If you have the clearance for it, putting rigid EPS or iso sheets above the roof deck is usually less than half the cost per unit R of closed cell foam (comparable to half-pound Icynene on installed cost). Putting the rigid foam above the roof deck also thermally breaks the thermal bridging of the trusses.
With 3" of cc foam and 15" of Icynene you're looking at ~R70 center-cavity, with some thermal bridging, but a very limited ability of the roof deck to be able to dry.
A combination of 5"/R30 of sheet iso above the deck and say 10" of Icynene on the underside of the roof would give you an R65 roof that thermally-breaks the trusses, performing thermally as-well or better than the 3"cc/15" Icynene solution, and raise the average January temp of the roof deck well above the dew point of the interior space air for very low wintertime moisture accumulation, and providing a moderately permeable path for drying toward the interior through the Icynene, as long as you avoid interior vapor barriers.
This would likely fly in the face of the Canadian building codes, which normally require interior vapor barriers unless the sheathing stays above the dew point at the heating outside design temperature, but it would in fact be more robust against rot than solutions that included a vapor barrier. To meet code without an interior vapor barrier would require ~2/3 of the total R to be exterior to the roof deck in Calgary, which I suppose you could do with 7" of exterior iso + 5-6" of interior Icynene. |