Air seal the ceiling or the attic?
I live in the East Bay part of the San Francisco Bay Area. Maps show me in Climate Zone 3B, but I’d say I’m a good deal cooler than that suggests. I’m on the east side of the Coastal Range/Berkeley HIlls, but still in the hills, not the hotter flat area further east. I’m also at the bottom of a valley near a creek with some tall trees and a big hill to the west and the house gets very little direct sunshine on all but one of its windows during the warmer months. We heat a lot in the winter and cool very little in the summer.
The house has no air sealing whatsoever and is only insulated at the ceiling and on one wall that was recently replaced. The attic insulation is about 4″ of blown in celulose. We have (somewhat) insulated flexible supply ducts in our HVAC system that are mostly in the basement crawl space with a few in the attic crawl space. Our return ducts are in the attic and totally uninsulated. I suspect the original asbestos insulation was removed during the abatement before we moved in and not replaced by the contractor that replaced all the supply ducts that were ripped out during the abatement.
With that long intro, I would like to air seal the top of my house at some point and the question is whether to try to seal the ceiling or the roof. Moving all the ducts out of the attic would be virtually impossible, although insulating the return lines would be doable. Sealing at the ceiling plane would be hard and I think would be almost impossible wihtout removing the blown-in celulose first. Also, adding thick insulation on top of the ceiling after doing the sealing work would probably cause code/safety problems with our electric wiring, most of which passes slightly below or just above the blown-in insulation. The more insulation surrounds a wire, the less current it can safely carry without overheating. 12 gauge wire is good for a 20 amp circuit up to a certain length, but not if it’s under 6 inches of insulation. For that, you need thicker wire.
So, I’m obviously marshalling my arguments in favor of sealing and insulating the roof when I have the roof replaced. At the gables, I was thinking of something like plastic sheets or sealed drywall inside of the batt insulation (probably rock wool), or a thin layer of spray foam outside of the batt insulation.
For the soffits and roof deck, I was thinking of a thin layer of rigid insulation outside of the decking, taped seams in the plywood deck, and a 1″ layer of closed-cell foam inside of the deck, with batts between the roof joists. And I’m inclined to remove the celulose in the ceiling.
So, the two questions are:
1. Would I be better off sealing at the ceiling, despite all of the issues I raise?; and
2. If I seal and insulate the roof, does the method of sealing and insulating it I mention sound close to the right way of doing that?
Thanks for reading my lenghty disertation.