@tim said:
If you haven't already done so, pop over to the FineHomeBuilding website and do some reading over there. Lots of good stuff.
There is an FHB related site:
http://www.greenbuildingadvisor.com/book/export/html/14955
http://www.finehomebuilding.com/PDF/Free/021189072.pdf
http://forums.finehomebuilding.com/breaktime/energy-heating-insulation/spray-foam-insulation-8
There are different solutions depending on climate. It does make a very tight house which is a very good thing but does require proper detailing and the aforementioned air exchange solution.
If you can afford it, it is the best insulation, it does require different approaches and solutions to ensure you don't have problems down the road. (this is also true of every other type of insulation)
With the metal roof you will likely not have to worry about roof issues but I would want to have an air gap for ventilation to reduce the amount of heat transferred to the structure. There are many ways of achieving this either by adding a second layer of sheathing with a spacer to create air channels or by the traditional chutes on the underside and then spray foam over them.
Creating a complete envelope and having the entire HVAC system in the conditioned space will ensure better efficiency and the sealing and insulation values you can achieve will offset any increase in the volume of the conditioned space.