I'm thinking for me that I want space between the metal roof (which may or may not be self-venting--the ag style roof without the foam under the ridge is, but for other reasons you don't want ag style) and my insulation.
possiblities include:
sheathing with OSB or something similar. putting the roofing on with or without purlins.
and/or those plastic foam channels,
somewhere John has posted instructions for venting the top of most any kind of roof--from Core-A-Vent?--that or one of its competitors.
http://www.cor-a-vent.com/Then put batts or one of the foam insulations or damp cellulose or..... then the ceiling of the kind I have down at the barn--half-lap joints in any of several widths of rough-sawn boards, I'm thinking that three different widths would be nice. That I may get around to whitewashing one of these years--barn sure hasn't been done yet..
Icynene and some soybean derived foam are the hot choice. The latter is supposed to use water as the blowing agent--nice if you might be sensitive to the blowing agent in Icynene (severe asthma when I worked near leaky paint booths do not exactly make that a welcome choice, although for most people since it must be professionally installed, there'd be none left after came in that night to inspect) I expect that the soy-derivitive is even more expensive than the Icynene. Both need professional installation.
