White cedar has an R-value of about R-1.4 per inch, so it isn’t too hard to calculate the R-value of white cedar siding. The trickiest part of the calculation is determining the siding thickness.
If we’re talking about cedar shingles, there are usually a maximum of three layers of shingles at any one point in the wall. The shingles are tapered, so the total thickness of the siding includes layers with different thicknesses. (The butt of the shingle may measure 3/8 inch; the top of the shingle may measure 1/16 inch; and the middle of the shingle may measure 3/16 inch).
Let’s be generous, and assume that the three layers add up to a total thickness of 3/4 inch. We calculate the R-value of the cedar this way:
(0.75 inch * 1.4) = R-1.05
Next, we’ll add a value (R-0.17) for the exterior air film:
R-1.05 + R-0.17 = R-1.22
What if the singles are thicker? Let’s do the math for 5/8-inch-thick shingles. We’ll be generous (again) and assume that the three layers add up to a total thickness of 1.25 inch. So the R-value of the cedar is:
(1.25 inch * 1.4) = R-1.75
Adding the R-value of the exterior air film, we end up with:
(R-1.75 + R-0.17) = R-1.82
No more than R-1.6 for standard shingles
So, if you’re using 3/8-inch shingles, the installed siding will have an R-value of (at most) about R-1.3. If you are using thicker 5/8-inch shingles, the installed siding will have an R-value of (at most) about R-1.9.
If any readers think that the R-value of cedar shingles might be higher, it’s worth consulting standard R-value tables for a reality check. Most such R-value tables ascribe an R-value of R-0.87 for wood shingle siding.
At least one cedar shingle manufacturer disagrees
If you were a shingle manufacturer, you might want to…