This report presents the findings of a new assessment of the techno-economic and policy-related efficiency improvement potential in the North American building stock conducted as part of a wider appraisal of existing buildings in member states of the International Energy Agency. It summarizes results and provides insights into the lessons learned through a broader global review of best practice to improve the energy efficiency of existing buildings. At this time, the report is limited to the USA because of the large size of its buildings market. At a later date, a more complete review may include some details about policies and programs in Canada. If resources are available an additional comprehensive review of Canada and Mexico may be performed in the future.