Tiny House Roof Assembly
First time builder Evan here, putting my sheathing up this week. Yahoo! Would love help solidifying my roof assembly for next week.
Tiny cabin, 80 square feet (8×10)
Vermont, Zone 6
Cathedral shed/skillion roof pitch 3:12
I had updated my roof assembly to be as follows: drywall, 2″ of fiberglass faced polyiso, 3″ of polyiso cut and cobbled in between 2×6 rafters, 2.5″ air for ventilation, OSB with taped seams, 30# tar paper, metal roof.
My limited beginner understanding, based on several articles on GBA and Building Science, was that I needed to do a vented assembly because I could not fit R-40 or more of insulation in the tiny house’s height restricted roof to keep the sheathing warm/dry and prevent ice dams. However, the expertise here on GBA from users Akos, Jason S, and others has recommended unvented tiny house assemblies in zone 6 before, both to me on an earlier thread and in Ari’s tiny house Q&A post back in July.
If I did the unvented approach it would look like this (thanks to Akos): Exposed rafters, 7/16″ OSB with taped seams, 3″ polyiso, 30 pound felt underlayment, 1×4 strapping, metal roof. Edges of the foam would be protected from critters.
My questions about the unvented assembly are: (1) If the cabin needs more insulation, how much air permeable (cellulose, fiberglass, wool, etc) insulation between the rafters can I add later before I develop condensation issues? I was guessing about R-16.5, or half the total roof insulation, but wanted to confirm. (2) Will 3″ of polyiso be enough to prevent ice dams? My limited understanding is that only 3″ would allow a lot of heat to escape and melt snow on the roof, leading to dams. (3) Should I use a peel and stick product on the roof like GAF StormGuard? Lastly, (4) is a roof overhang necessary? I would love to have no/minimal overhang to stay within the 8’6 width for road transport and thought if the roof is all the same temperature without an overhang that perhaps melting snow would not have a chance to freeze at the eaves and dam up.
Thank you so much for your continued support and expertise! I would not have gotten this far without you all.
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