Re-thinking green roof design : The prospect of a carbon sink structure investigated through a life cycle cost-benefit analysis

Sammanfattning: Green Roofs (GRs) have been used worldwide as a solution for greening cities, while providing useful services to society such as storm water management, improved air quality, energy savings, and increased biodiversity, to name a few. However, the environmental impact of their structure has been understudied, and more importantly, the carbon cost related to the use of materials that have a high carbon footprint has received little to no attention. In the context of an aspiring carbon neutral Sweden, public and private decision makers need better guidance when considering the implementation of GR systems. While cost-benefit analysis (CBA) is a decision support tool that has been commonly used to assess GR projects from an environmental, social, and economic point of view, the carbon impact of the structure layers has been left unaccounted for. This thesis aims to propose an analytical framework which incorporates the carbon footprint of each component used in the GR structure into CBA, to support more informed decision making. This analytical framework is tested on two extensive GRs, one made with commonly used materials and one experimental prototype which is almost entirely made with carbon sink materials. This case study is designed to reveal the socio-economic trade-offs resulting from the use of conventional materials versus materials that have a lower carbon footprint. This comparison is also made to support future innovations in GR design and to bring forward the most cost-effective materials from a socio-economic perspective. The results of this thesis show that the two GR alternatives have a positive net present value (NPV) indicating that both projects bring more socio-economic benefits than they incur costs in their lifetime. However, the key findings reveal that while the structure of the conventional GR is a source of carbon which incurs socio-economic costs of 42 SEK/m2, the structure of the prototype is a carbon sink which brings socio-economic benefits of 63 SEK/m2. A comparison of the two extensive GRs shows that the avoided emissions from using the prototype instead of a traditional GR has a value of 105 SEK/m2. Moreover, when considering each component of the GR structure it becomes evident that while it is possible to replace traditional materials with carbon sink materials this generally comes with higher material costs. However, once the avoided CO2 emissions are accounted for, this choice seems of potential interest from a socio-economic perspective.