 3 done with only open cell foam, but often enough that it's worth the inch of closed cell to prevent it.

Spider isn't the only fiberglass that would work, L77 or Optima would work too, but Spider can be damp-sprayed without netting due to it's water-activated adhesives, whereas the others have to be blown in mesh, which makes it somewhat more expensive.

Damp sprayed cellulose can work too, with R/inch performance very comparable to half-pound foam. You'd need the flash-inch of closed cell + 7" of 0.5lb foam or cellulose, which is a bit tough to do with edge strips. If going with either of those it's better to run a set of 2x3s 16" or 24" o.c. perpendicular to the rafters to achieve the necessary depth, with a bit of thermal break, "Mooney Wall" style, on which to hang the wallboard.

http://www.builditsolar.com/Projects/Conservation/MooneyWall/M01.png

In many ways fiberglass or cellulose for the majority of the fill is less risky. Open cell foam (closed cell too) is sensitive to installation temperature and chemical mix, and it they are off a bit there are long term outgassing issues. The maximum safe depth per lift with either 0.5lb or 0.7lb foam is 5.5", which means that in any of these scenarios it needs to be installed in two lifts with a cool-off period. If you install 6"+ in one go there is both a very real fire risk during the curing period for hours after installation, and uneven curing, shrinking and adhesion problems.

My personal preference would be to use cellulose, for it's lower environmental impact, and also for it's thermal mass and moisture buffering/distribution characteristics. A 1" flash of closed cell and the rest as either fiberglass or cellulose is a VERY safe stackup in zone 3. Read this 14 page document, and note the risk factors for zone 3 locations in the summary tables toward the end.

http://buildingscience.com/file/5809/download?token=18Y6NJQ8

> Cut some 1" EPS or polyiso strips and tack/glue them to the roof deck adjacent to the rafter element as stand-offs ...
.. I am required to have an unvented attic/living space

Aren't there shingle vents that go under the shingles and provide exit vents short of the ridge? Consider similar to above but completely above the rafters - ie, OSB roof decking then 1" vent gap created with furring strips then taped rigid foam (preferably staggered) then rafters. Very little air movement is needed to remove the small amount of moisture that diffuses through an airtight < 1 perm partition. No spray foam or furring-out needed.


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