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.

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    "Eco-friendly" is a vague and subjective term. Some consider long-lasting solutions eco-friendly for their small average annual footprint. Also, some plastics are plant-based. You'll have to specify your criteria, and even then we're verging on a shopping assistance question. – isherwood May 14 at 15:08
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    Plastic vapor barriers that use waste oil products keep that material from going into landfills so this is really an opinion type shopping question (wood rot is natural and you want to stop that?) – Ed Beal May 14 at 15:18
  • Please put all that in your question post. It doesn't belong down here. – isherwood May 14 at 16:37
  • where is the cabin? Ecofriendly means local materials if possible. – Willk May 15 at 0:19
  • It's in Massachusetts. – Jesse Yishai May 16 at 1:04

Red Rosin Paper (which is a bit old-school, but can still be found) would fill the bill, I believe. It's plant based. Seems to be getting sold for floors now, but was the thing people used before housewrap on exterior walls under siding.

However, depending on type and installation of insulation, all you really need is an airspace between the insulation and the roof. You need a vapor barrier between the ceiling and the insulation.

Some sort of cloth could also work, just to contain the top of the insulation and maintain an airspace. Boards also work, but may cost more than you want to spend.

You will want metal screening on the vent openings to discourage rodents (and perhaps other things) from nesting in the insulation.


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