Conventional versus modular construction methods: A comparative cradle-to-gate LCA for residential buildings

Abstract

Traditional on-site and modular off-site methods are the main construction methods in the residential building sector. Regardless of different advantages and disadvantages offered by each of these two construction methods, the overall environmental trade-offs between them are still unclear. This paper applies a cradle-to-gate life cycle assessment (LCA) to benchmark the environmental performance of conventional and modular construction methods. To this end, three single-family buildings (one conventional and two modular) designed and constructed in the Okanagan, British Columbia, Canada, were used as representatives of typical houses built by on-site and off-site methods in the region. The required data such as raw materials, material and workforce transportation, and energy resources was collected from the corresponding homebuilders through a questionnaire survey. Then, the environmental impact measures such as global warming potential were calculated through LCA analyses. Subsequently, the calculated impact measures along with their importance weights were incorporated into an analytic hierarchy process (AHP) based framework to develop a set of environmental impact indices for the material production phase, the construction phase, and summation of these phases (i.e., cradle-to-gate). Furthermore, a sensitivity analysis was performed to eliminate the human subjectivity involved in AHP. Eventually, individual impact measures and also the developed impact indices were compared and contrasted between the benchmarking buildings. The analyses revealed that construction of the first modular building resulted less environmental impacts than the other two buildings. Nevertheless, the lowest environmental performance was due to the second modular building. This disclosed that none of the modular and conventional construction methods is the absolute option for environmentally friendly construction. However, implementation of strategies such as optimal designs and decrease in material and worker transportation of both modular and conventional building projects and increase in annual production of modular manufacturing centers can reduce the material and energy consumption and the subsequent environmental impacts. This paper also provides a basis to create a benchmarking database by adding future LCA studies for new modular and conventional building projects.