1. What are the differences (if any) in terms of airflow and insulation required below the metal roof surface (below the decking)? Does the decking get enough airflow from under the metal roof, or does a metal roof still require airflow under the plywood as well? This would be over a finished attic area, so I'd have minimal area for venting and I'm not sure if or how this would affect my need to powervent or ridgevent to have airflow. Ideally I'd love to use that expanding spray insulation to insulate the attic roof space, even if I need to use those vent spacer boards to leave enough airflow on the underside of the decking.
2. It has been a while since I've done price comparisons; what is the general installed price per square (simple roof) as compared to the price per square of asphalt? My old asphalt roof needs to be removed in either case, so it would be for an install on just the decking. I'm not looking for a to-the-dollar accurate answer, just a general estimate of how much more I might pay per square if I went with something like galvalume (I doubt I could afford copper).
Thanks for the info, Keith