Conventional wood-framed houses insulated with R-19 fiberglass batts won’t have R-19 walls. Why not? Because the studs and rafters that form the skeleton of the house have an R-value of about 1 per inch—less than one-third that of a fiberglass batt. When framing and insulation are considered together for “whole-wall” R-values, numbers will be lower than for the insulation alone. This effect, called thermal bridging, can be significant. The Oak Ridge National Laboratory (ORNL), for example, pegs the R-value of a 2-by-6 wall on 24-inch centers with R-19 batts at R-13.7. ORNL maintains an interactive calculator on its website where you can plug in various values to see how a given wall would perform (bit.ly/WallCalc).
Another issue is the installation itself. Fiberglass must be installed between studs and cut to fit around window openings and wiring. This process can never be perfect and often leaves gaps where there is no insulation at all. Even very small gaps reduce the effectiveness of the insulation significantly, and perfect installation by insulation subcontractors is rare. That’s one reason that high-performance houses often have blown-in polyurethane foam, cellulose, or fiberglass, which fills stud and rafter bays more effectively.