T. Barker is planning a high-performance house in Climate Zone 7A, where the number of heating degree days reaches 10,000 a year and the January design temperature hits 20 below zero.
Although he is still tinkering with his plans, Barker is leaning toward a design with R-50 exterior walls to make the house as comfortable as possible and to reduce the cost of the mechanical systems. The question is how best to get to that insulation value.
In a Q&A post, Barker asks, “Are there any good reports using actual construction experience comparisons for the cost to build double-stud 2×4 (or 2×6) walls compared to single 2×6 with exterior insulation?”
What reading he’s done so far suggests that most double-stud walls are framed on 24-inch centers. “I would think for the minimal cost difference to stay 16 inches on center you get better drywall finish and shear wall capacity for 2-story construction,” he says.
What does the most recent research show in terms of material costs, construction labor, and pros and cons of each of these two options? Further, are there insulation materials on the market with very high R-value that might be a good fit for his house, such as silica aerogels? Those are the questions raised in this Q&A Spotlight.
Double-stud walls are more practical
Reaching R-50 with a 2×6 wall plus exterior foam takes a lot of foam, says Dana Dorsett, something on the order of 6 inches. That makes construction awkward, and may be a good reason to choose the double-stud option.
“A double stud wall is far more practical, since the distance between walls is flexible — two feet thick isn’t substantially more difficult than a foot,” Dorsett writes. “You have to pay attention to the thermal bridging of the subfloors and band joists, etc., and dense packing…