What is the effective insulation value of a 2×6 stud, 16″ center wall with spray foam? I can find the R value of the stud – 6.88, and the R value of (open-cell) spray foam at 5.5″ = 19.8. But it seems that it is a calculus problem to figure out the effective R value since the stud has a lower R and so provides an easier path for heat transfer for at least some of the space between the studs.
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