Hello, I was hoping to get your advice on how to insulate a tricky part of my attic.

I had the underside of my roof sheathing sprayed with closed cell foam the other day, but the company said they weren’t able to get to parts of the roof that were made inaccessible by an arched barrel ceiling construction. There’s a room on the second floor that has an arched barrel ceiling made of lath and plaster. It’s like a bowl that fits into the V-shaped roof above it. There’s clearance of maybe four feet from the apex of the bowl to the apex of the roof, but there’s a space at the sides of the attic where the foam guy couldn’t reach, a void sandwiched between the roof and barrel ceiling.

The attic guy said I need to have 3’ of the barrel ceiling cut away along the entire length of the room, on both sides of the arch, so they can access the roof from the room. But lath and plaster is very expensive to repair, anywhere from $4-$6K to repair the sides of the arch. I’m wondering if there’s some other way I could achieve air impermeability without cutting up the barrel ceiling, for example, insert sheathing sheets into those narrow v-shaped voids, stuff them with insulating material ?

thanks for any creative ideas!