Re: 1921 Church/vaulted roof
If you are planning to reroof, you can place rigid foam insulation on top of the existing framing or skip sheathing, cover it with plywood, then place your roofing material on top of that.
That's how it was done on my parents' house many years ago. The framing did not allow for insulation; there was no ceiling cavity. I don't know that it's the BEST way to do it, and there may be good reasons not to do it that way, but it might be a possibility to consider.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.