Jesse Lizer’s new house will be in Climate Zone 6, where he can expect 7,400 heating degree days a year. High R-values in the building envelope are a high priority.
As Lizer explains in a Q&A post at GreenBuilding Advisor, there are three possible scenarios for constructing outside walls:
- A double-stud wall, between 10 in. and 12 in. thick, filled with dense-packed cellulose and sheathed with the Zip System OSB.
- A 2×6 wall framed 24 in. on center with 1/2-in. OSB or plywood sheathing, insulated with wet-blown cellulose and wrapped with two layers of foam on the outside of the wall — 1 in. of XPS and 1 in. of polyisocyanurate.
- A 2×6 wall with 1 in. of exterior polyiso over 3 in. of EPS, also with 1/2 in. sheathing and wet-blown cellulose.
In an unusually detailed post, Lizer lists two other important pieces of information: the cost of each option, and the nominal R-value of each wall system. The double-stud wall ($2.15 per sq. ft. in material costs) has an R-value of 40; the 2×6 wall with a total of 2 in. of foam ($2.05 per sq. ft.) is R-32; and the wall with a total of 4 in. of foam ($2.15 per sq. ft.) is R-41.
“Any big thoughts on the best approach?” he asks. “My thinking is leaning towards exterior thick foams would do a better job at sealing up the house from both vapor and air vs. the Zip and thicker wall. Since I will be building it (yes, I do have years of construction experience) I am thinking the 2×6 and thicker foam will be faster and easier to detail?”
That’s the topic for this week’s Q&A Spotlight.
The double-wall option
All three of Lizer’s proposed walls minimize thermal bridging, but the double 2×4 wall does so without the use of…