Why not insulate at the ceiling joist level. We found a 1980 standing seam roof over an 1851 Cypress Shingle roof. I wouldn't spray foam against the wooden shingles. Not that it matters, but I doubt they're Cedar. Not sure about what was used up there in 1842, but around here everyone that doesn't know better also calls them Cedar Shakes.
"Shakes" is probably a 20th Century term. Back then, they were all made by hand, including splitting, but they were called shingles.
Here's a link to the page on my website about that roof we found. You can see the old roof in the first three pictures, which include one edge of the metal roof on top of the shingles.
http://historic-house-restoration.co...sShingles.html All those pictures are thumbnails. You can click on one, and get a full sized version.
Historic Associations don't like spray foam, but they're fine with the way we do it by wrapping a vapor barrier over, and between ceiling joists, and blowing in insulation. The blown in stuff can be removed later without damaging any of the original parts.
It was not unusual for these old wooden shingle roofs to last 150 years. These days, they've engineered the life out of them by interlacing rows of tar paper. If one is put on according to specifications by the Cedar Shingle and Shake Bureau, they typically last about 35 years. The old ones would have lasted longer than that, had the fasteners not started to fail. The reason there were enough buildings left standing to save in the restoration of Colonial Williamsburg was because they still had the original Cypress shingle roofs on them.