Assessment of GHG emissions from materials during building design - Methods for improved reliability and quality

Abstract

Construction materials in buildings contribute significantly to climate change. Globally, nations have committed to drastic reductions of greenhouse gases (GHG), among them Norway which has committed to 50-55% reduction by 2030 and 90-95% by 2050. Building material use and the related construction activities are among the areas that must drastically reduce impacts to achieve those goals.

Such mitigation efforts are limited by current assessment methods. Improved methods for esti- mation of these climate change impacts are needed in the early project phases to improve the design and planning. Methods for benchmarking results against reference values are needed both to improve design and for effective regulation. Quantification methods are also immature. Material use in buildings affects the climate over centuries, however, temporal aspects are often ignored in Life Cycle Assessment (LCA). Results too often promise uncontested precision of impacts occurring far into the future. Additionally, the validity of building LCAs is being ques- tioned over inadequate scope and inventory. The goal of this thesis is to contribute with methods addressing those limitations, to enable effective reduction of building materials’ contribution to climate change.

The body of research on the impacts of individual buildings is growing, but the results and data remain inaccessible and incomparable due to insufficient reported information, differences in system boundaries, assumptions, methods, and data used. This inhibits further utilization of results in statistical applications and makes interpretation and validation of results difficult. A database of empirical material use and emission data from building LCA case studies was developed to mitigate these challenges, providing a framework for impact assessment in the planning and design phases. Systematizing and storing relevant information for these case studies in a compatible format enables comparison and harmonization of results across system boundaries and assumptions, improves the transparency and reproducibility of the assessments, and makes utilization of the results in statistical applications possible.

A dynamic LCA method for material use in buildings was developed. It addresses uncertainty and temporal effects arising from the long lifetime of buildings. In particular, novel solutions for accounting for delayed emissions and future emission reductions due to technological im- provements are proposed. Climate change effects of material use in construction, operation, and end-of-life phases are estimated, from production, transport, construction-waste incinera- tion, biogenic carbon-sequestration, and cement carbonation. The importance of choosing a normative time horizon for the estimated climate change impacts is emphasized.

A method was also developed for evaluating and visualizing the climate change impacts of material use by linking the material inventory data with the aggregated results through a set of metrics for a building and its subparts. These subpart metrics can be compared to the rest of the building and to results from other buildings, and statistical benchmarks can be established. This intermediate calculation step simultaneously serves as a breakdown of the results and an aggregation of the building’s inventory data. The subpart metrics lay the foundation for applications throughout the project phases by enabling combined use of case-specific data and statistical data.

Uncertainty is estimated from variation in the dataset, and further, from sampling results while varying assumptions. Parameter influence is assessed with global sensitivity analysis. The time horizon for the impacts, the building lifetime (long time horizons only), and the construction waste parameters are found most sensitive. The method reduces uncertainty of postulated future impacts; an important step in the direction of policy-relevant modeling. It is recommended that building LCA modeling practice adopts the presented methodological concepts to gain trust and policy-relevance.

Case studies are used to demonstrate the methods and to generate statistical results. Rarely have the climate change impacts of material use in buildings been studied by statistical methods, and never this sophisticated. In the early phases of a building project, empirical statistical emission profiles of construction materials can inform mitigation efforts. However, engineers and archi- tects do currently not have sufficient information at disposition. The climate change impacts of building material use in 20 Norwegian case studies of low-emission buildings are made com- parable, harmonized, and then studied statistically to find how the impact varies with building types (typology, timber/ concrete), building subparts (building elements, material categories), and time horizon. Anticipated future technological development, and delaying emissions in the coming decades, will together lead to significant reductions of accumulated impacts and thus reduce the importance of future replacements and end-of-life. Results show that global warming policy targets require that the building industry focuses on interventions with short-term effects, such as low immediate impact of materials in the construction phase, as well as demonstrating the importance of reducing impacts from construction waste throughout the building lifetime.