Hello, I’ve been digging through the wealth of information here at GBA and have gotten some really good ideas and a much better understanding of theory that is really helpful, but I have a few questions. Im making plans to build a simple 16×30 1.5 story cabin, with balloon framed 12ft walls and 12/12 roof in Homer, AK, an oceanfront town which is in zone 7 from what I can find. I want to build it to be energy efficient and tight, but not extreme. The plan is for it to be a rental, so durability is the biggest concern. I have drawn up a sketch with 2×4 double walls and 12″ TJI rafters, but people are trying to convince me to go with scissor trusses, which seem hard to insulate with cellulose and inefficient unless they have an energy heal. So my questions are:
-What are the best strategies (wall stack, air barriers etc.) to use if high durability (including moisture control) is the top priority? With renters, who knows what people will do; boiling water on the stove for hours without using a vent, poking holes in airtight drywall etc. I know single stud with exterior rigid insulations is often recommended, according to the charts I can do: single 2×4 wall with cellulose plus 2″ foam, which barely goes over the Zone recommended minimum R value; a 2×6 wall + foam, but the minimum exterior foam required gets excessive; or stick with a double wall 2×4, and decide the best way to place the barriers.
-I know how you feel about new people inventing new wall stacks, but I have to ask. I watched the video Martin posted made by the CCHRS on blown in cathedral ceiling insulation. They layered (from the middle out) TJI rafter, Tyvek to retain the blown insulation, 2×4 to provide vent channel, then roof sheathing. I’m sure theres a reason its not done, but why not use this on a wall? Airtight drywall, double 2×4 frame, house wrap, 2×2 wood to provide vent channel, then sheathing and siding. It seems like this would completely do away with any cold sheathing worries.
-The much hated interior poly vapor barrier is very common here, most of the anti vapor barrier references I’ve read say it’s not needed anywhere in the U.S. except maybe in parts of Alaska. Is this a rare exception? I think with the exterior foam route the poly is a problem, but I’m sure anyone in Homer will try to convince me it’s necessary.
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