Hi there. I have an adobe garage with metal roof and wood rafters. Want to insulate under the roof (interior) with eco-friendly easy to use material (batts but not fiberglass?). Should I put something (water barrier) against the metal roof first before laying in insulation? Any suggestions on kind of R38 insulation to use. Rafters are 4" so lots of the insulation seems like it would be thicker than the rafters. Then need to put drywall over that? Want to finish off with rolls of bamboo poles for an island feel. Thanks for any suggestions, including 'just get a contractor'.