Hello,

I have a late 60’s cathedral ceiling, unvented, with no effective air barrier on the interior. The interior ceiling is plywood with no plastic or sealing of any sort, and lots of gaps. Currently there is some 8″ of batt fiberglass between the 2 x 12″ rafters. Thankfully, the house was used almost exclusively in the non-heating seasons, but that may be changing. Climate zone is 5a.

We are doing a renovation, and plan on removing the exterior roof sheathing, as some rot is expected, and are replacing the roof and sheathing.

Because we don’t want to remove the finished ceiling in the interior, for various reasons, I am considering my options. I would welcome any response, thoughts, and suggestions to my rough plan.

Plan For an Unvented Roof Assembly:

1) Remove all sheathing and batt insulation from the exterior side.
2) To prevent foam incursion into living space during application, lay down cardboard rectangles between rafters or lay house wrap strips over the existing ceiling, coming up the side of each rafter an inch and stapling to the rafters.
3) Spray closed-cell foam between rafters from exterior, enough to either meet code or match the prior R-value.
4) Install sheathing and roofing.

My main question is two part:

A) Could I get away with 2″ of closed-cell foam (again, sprayed between rafters from exterior), and then add fiberglass batt to fill the bay, instead of expensive closed-cell for entire bay?

B) If there is an air space above the insulation and below the sheathing, would this be a problem? Again, this is an unvented cathedral ceiling assembly. I have heard differing answers to this question.

Again, thanks to this community for any thoughts.