You can also insulate from below. You can never have TOO MUCH insulation. Cut polyiso (4X8 foam board sheathing) to fit and then fill in with can foam... you might want to skin over that with lightweight aluminum flashing (cut with a pair of household scissors) or perhaps Tyvek if you can figure out how to keep it up, where exposed to protect from dirt/bugs/critters. Manufactured RV's use something that looks like commercial greenhouse ground cover "cloth" (but it's not as sturdy). you can add insulation to the ceiling from out side as well (George Myers did although I would have used more than 1" polyiso and used a vinyl sheet with elastometric roof coating rather than metal... some of the things George has come up with scares me... I have his "Bus Converters Bible") http://www.busnut.com/bbs/messages/1...tml?1165275072
. If you are planning on putting a roof deck up, you can cut down on the heat gain/loss by putting a couple sheets of THICK foam insulation under the decking... think building a floating deck on a lake.
Expandable foam needs AIR to help it cure and expand. While it sounds neat to be able to squirt a bit of expanding foam (like Great Stuff) into a hole in the ceiling and upright tube, the fact remains that you have to drill a lot of holes in order for the foam to have enough air for it to expand. Been there, done that (after listening to others -- engineers -- that I thought knew better) only to discover the stuff didn't expand more than a couple of inches (but pumped out the entry hole rather nicely) unless there was a gaping hole close to where I was squirting the stuff in at.