Spray foam roofline in stone house
I have a house made of dry-stacked stone (with a continuous layer of concrete parging on the outside, and mostly plaster on the inside of the walls, with some exposed dry stone in places the plasterers couldn’t reach. I regard the concrete on the outside as a pretty good air barrier, and I am debating whether to complete the envelope by spray-foaming the underside of the roof-deck to R-32. Currently the roof is insulated haphazardly with R-13 batts behind a finished drywall dormer ceiling.
I worry about a few things before proceeding with the plan:
1. My house has no central air conditioning (because it is basically a cave and it is already very cool in the summer, except in the attic.) It has hydronic heating, too, so there is no air handling in the house at all. Will moisture levels in the house become a problem if the roof is sealed up? For example, will this worsen the problem we already have sometimes have with condensation on aluminum windows in the winter? Will water vapor, no longer to escape through the roof, instead migrate into the stone walls and cause mold? We often get some water in our basement after spring thaw, so that has me concerned about humidity problems too.
2. Will all the old-house smells all of a sudden become more noticeable because they cannot escape? For example, smells from within the stone walls?
3. The attic is a finished room with original wide pine plank flooring. Under the floor is lots of loose-fill fiberglass. I worry this fiberglass will produce dust that has nowhere to escape to after tightening up the attic space.
4. More than anything I worry this is an unreasonable expense that will not pay itself back soon. We pay $2800 a year to heat the house with oil and wood heat, and the project will cost $5800. (The expense is because we plan to remove existing shoddy drywall on the ceiling before spraying, then replacing it after.) We could instead spend the money on some other project, like upgrading our oil furnace to propane or insulating the foundation or something.
Any advice would be greatly appreciated, as this is starting to cause me some anxiety. I hate wasting so much oil on heat each year, but an antique stone house is such a challenge for green upgrades!