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Rental wall

user-6645241 | Posted in Energy Efficiency and Durability on

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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  1. Reid Baldwin | | #1

    Are you regarding scissor trusses as an alternative to 1.5 story, turning it into either single story or two story, or do you have some plan for building a 1.5 story with scissor trusses?

  2. user-6645241 | | #2

    This is what I'm calling 1.5 story. Two lofts separated by 6 feet, connected by a catwalk. With 12' walls balloon framed that gives me about 3'10 wall upstairs. The simplest way for me to think about it is my original plan: structural ridge beam and TJI rafters. If I used scissors trusses, I would want similar headroom, so whatever that would entail, probably raising the wall height to compensate.

  3. user-6184358 | | #3

    It looks like you need min R49 in the ceiling. The 12" joist likely won't have enough space. You can use deeper I-joist to get the needed depth plus an ventilation gap, say an 18" or 24" I-joist. This would keep your ridge beam plan. If you went trusses, they can make parallel chord trusses with a raised heel. This would save onsite labor and eliminate the ridge beam. The truss would also make venting at the ridge easier. You could look at Henry BlueSkin VP100 peel and stick for the air barrier and waterproofing on the outside of the walls. The stairs in you drawing looked odd. Like a ships ladder is that allowed?

  4. user-6645241 | | #4

    Thanks for the replies. I read here:
    that there is a minimum 49 or 38, 38 being allowed with an energy heal truss. I was thinking I would pass for that with the rafter insulation going full depth all the way to the outside of the wall. Raised heal parallel chord trusses seam like the best of both worlds, but I've read that they still apply some outward thrust to the top of the walls, which would be ok where I have the walls tied together with loft floors, but I don't know about in between the lofts in the catwalk space. I guess I'd have to talk to a truss company to see if they would work for my building. The stairs are an alternating tread space saver design, I don't know if I'll use them in the end or just a steep set of standard stairs; they look weird, but I think they are actually more comfortable to use once you are familiar with them. There are no enforced building codes here.

  5. Expert Member


    Having the rain screen gap behind the sheathing is an interesting variation and I'd be interested to see what people think of it. It does provide protection from interior moisture damage to the sheathing, but negates some of the other advantages of traditional rain screens, which are the capillary break between the siding and sheathing, and providing a drainage channel for bulk water. Depending on where you are the sheathing, being only attached vertically, may not meet shear requirements either.

    Although you live in an area without code enforcement, I'd caution against including features that don't meet codes for liability and insurance reasons if nothing else. Apart from the stairs, it's hard to tell if your bedroom windows meet egress requirements, and I don't think angled top plates on the walls would pass either.

    Good luck with your build. It looks like a nice place!

  6. GBA Editor
    Martin Holladay | | #7

    This article summarizes what you need to know about insulating walls: How to Design a Wall.

    It is routine to have a vent channel on the exterior side of a wall. The vent channel is called a rainscreen gap. However, you don't need to create this vent channel between the studs. The rainscreen gap usually goes on the exterior side of the wall sheathing (because that way, it's easier to build). Here is a link to an article with more information: All About Rainscreens.

    In Alaska, an interior polyethylene vapor barrier usually doesn't cause problems -- but if you are planning to install air conditioning in the house, and if the house is located in a (relatively) mild location like Homer, then I would advise using a so-call "smart" vapor retarder like MemBrain instead of polyethylene.

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