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

Sammanfattning

The construction sector accounts for a large share of global greenhouse gas emissions and embodied emissions make up a growing part of this impact. One way toreduce embodied emissions is to reuse building components, and windows have beenidentified 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 operational 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 beassessed through a life cycle assessment (LCA), but no established method exists formodeling reused building components. This assessment is also needed already in theearly design phase since the choices made at this stage have the greatest influenceon the building’s overall carbon footprint. Conducting a full LCA at this point ishowever not feasible, as many alternatives must be compared quickly and the dataavailable on the reused window is often limited.This thesis therefore develops a simplified LCA-based method for comparing theglobal warming potential of a reused and a newly manufactured window in the earlydesign phase. The method follows the cut-off allocation method, covers modulesA1-A4, B4, B6 and C4, and reports GWP-fossil per m2 of window over a 50-yearreference study period. As no environmental product declaration (EPD) is typicallyavailable for reused windows, a geometry-based framework is used to estimate thematerial quantities from the window dimensions. The method was developed andtested through a case study of a 1960s coupled wooden window, complemented bya sensitivity analysis of the governing parameters.The case study demonstrates that the method can be applied under limited dataavailability. The sensitivity analysis identifies a small set of parameters that governthe result and must be specified by the user, namely the U-value and referenceservice life of the reused window, the local district heating emission factor and theannual degree-hours. The transport distance has a smaller influence and only needsto be roughly estimated, while the remaining parameters have a negligible effect andcan be assigned default values. The framework additionally requires a few windowdimensions, but these can easily be measured directly from the window without anymaterial documentation.