A single-family residence had its siding removed and replaced with an exterior insulation and finish system (EIFS) without changing any other feature of the house or its operation. Factoring out the variations in weather, energy records revealed an unexpectedly large reduction in space conditioning energy following the EIFS installation. Analysis revealed that energy efficiency improvement predictions based on thermal resistance values alone can substantially underestimate the actual improvement achieved by the addition of an EIFS. Quantitative examination of wall thermal characteristics based on energy usage is presented, and air infiltration effects on space conditioning energy are quantified from standard values. These are assessed to see if they account for the overall magnitude of the difference in the thermal performance of the walls before and after being retrofitted. The assessment shows that the customary emphasis on improving wall R-values should be supplemented by consideration of the air infiltration reduction that can accompany an EIFS retrofit.