Residential weatherization programs have become a major component of U.S. energy policy. Through these programs, households receive heavily subsidized energy efficiency upgrades as well as informational and behavioral treatments designed to encourage conservation. While previous work demonstrates that weatherization programs provide sizable energy savings, all have measured the composite effect of efficiency upgrades and behavioral treatments. In this paper, we present the first estimates which disentangle the energy savings provided by each of the individual interventions. Our results reveal that the actual energy savings achieved by the efficiency upgrades are substantially smaller than ex ante, engineering predictions. Moreover, we present evidence that the energy savings provided by the simple behavioral interventions can exceed the savings resulting from the much more costly efficiency upgrades.