This study develops a thermal performance metric for vegetated roof systems. The Dynamic Benefit of Green Roofs (DBGR) is the ratio of Heating, Ventilation and Air-Conditioning (HVAC) energy use for a building with a conventional roof to that of a building with a green roof. If the green roof results in lower energy use than a conventional roof with the same level of thermal resistance the value of DBGR is greater than unity. Data from a field study in Portland Oregon were used to validate the green roof model incorporated within a whole-building energy simulation program. This model was then used to estimate the DBGR for a new construction office building in four climates: Portland, Oregon; Chicago, Illinois; Atlanta, Georgia; and Houston, Texas. Results suggest that a green roof in Atlanta and Houston would provide net annual HVAC energy savings compared with a traditional roof. The Chicago case, with severe winter and mild spring/summer/fall, resulted in a smaller energy savings. The DBGR for Portland was less than unity, suggesting a net energy consumption penalty associated with the green roof. This was due, in part, to the undesirable evaporative cooling in the shoulder seasons which led to increased building heating loads. (C) 2013 Elsevier B.V. All rights reserved.