Posted By Bob I on 01/04/2010 10:21 AM
Yes but:
That should work providing they spray a sufficient amount of insulation in the attic to keep it warm - I'd look for 4-6" of closed cell foam. The "but" is that moisture issues should be addressed at the same time by air sealing the attic floor/living space ceiling from above by pulling the batts aside and sealing all penatrations and framing joints with spray foam. A stairway or attic hatch should be air sealed with weatherstripping. If there is only a few inches above the top of the eave wall to the roof sheathing, pull back the batts & insulate it well with the spray foam.
4-6" of cc foam would make it a class-II bordering on class-I vapor retarder, and if the vapor retarder at the ceiling plane is class-II or higher you'll have created a vapor trap. It's more than 40% of the total R value, so of the foam were the ONLY vapor retarder in the stackup it would reliably dry toward the interior, but since it isn't you're likely to end up with seasonal high humidity/mold conditions in the attic. Without knowing the condition & vapor retardency of the existing vapor retarders, don't go there. It's always the unknowns that bite you in the end, and since it was set up to dry toward the exterior it's wise to preserve that unless you have certainty about the rest.
If you went with 6-10" of
open cell foam it would be as-warm (about the same R value as 4-6" of closed cell), but highly capable of drying toward the exterior. (It'd be cheaper & lighter than closed cell foam too.) Open cell foam is an extremely good air barrier, but has relatively high water vapor permeance.
For performance reasons you'd still want to air seal all soffit & ridge venting with foam, but you could be somewhat less fussy about air-sealing between the ceiling & attic, hatch doors, etc. (Although it never hurts, and always helps, to air-seal that boundary.) It may still be worth using closed cell for air sealing at the soffits/eaves if it's a thin spot in the insulation (for the higher R value per-inch ou get with closed cell).
The other issue with closed cell at the roof deck is that it makes roof leaks difficult to locate as it's water proof can trap water. With open cell foam a roof leak may saturate an area of insulation at the point of entry, but it's location will be dead-obvious, and it'll dry on it's own to return to it's full R value once the leak is repaired.