I think plastic has the tendency to form condensation more so than any other material and then the condensation freezes...that's why I didn't plastic my whole coop under the tarp and used tarps that could breath a little. I don't know how much the canvas tarps would breath with the water proofing, if at all, nor do I know how much they would hold potential moisture from inside humidity.
I looked up the materials they use on yurts and found one company, but I bet they don't sell small quantities: http://duro-last.com/
I looked up the materials they use on yurts and found one company, but I bet they don't sell small quantities: http://duro-last.com/