I am renovating this old porch addition and want to insulate it properly. There must have been some moisture issues before because some of the OSB has what looks to be mold or something moisture related. Luckily it doesn't seem to smell and it still feels firm. I'm speaking with a spray foam installer today but I have the feeling that cost for the size of the ceiling (145sq.ft.) is going to be rather... expensive.
These are 2x4 rafters on OSB with a layer of roofing felt and then a corrugated locking steel roof. There should be some ventilation due to the nature of the roof. I've attached some photos. My thoughts are 2x4 batts with a layer of 6mil poly and then 5/8" drywall. The walls have 1" foam on the outside w/ stucco.
If I don't go with spray foam what's my next best option for method and materials?
Thanks for your help.