While government policies aim to increase residential energy efficiency, policymakers know little about the efficiency of the U.S. housing stock and improvements in efficiency over time. This paper estimates the average space heating efficiency of U.S. homes, improvements in residential space heating efficiency over time, and impacts of building energy codes on residential space heating efficiency. Homes built in the 1990s were about 30% more efficient than those built before 1950. Improvements over time in design and construction of new homes can explain about 80% of this difference, and most of the improvement occurred during and after the 1970s. Implementation of building energy codes by states increased home space heating efficiency, accounting for about 30% of the efficiency gains attributable to improvements in design and construction of new homes. Codes saved about 7% of annual residential space heating energy consumption or about 3% of overall annual residential energy consumption and homes saved on average about $60 per year in space heating energy costs. Greater knowledge about home space heating efficiency will enable policymakers to design better policy instruments and to direct resources where they are most needed.

The paper has several implications for energy efficiency policy. First, it provides additional evidence that residential building energy codes saved energy and reduced GHG emissions. Second, the paper suggests that policymakers may want to research the cost-effectiveness of directing more efficiency resources towards improving the efficiency of exterior walls of existing homes. Finally, this paper presents a new way of evaluating energy savings from residential building energy codes.