What is the best method to upgrade a 3-season cathedral ceiling to 4-season in zone 7a?
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From inside to out my roof consists of 1 1/4 x 5 1/4″ tongue-and-groove cedar, then a layer of poly, 2×4 strapping with Buffalo board filled in the gaps (1 1/2″), then roof sheathing, then shingles. Need to upgrade this and maintain the cedar on the inside. The roof is laid on large beams inside.
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Keith,
I've never heard of Buffalo Board. In the U.K., it seems to be a brand of birch plywood -- but I'm guessing that you are talking about something else.
For all intents and purposes, your roof assembly is uninsulated. If you want to keep your ceiling looking the way it does now, that means that your insulation has to go on top of the existing roof sheathing.
In your climate zone, the code requires a minimum of R-49 insulation. You can achieve that with 8 inches of polyiso (rigid foam) or about 12 inches of EPS.
You might find it easier to install a SIP roof or nailbase rather than assembling a foam sandwich on site.