Originally Posted by funfool
rigid foam is good for some situations.
Problem is, only get about r-7 from it.
If your rafters are at least 2x4, rolled fiberglass will give you r-13.
2x6 rafter you can get r-19.
You want to get as much r value as you can, and rigid foam is not cheap.
Rigid foam gets you and R-7 per inch in some cases. The R-13 you are referencing is from 3.5" of fiberglass that also has no air barrier aspect or moisture barrier (if unfaced).
The best and easiest thing (usually equals the most expensive as well) would be to condition the crawl and spray foam the stem walls and the inside of the roof with CC SPF.
Vapor barrier and insulation layer in on shot.
You will still have cold rafters and they will be condensation locations but you could cover the rafters with a rigid foam and drywall the ceiling if you have already sprayed the rafters with foam.