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:
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 rigid foam insulation. Two conventional 2×4 walls are separated by an insulation-filled gap that dramatically reduces the amount of heat escaping through the framing. Building techniques are familiar and straightforward.
“Having built both double walls and exterior foam, I’d say the overall cost savings are with double wall, easily,” writes Dan Kolbert. “The interior walls are easy to build and go up quickly. Exterior foam is slow to install, expensive, and makes all the…