I am building an octagonal cabin with a 200 sq ft metal roof (Grandrib 3) at a 30 degree pitch; the roof will sit on wooden strapping and will eventually have insulation below it contained by a ceiling.
It was only recently brought to my attention that because I am including insulation (type yet to be determined) I should have an underlayment - if this is indeed the case? My friend told me the cold air should flow between the roof and the underlayment, underneath the ribs essentially. This is to keep out moist air.
However, I am trying to make this cabin as eco-friendly as possible: so far I have used no plastic or asphalt materials, no plastic wrap, I expect the 100 sq ft cabin to dry out when I use its wood stove. By this I mean the underlayment would decompose over time if it got wet. So, if this cabin were left in the woods and no one tended to it eventually it would disappear. I don't necessarily need shopping guidance but maybe a creative solution with another material - can floor underlayment be used in my situation, for example? Can I combine materials in some way? I have searched for eco-friendly underlayments and they are either for floors or are not really that eco-friendly, i.e. they include plastics.
So, my question is has anyone come up with a creative solution for an eco-friendly roofing underlayment? I am not worried about water getting in from above, just moist air. I would be happy to hear about solutions that didn't require an underlayment too.