Blower doors are used to measure the airtightness and air leakage of building envelopes. As existing dwellings in the United States are ventilated primarily through leaks in the building shell (i.e., infiltration) rather than by whole-house mechanical ventilation systems, quantification of airtightness data is critical in order to answer the following kinds of questions: What is the construction quality of the building envelope? Where are the air leakage pathways? How tight is the building? Tens of thousands of unique fan pressurization measurements have been made of U.S. dwellings over the past decade, and the available data have been collected into an air leakage database. This report documents what is in that database and then uses the data to determine relevant leakage characteristics in the U.S. housing stock in terms of region, age, construction type, and quality.