Global warming potential comparison between green and conventional roofs in cold climate using life cycle assessment

Buildings are major contributors to climate change because of the greenhouse gas emissions released during their life cycle. Among building systems, green roofs are known for their potential environmental benefits. Many studies have focused on the environmental impacts of green roofs without considering their ability to sequester carbon. Furthermore, this environmental benefit is not well characterized in the cold climate conditions of the province of Quebec, Canada. The aim of this study was to identify the decoupling point between the environmental costs and the environmental benefits of green roofs compared to a conventional roof in a renewable energy mix, namely hydropower, and a cold climate context, namely subarctic climate. Few articles have modeled a comparative LCA taking into account a subarctic climate. Four roof assemblies were compared: extensive, semi-intensive and intensive green roofs, and one conventional roof. First, a cradle-to-grave life cycle analysis was conducted in Simapro (c) to assess the environmental impacts of the different roof assemblies. Then, a dynamic life cycle analysis was performed to quantify the environmental benefits. At this stage, the rate of carbon sequestration by plants planted on the three types of green roofs was taken into consideration, a factor not considered in many articles. The results indicate that the global warming potential of conventional roof, extensive, semi-intensive and intensive green roofs were 3328.54 kg CO2 eq, 3113.15 kg CO2 eq, 3147.45 kg CO2 eq and 3432.05 kg CO2 eq, respectively, over 45 years. Considering environmental benefits, the global warming potential of extensive, semi-intensive and intensive green roofs were reduced to 3051.5 kg CO2 eq, 3007.5 kg CO2 eq and 3292.1 kg CO2 eq, respectively. These results suggest that for a cold climate, the environmental benefits of green roofs are greater than the environmental costs of semi-intensive and intensive green roofs after 14 and 35 years of installation, respectively. For the extensive roof, it shows a carbon benefit for all 45 years. The highlights of this study are that green roofs have significant intrinsic impacts due to the materials they require. Another point is that carbon sequestration is negligible in reducing CO2 emissions in cold climates. A final point is that green roofs with a lifespan of less than 45 years increase their carbon impact.