In this study, the U.S. Department of Energy’s team Building America Partner­ship for Improved Residential Construction (BA-PIRC) worked with Kinsley Construction Company to evaluate the real-world performance of insulated sid­ing when applied to an existing home. A 1960s home was selected for analysis. It is located in a cold climate (zone 6) where the addition of insulated siding and a carefully detailed water-resistive barrier have the potential to offer significant benefits. In particular, the team quantified building airtightness and heating energy use as a function of outdoor temperatures before and after the installa­tion of the insulated siding.