What is the R rating?

R-4 It's on the label

4

Assuming 42-inches around means circumference. That suggests a duct diameter of 13.3 inches, probably not the case. Rectangular duct probably 9" x 12". 42" = 3-1/2 ft A straightforward calculation would be: (15-ft x 3-1/2 ft x 2 ducts) / (15 sq ft / package) = 7 packages But it is rectangular, so you want full depth at the corners: imagine first laying two pieces flat on the 12-inch sides, now lay one length on each of the 9-inch sides with 1-1/2 inch extending to the outer faces of the first two lengths. Tape the seams. You will need 8 packages this way. See if you can get wider roles of insulation and run it around the duct rather than lengthwise. The square feet you need (120 sq ft) will be about the same.