Double stud wall or 2 X 6 wall with foam, need help on decision
My wife and I are getting ready to build a house just on the lake in Chattanooga, TN. I’ve been going back and forth on construction of the house. The house is 2,700 on one level that will be over a crawlspace. I’ve gotten a quote going with ICF up to the roofline and am wondering if that is the most effective way to get a super energy efficient envelope that I want. I’m considering a double stud wall system but I’m worried about moisture issues. The house will be all brick for the exterior except for a couple of gables under the front and back porches. In talking to builders and others I’ve already been asked multiple times why I’m even considering these when the payback is so long. We plan on living in the house 20 – 30 years. I’m interested in hearing what the best wall system for my situation would be. I’m planning on encapsulating the crawl space and using spray foam for the attic.
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Replies
Matt,
If you are cladding the home with brick veneer, I urge you to choose the 2x6 wall with exterior rigid foam, because the rigid foam does an excellent job of addressing inward solar vapor drive. For more information on this issue, see When Sunshine Drives Moisture Into Walls.
If you haven't seen it yet, you may wish to read this article, too: How to Design a Wall.
In a zone 4A climate it's possible to get to Net Zero Energy with a PV array that fits on the roof with the following "whole assembly" R-performance:
R25 walls (For a 2x6 + foam wall that would be R20 cavity fill + 1.5-2" of foil faced polyiso facing the masonry cavity.)
R60 attic (Trussed roof with 18" of cellulose, with no ducts or air handlers in the attic)
R15 basement/ crawlspace walls (Minimal 2" + 2" ICF, or 1" of polyiso trapped to the wall with a 2x4/R13 studwall.)
R7.5 sub-slab (Full area, not just slab edge. going with 2" of EPS is cheaper, greener and higher R than 1.5" of XPS.)
U0.30 or lower windows, with an SGHC of 0.35 or lower.
Going much beyond that has a higher lifecycle cost than net metered rooftop PV driving heat pumps.
With brick veneers there is always a thermal bridge at the top of the foundation to contend with. Purists will sometimes include a row of autoclaved aerated concrete block between the foundation sill and foundation to manage that. Short of that designing the foundation with at least a foot deep step between the section supporting the brick and the section supporting the foundation sill, running the cavity foam the full foot below the sill, and the interior foam all the way up to the sill, and insulating the sill & band joist offers some amount of thermal break.
https://www.greenbuildingadvisor.com/sites/default/files/Brick%20veneer%20detail%202.png
(Draw the interior foundation insulation in with your mind's eye. :-) )
what is your recommendation on the thickness of the exterior insulation? From reading it seems that a minimum of 1" is what is recommended. For where I am taking cost into consideration what do you recommend? I would plan on using the dense pack cellulose in the wall cavity.
Matt,
I agree with Dana that polyiso that is 1.5 inch thick or 2 inches thick makes more sense than 1-inch foam.
Remember, it's the same amount of labor in any case -- so if you can afford the thicker foam, you should specify the thicker foam.
I see from the link that Dana posted how it looks with a sheet of 1 1/2 " rigid sheet. Is there a diagram of how it would look with 4" ( 2 sheets doubled up)?. Trying to picture in my mind how it works where the stud wall meets the foundation with the extra thickness to contend with.
Matt,
There are lots of ways to detail exterior rigid foam. I'm not sure what your questions are, but below are four illustrations to get you thinking about different ways to do this.
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Matt,
If you are unfamiliar with methods for attaching and detailing exterior rigid foam, you may want to watch these two GBA videos:
How to Install Rigid Foam Insulation Outside a House
Superinsulating a Home With Rigid Foam
I used rigid foam with brick veneer and a poured concrete foundation. The top of the foundation was a difficult area to detail due to the need for a brick ledge. I couldn't get over 2" of foam without either pushing the bricks too far off the brick ledge or pushing the wall too far inside the foundation. I think there are bolt-on brick ledges available but I didn't investigate that because I decided 2" was enough. I have no idea how brick ledges are formed in ICF basements.
The question I have is best shown in the pic attachment by Reid in answer # 8. Is a much thicker poured wall necessary to get the brick ledge and the necessary width for the wall to sit on?
This attachment is what I was thinking about had I decided to pursue thicker foam. A steel plate is bolted to the brick ledge to extend it out. I didn't do any engineering or other feasibility assessment. It seemed less wasteful of material than making the whole wall thicker.