An Integrated Assessment of the Sustainability of Green and Built-up Roofs

There is growing demand to develop methods that integrate environmental and economic assessment of more sustainable technologies incorporated into commercial and residential buildings. In this paper, we incorporate economic and energy use data obtained for a green roof operating in the Midwest U.S. at latitude 42.94N into an integrated approach to estimate and compare the economic and environmental impacts of an intensive (or extensive) green roof with a built-up roof. The life cycle stages included in the analysis were material acquisition life stage which including the transportation effects from material extraction through manufacturing to the finished products, and the use and maintenance life stage of the building. Environmental impact analysis indicates that green roof emits three times more environmental pollutants than built-up roofs in the material acquisition life stage. However, in the use and maintenance life stage, built-up roof emits three times more pollutants than a green roof. Overall, when emissions from both material acquisition life stage and use and maintenance life stage are combined, the built-up roof contributes almost 3 times more (or 46% more) environmental emissions than green roof over a 45-year building life span. Furthermore the overall energy use, specifically energy involved in the transportation from material extraction through to the finished product indicate that green roof uses 2.5 times less energy than a built-up roof. An Economic Input and Output life cycle assessment (EIO-LCA) was used to estimate the environmental impacts. The economic impact over an assumed 45-year building life was determined using life cycle costing, taking into account Net Present Value (NPV) calculations. Life cycle costing results indicate that green roof costs approximately 50% less to maintain over a 45 year-building life than a built-up roof. A Monte Carlo simulation is also performed to account for any variability in cost data. In addition, the paper presents a method to quantify the value incentive that a decision-maker has in adopting green technology. Results from the study indicate that when a green roof is compared to the Midwest regional NPV of a built-up roof, we find that the cost to maintain it ($35 per square foot) lies well below the average regional NPV of $59 per square foot of a built-up roof.