Feedback on 2×8 exterior walls with 2×4 studs to limit thermal bridging
I live in Eastern Washington, Zone 5 A. 100 degrees in the summer to zero in winter with 1-3 ft of snow. I will hopefully be building a home this summer, 2016. I originally looked at SIPs but am convinced that if they are not installed perfectly I may end up with moisture problems later on. My brother suggested 2 X 8 top and bottom plates with 2 X 4 studs, (1/2 inch gap between them to break the thermal bridging), and blown cellulose insulation. Then two separate pieces of 1 inch ridgid foam insulation on the outside.
I am not a contractor and pretty much know nothing about what I am proposing other than what I read in forums like this and other post’s. I know that for what I am about to spend on new home construction I want an efficient home. What are your thoughts on this, problems I may encounter, moisture problems?
Thanks in advance.
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No mention of sheathing on/in your proposed wall! I'll assume you have 7/16 OSB or plywod in there somewhere so you'll have about a 9 3/4" thick wall and about (r30). The 2x8 plates, add expense and reduce the flexibility compared to separate 2x4 plates. I'd switch to separately framed inner and outer walls, sheath the inner wall's outer face with OSB detailed as the air barrier, and space them at 2 1/2" and switch to mineral wool bats, NO FOAM! The result is 10" thick, about r30 after fully accounting for the"thermal bridging" of the wood. Housewrap over the outer studs then vinyl siding (NO EXTERIOR SHEATHING) as a simpler, cheaper,, more robust alternative. With the double stud design window installation as ""outies" is not complicated by exterior foam.
Thomas,
You wrote, "I want an efficient home." You also wrote, "I am not a contractor and pretty much know nothing about what I am proposing."
Here's my advice: You are the type of person who should hire an architect, designer, or energy consultant to help you with your project. This person will be worth every penny you pay him or her.
If (instead of taking my advice) you end up designing your own house, you should spend many hours on the GBA site so that you learn as much as possible before proceeding.
You might want to start with this article: Green Building for Beginners.
A half inch of cellulose isn't much of a thermal break- it's less than R2. If you're going to do double-studs on a single plate, use finger-jointed 2x3s on the non structural studs, and stagger the sets of studs.
This makes dense-packing celluose difficult, but you can still damp spray it.
double studs on the single plate only thermally breaks part of the framing fraction- the stud plates are still thermal shorts. For about the same wall thickness it's easier and cheaper to build a 2x6 wall with 2" of exterior continuous rigid insulation, and it can be somewhat higher performance to boot.
Thomas you should do a bit of research on Larsen Trusses which is similar to what you propose without the lack of thermal break on your top plates and rim joists. Basically a normally framed house with a balloon framed shell to hold the dense pack cellulose. You will come across other systems of insulating like REMOTE and PERSIST which are also viable options. Basically spend a few weeks reading every blog on this web site which will cover almost every facet of energy efficient construction methods. The actual hardest part will be finding somebody who can build outside of their comfort zone for you..