Levenscyclus milieu benchmarks voor nieuwbouw woongebouwen in België - Een vergelijking van bottom-up en top-down benaderingen

Building environmental impacts are increasingly quantified with life cycle assessment (LCA). In recent years, several countries have implemented LCA requirements in their building regulations. These regulations often include benchmark values, i.e. reference values with which designers can compare, or even binding limit values that must be achieved. Currently, most building regulations, including benchmark values, only focus on one environmental impact category, namely climate change. However, buildings contribute to multiple impact categories, some of which are currently critical at the global level. Two approaches exist to develop environmental benchmark values for buildings: (1) bottom-up, based on a statistical analysis of reference buildings, and (2) top-down, where global environmental targets are translated into target values for buildings. The comparison of both reveals if the environmental impacts of typical buildings exceed global environmental targets that define the Earth's carrying capacity. However, the current focus on climate change risks that other critical global carrying capacities are neglected. This research compares bottom-up and top-down benchmark values of new-built Belgian residential buildings for multiple environmental impact categories, specifically all LCA impact indicators recommended by the European standards. The benchmark values obtained are applicable in the context of the TOTEM tool, i.e. the Belgian LCA tool for buildings. The bottom-up benchmark values are derived from a statistical analysis of the TOTEM LCA results of 117 reference buildings, derived from 39 geometries and three materials applied to each. Three benchmark types are calculated, i.e. a limit (90th percentile), reference (median), and best-practice value (10th percentile), for all individual impact indicators and a single aggregated environmental impact. The top-down benchmark values are derived by allocating global environmental target values to buildings in different steps: from global to Belgium, to the Belgian residential building sector, to new-built residential buildings in Belgium. In each step, multiple allocation principles are applied. Based on data availability and quality, ethical judgements, and recommendations from the literature, the preferred principles are: equal per capita allocation to Belgium, financial consumption expenditure allocation to the Belgian residential building sector, and an egalitarian allocation to m² of new residential building floor area. The three bottom-up benchmark values of 15 individual indicators are compared to the target values resulting from the different allocation principles, focusing on the recommended allocation approach. For five indicators, the bottom-up benchmark values exceed the recommended top-down target value, i.e. particulate matter emissions, eco-toxicity (freshwater), climate change, and abiotic resource depletion, both fossil fuels and minerals and metals. The main drivers for these impacts are operational energy use and conventional building materials. Key mitigation strategies thus include decarbonising the electricity mix, increasing renewable energy use, and adopting low-impact building materials. Impacts should also be reduced by minimising material use per m² building floor area, extending building and material lifespans, and reusing materials to limit raw material demand. Reducing floor area per resident was identified as most effective but challenging to implement in practice. Therefore, a combination of innovative mitigation measures is needed to reduce building environmental impacts to achieve the targets.