I suggest you get some recycled polyiso, cut it to fit the cavities and spray foam it in place. That will give you a good air seal and an immediate R 16. Then I'd Larsen truss to the inside with plywood gussets and 2x3's and dense pack. You could easily get R 40 that way and it would be a lot cheaper/healthier than a full load of spay foam with a much higher R value.
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