Comparative Life Cycle Assessment of Green Roofs in Urban Buildings

Thesis Abstract

Green roofs are increasingly adopted in urban environments to mitigate stormwater runoff, reduce urban heat island effects, and enhance building energy performance; however, comparative Life Cycle Assessment (LCA) across different climate zones and soil–substrate configurations remains underexplored, limiting evidence-based decision-making for urban retrofit strategies. This study aims to evaluate and compare the environmental performance of green roofs installed on mid- to high-rise residential buildings in three distinct metropolitan climates, identifying how substrate depth, irrigation practices, and local energy mixes influence overall life cycle impacts. Specific objectives include (1) quantify cradle-to-grave environmental impacts (global warming potential, cumulative energy demand, eutrophication, and human toxicity) of extensive, semi-intensive, and intensive green roof systems; (2) compare impacts across cooling and heating-dominated urban contexts; (3) assess the influence of substrate depth (60 mm, 120 mm, 180 mm) and irrigation regime (passive, low-quantity, rainfall-driven) on LCA results; (4) evaluate sensitivity to electricity grid mix and maintenance routines; and (5) develop actionable design recommendations for climate-responsive green roof configurations with the lowest environmental burden without compromising performance. The research adopts a comparative cross-sectional design using a functional unit of 1 square meter of green roof over a 40-year service life, integrating primary and secondary data to construct detailed life cycle inventories. The population comprises green roof installations on 15 mid- to high-rise residential buildings across three global cities representing temperate, hot-humid, and Mediterranean climates. A purposive stratified sample of 45 green roof systems (15 per climate) is selected to cover the three substrate depth categories and three irrigation regimes, ensuring representation of extensive, semi-intensive, and intensive typologies. Data collection instruments include on-site measurements of substrate properties, irrigation water use records, energy consumption for irrigation pumps, and maintenance logs, complemented by manufacturer specifications and building energy reports. Secondary data from life cycle databases (ecoinvent 3.8, GaBi) and regional electricity profiles are used to populate the LCA model. The LCA follows ISO 14040/14044 standards, employing SimaPro as the analytical tool and using the consequential and attributional approaches to explore foreground and background system changes. A hybrid LCA framework combines process-based inventories with input–output data to capture economies of scale and regional electricity impacts. Impact assessment uses ReCiPe 2016 Midpoint (H, F, M) indicators for global warming, abiotic depletion, acidification, eutrophication, and human and ecological toxicity, with normalization against regional benchmarks. Uncertainty and sensitivity analyses include Monte Carlo simulations (5,000 iterations), scenario analysis for substrate depth and irrigation variations, and a tornado diagram for key drivers. Regression analyses identify statistically significant predictors of environmental impact across climates, while ANOVA tests assess differences between roof typologies and configurations. The study also integrates a life cycle cost perspective to juxtapose environmental and economic performance. Expected findings indicate that deeper substrates and irrigation-intensive regimes exhibit higher embodied impacts, particularly in hot climates with electricity-heavy grids, while passive irrigation and shallower substrates yield lower global warming potential and energy demand; the degree of grid decarbonization markedly modulates comparative advantages across climates. Differences among roof typologies are anticipated, with extensive systems delivering favorable balances between cooling benefits and cradle-to-grave impacts in temperate cities, whereas intensive systems may be preferable where stormwater retention and biodiversity gains justify higher energy inputs in Mediterranean contexts. The study contributes to knowledge by providing a location- and design-sensitive LCA of green roofs, highlighting the interaction between climatic context, substrate choice, and irrigation strategy on environmental performance. It advances methodological practice by applying a robust hybrid LCA approach, integrating sensitivity analyses, and presenting policy-relevant guidelines for climate-responsive green roof design. The main conclusion underscores that there is no one-size-fits-all solution; optimized configurations depend on climate, electricity mix, and maintenance practices. Recommendations include adopting shallow substrates with passive irrigation in temperate and cool climates to minimize impacts while preserving thermal and hydrological benefits, and prioritizing modular, irrigation-efficient designs with robust insulation in hot climates to reduce cooling loads and life cycle emissions.