LCA framework for assessing the climate impact of window reuse in the early design phase

Abstract

The construction sector accounts for a large share of global greenhouse gas emis sions and embodied emissions make up a growing part of this impact. One way to reduce embodied emissions is to reuse building components, and windows have been identified as one of the components with the greatest potential for reuse. However, as windows are part of the building envelope reusing them may increase the opera tional energy use of the building, which can offset the savings in embodied emissions. Whether reuse of windows actually lowers the climate impact therefore has to be assessed through a life cycle assessment (LCA), but no established method exists for modeling reused building components. This assessment is also needed already in the early design phase since the choices made at this stage have the greatest influence on the building’s overall carbon footprint. Conducting a full LCA at this point is however not feasible, as many alternatives must be compared quickly and the data available on the reused window is often limited. This thesis therefore develops a simplified LCA-based method for comparing the global warming potential of a reused and a newly manufactured window in the early design phase. The method follows the cut-off allocation method, covers modules A1-A4, B4, B6 and C4, and reports GWP-fossil per m2 of window over a 50-year reference study period. As no environmental product declaration (EPD) is typically available for reused windows, a geometry-based framework is used to estimate the material quantities from the window dimensions. The method was developed and tested through a case study of a 1960s coupled wooden window, complemented by a sensitivity analysis of the governing parameters. The case study demonstrates that the method can be applied under limited data availability. The sensitivity analysis identifies a small set of parameters that govern the result and must be specified by the user, namely the U-value and reference service life of the reused window, the local district heating emission factor and the annual degree-hours. The transport distance has a smaller influence and only needs to be roughly estimated, while the remaining parameters have a negligible effect and can be assigned default values. The framework additionally requires a few window dimensions, but these can easily be measured directly from the window without any material documentation.