Strawbale buildings as carbon sinks: influence of the design and construction process on carbon emissions

Strawbale buildings have drawn attention as a prime example of carbon storage using bio-based materials in construction. However, the question on how to efficiently use the potential of this material in the design and construction processes to achieve carbon negative buildings remains open. The reflections proposed in this paper arise from the comparative analysis of two case studies: Bombasei, a residential settlement located in the outskirts of Zurich, and Biotal, an organic farm shop in southern Germany. While both buildings are designed by Swiss architect Werner Schmidt and use the same materials, they present profoundly different design and construction processes. Bombasei is composed of three timberframed strawbale buildings of four storeys each, meticulously designed and efficiently built, partly through prefabrication. Biotal is a single storey hybrid strawbale construction, mainly self-built by the staff: the design differs significantly from the as-built situation. We performed a systematic investigation based on verified sources to draw bills of quantities and 3D models representing as-built situations. The comparative Life Cycle Assessment, following EN15978 through Ecoinvent 3.9 database and SimaPro software, is limited to A1-A3 phases, to avoid assumptions and uncertainties. Carbon emissions were evaluated through IPCC2021, and biogenic CO2 assessment followed EN 16449. The results show that the material production for both constructions has an overall low impact, with the main contributors being energyintensive products. Biotal tends to perform better, and is a negative carbon building, mainly due to the choice of less-processed materials compared to Bombasei, which is a near net-zero construction. However, the use of large quantities of bio-based materials allows for the storage of significant amounts of biogenic CO2 in both case studies. In conclusion, strawbale buildings have a regenerative potential for the built environment, suitable for both self-built and scaledup constructions and adaptable to specific use scenarios.