Development and application of a building energy performance metric for green roof systems

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.

► It is wrong to represent the energy effects of green roofs simply as added insulation.
► The complex green roof energy balance does not always lead to building energy savings.
► A new metric – DBGR – has been introduced to quantify green roof energy performance.