5 questions: exterior wall insulation
My wife and I are thinking of buying a hard-walled “yurt” to put in upstate NY. Looks beautiful, now I am trying to see how green I can make it. Basically, we would have a 35 foot diameter “yurt” that is made up of 4′ x 10′ wall sections that are screwed to each other and then secured to the floor. A cable wraps around the tops of the walls to turn it into integral unit (this is common in yurts). The company sends the walls and roof that we would attach to the floor. I am figuring out the insulation system for walls, floor, and roof, so then I can estimate the heating and cooling demand. For this post I just want to focus on the walls. The company website says each 4 x 10 section is framed using 2 x 6s, 1/2″ plywood shear panels, drain plane type vapor barrier, and 1x 6 tongue and groove red cedar shingles. They can leave the shingles off, and send separately.
Before installing walls I would put in Triple Guard sill sealer in two spots: 1) where floor meets foundation, 2) where yurt wall meets floor. Q1: Bad/good idea? I also wanted to add exterior insulation to avoid thermal bridges etc. I wanted to avoid XPS and EPS because of the GWP and other inputs. Because we’re in a cold climate, I don’t think polysio would be a good choice. If that’s right, I would remove the water/vapor barrier so the wall can breathe, tape the plywood seams (ZIP tape?) and replace with a “smart” membrane like Plus and ProClima DA before finally putting Roxul mineral wool on the outside with vertical furrows. Then attach the cedar shingles to the furrows (3 furrows 2 foot apart on each 4 foot wide wall section). Q2: Is this a reasonable idea? If so, then for my climate to control moisture buildup/”condensation” I am under the impression that the R value for external rigid foam should be at least 11.25 (I’ll round to 12), and that at least 36% of the insulation needs to come from the board (https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing). Q3: Does the same hold true for Roxul CavityRock (~R value of ~ 4.2/inch, so three inches total thickness giving 12.6 total)? Do you all have a preference for CavityRock MD or DD? For the interior of the wall, I was thinking of using Roxul or Owens Corning mineral wool depending on cost and what you all recommend. This will add another ~20 – 30 in R value (budget dependent). Q4: Should the thickness of the rock wool be a full 6 inches, or, is it OK for their to be an air gap with wallboard/sheetrock? I was going to finish the interior with an earth plaster. I believe plywood as an R value of .62, one inch cedar is 1, so final wall value is 1.4+.62+12.6+30 = 35 – 45. Q5: Overall, is this a solid design or foolish? I read here (https://www.greenbuildingadvisor.com/green-basics/insulation-overview) that some recommend R40 or even R60 on the walls, but I am well above state codes (20 or 13+5). Thanks for any insight, I just signed up to this website and it is a phenomenal resource.
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