2018 energy code r values | Int'l Energy Conservation Code | Forum

R13 = 3.8 ci or R-20

The first number r-13 is the minimum r value of between the studs, using insulation such as fiberglass in a 2x4 wall, 3.8 ci is for R-3.8 continuous insulation (foam board, rock wool, etc) continuous on the exterior or interior of the framing. The total R of this configuration would be ~16.8 but due to the continuous 3.8 not being interrupted by framing the code considers this as compliant to R-21 as in a 2 x 6 wall with just batt insulation.