A credible model to account for the overall energy benefits with retention of historic buildings has been needed since preservation became national policy in 1966. The initial need to measure energy capital in buildings arose from the two energy crises in the 1970s, with a second need to address the sustainability goals of the 1990s/2000s. Both responses measure overall energy efficiency of historic buildings by attempting to account for the "energy capital." The Advisory Council on Historic Preservation introduced the first model in 1979, focused on measuring embodied energy and it has remained embedded in preservation vocabulary and is a reflexive argument utilized to advocate for the retention of historic structures over new construction. The second model, the life cycle assessment/avoided impacts is a response to the evolving metrics and currency of sustainability. The Preservation Green Lab further matured the capabilities of the life cycle assessment/avoided impacts model in 2012 with their innovative report, The Greenest Building: Quantifying the Environmental Value of Building Reuse. This thesis evaluates the future of the preservation field to communicate with a common currency regarding retention of historic structures.