Comparative LCA of Urban Green Roofs in Mint and Madrid

Thesis Abstract

Urban green roofs (UGRs) present a promising climate mitigation and ecosystem service strategy for dense urban areas, yet their environmental performance remains uncertain when comparing distinct climatic and urban contexts. This study addresses the knowledge gap by evaluating and contrasting the life cycle environmental impacts of UGRs implemented in Madrid, Spain, and Manchester, UK, with a focus on material composition, maintenance regimes, and end-of-life pathways to inform policy and planning decisions. The aim is to quantify cradle-to-grave environmental burdens and benefits associated with UGRs in two metropolitan settings that differ in climate, vegetation types, and urban density, thereby assessing the transferability of LCA results across European cities. Specific objectives include (1) to develop a harmonized LCA framework for UGRs that accommodates regional construction practices and maintenance schedules; (2) to quantify and compare greenhouse gas emissions (GHG), embodied energy, and potential water-energy nexus co-benefits over a 40-year life cycle; (3) to identify key drivers of environmental performance through sensitivity and scenario analysis; (4) to evaluate the influence of substrate depth, irrigation regimes, and plant selection on overall impacts; and (5) to provide evidence-based recommendations for design strategies that optimize environmental outcomes in temperate urban contexts. The methodology adopts a comparative positivist research design grounded in Life Cycle Assessment (LCA) per ISO 14040/44 standards. The population comprises urban green roof installations in Madrid and Manchester, with a stratified sample of 20 representative projects per city, totaling 40 case studies. Data collection employs primary instruments including site surveys and semi-structured interviews with 30 design engineers, facility managers, and horticultural contractors, complemented by secondary sources such as bills of quantities, maintenance logs, and municipal infrastructure records. The functional unit is defined as 1 square meter of fully operational green roof area over a 40-year life cycle. Process-based LCA (gate-to-grave) captures cradle-to-gate construction inputs, ongoing operational energy and water use, maintenance activities, and end-of-life disposal or recycling pathways. Characterization and impact assessment utilize the ReCiPe midpoint method (H set) and the Tool for Reduction and Assessment of Environmental Impacts (2.0), implemented in SimaPro v9.2, with uncertainty quantified via Monte Carlo simulations (10,000 iterations). Regression analysis and multivariate ANOVA will identify statistically significant differences between cities and influence of substrate depth, irrigation intensity, and plant assemblages on key impact categories such as GHG, acidification, eutrophication, and cumulative energy demand. A scenario analysis explores climate-related variations and alternative maintenance regimens. The study integrates a theoretical lens drawing on the Theory of Planned Behavior to interpret stakeholder decision-making processes in design and maintenance and the Urban Metabolism framework to situate UGRs within city-scale material and energy flows. Expected findings anticipate that Madrid’s semi-arid conditions and higher irrigation usage will yield lower water-related buffering benefits but higher embodied energy due to substrate depth and irrigation infrastructure, while Manchester’s cooler, wetter climate may produce greater stormwater retention benefits and different nutrient loading dynamics. Relative contributions of substrates, vegetation selection, and drainage layers to life cycle impacts are expected to diverge between cities, with sensitivity analysis highlighting substrate depth and irrigation frequency as primary leverage points. The study aims to demonstrate that cross-city LCA results require context-specific calibration and that standardized generic models may misrepresent environmental trade-offs of UGRs. The contribution to knowledge lies in delivering a robust, harmonized cross-city LCA framework for UGRs in temperate European cities, enriching methodological guidance for comparative assessments, and identifying design parameters that optimize environmental performance. Policy relevance includes informing municipal incentives for substrate choice, irrigation management, and maintenance scheduling to minimize life cycle impacts while maximizing ecosystem services. The main conclusion is that context-aware design and operation significantly shape the environmental viability of UGRs, and recommendations emphasize modular, climate-responsive specifications, reduced irrigation dependency through drought-tolerant plant palettes, and end-of-life recycling strategies to enhance circularity. Suggestions for further research include extending the framework to additional cities with diverse climates and exploring dynamic LCA incorporating temporal fluctuations in maintenance regimes and climate variables.