I would build the rafters down with 2x4 or 2x6 and retain airflow against the roof with plastic or cardboard channels. You don't want to create a "hot roof". A simple way to do that would be to counterbore most of the way through the two-by lumber and use 3" construction screws.

If you get about R-40 on the top, say R-30 on the slopes with fiberglass batts, and R-19 on the walls*, you'll have a reasonably modern building envelope.

_{* They are just walls. Don't go overboard with twice the insulation the rest of your walls have.}

I would build the rafters down with 2x4 or 2x6 and retain airflow against the roof with plastic or cardboard channels (baffles). You don't want to create a "hot roof". A simple way to do that would be to counterbore most of the way through the two-by lumber and use 3" construction screws.

If you get about R-40 on the top, say R-30 on the slopes with fiberglass batts, and R-19 on the walls*, you'll have a reasonably modern building envelope without the expense and hassle of rigid foam.

_{* They are just walls. Don't go overboard with twice the insulation the rest of your walls have. Since the slopes are halfway between walls and ceiling, they don't really need full attic insulation either.}