I will start with the problem statement: For our unvented roof (vaulted ceilings with skylights) we have no ridge vents and the rafter bays are closed off, all on purpose - but now we are unsure if it is safe to seal off the interior roof sheathing with closed cell spray foam (CCSF) because the roofing underlayment that we ignorantly allowed has such a low permeance value. I feel that we are now in an unsolvable situation? Is Martin Holladay or Joe Lstiburek around to help with this??
My husband and I are rebuilding a 3000 sq ft home that has vaulted ceilings (2 x 10 rafters) with 7 skylights and we'd like to use LED canned lighting as well. We are in the Pacific NW in the zone 4-5 region would like to make a well-sealed, energy efficient home. After researching the topic of vented vs. unvented roofs, it appeared that many FineHomebuilding articles and GreenBuildingAdvisor articles stated that sealing and insulating with CCSF directly against the interior side of the roof sheathing would be the best thing to do for vaulted ceilings with skylights.
We chose to do the unvented roof with spray foam (at least 3in CCSF + 6in FG batts). We haven't yet insulated but we purchased a fairly expensive roof (stone-coated steel on 1in battens) because of the possibility of fire in our area. The roofing company knew that we were doing an unvented assembly (obviously, no ridge vents), but put Owens Corning deck defense down on the top of the roof sheathing before nailing down the battens. It's a semi-impermeable vapor retarder with perms=0.23. We didn't know that this might matter because the exterior roof underlayment had not been mentioned in any of the articles we had read. All the articles were about sealing off all the small air leaks (for which CCSF is great) and keeping the warm, conditioned air away from the cold roof sheathing to prohibit condensation/moisture from forming. We put the roof on last year.
Now we are getting ready to insulate walls and ceilings with spray foam. We were told by a premium homebuilder (who came to bid on the staircase) that the CCSF directly applied to the interior side of the roof sheathing would cause us moisture problems. After reading some more recently found articles about underlayments and some Building Science articles by Joe Lstiburek, I see that the deck defense underlayment may be a problem, since we now have no outward drying path to the exterior, and the CCSF on the interior underside of the roof sheathing will certainly be a vapor barrier and not allow drying to the interior. The steel roofing material is 1" up off of the roof sheathing so there is air flow between the exterior roof underlayment and the steel roof shingles, but if moisture gets under the deck defense underlayment (is this a concern? moisture is sneaky stuff....) it won't have a way to dry out.
We do not know what to do now to keep our unvented assembly - We must insulate in November but we don't want our roof to rot over time. Any help out there???
Thanks for any help/ideas,