Comparative Assessment of Urban Green Roofs on Heat Mitigation Across Cities

Thesis Abstract

Urban heat islands (UHIs) intensify cooling-energy demand, urban discomfort, and air pollution exposure in dense city environments, with green roofs proposed as a nature-based intervention to mitigate heat in the built environment. Yet comparative evidence across diverse urban contexts remains fragmented, limiting transferable design guidance for policymakers and practitioners. This study aims to evaluate and compare the effectiveness of green roofs in reducing surface and ambient temperatures across four distinct cities representing varied climate zones, building typologies, and maintenance regimes. The specific objectives are (1) to quantify the cooling performance of retrofit and new-build green roofs during peak summer periods; (2) to compare temperature reduction effectiveness across cities and roof configurations (substrate depth, vegetation layer, and irrigation status); (3) to identify modulating factors such as roof albedo, building height, solar exposure, and urban morphology; (4) to assess ancillary co-benefits including stormwater retention and energy use implications; and (5) to formulate transferable design guidelines for scale-up and policy integration. The study adopts a cross-sectional, multi-city comparative design combining empirical field measurements with simulation-based scenario analysis. The population comprises built-up rooftops with green interventions in metropolitan areas spanning temperate, arid, tropical, and continental climates. A stratified sampling strategy selects 120 representative green roofs (30 per city) categorized by retrofit age (<5 years vs. ?5 years), substrate depth (?100 mm, 101–200 mm, >200 mm), and irrigation regime (rainfall-only vs. supplemental irrigation). Data collection employs in-situ thermal imaging, surface and near-surface temperature loggers (hourly readings over summer weeks), micrometeorological stations for ambient conditions, and roof-specific metadata collected via structured surveys of facilities managers. Additional data include energy consumption metrics for adjacent non-residential envelopes and satellite-derived urban morphology indicators. To capture processual insights, semi-structured interviews with architects, building owners, and municipal planners are conducted (n=40). Analytical approaches integrate quantitative and qualitative methods. Descriptive statistics summarize temperature differentials and roof characteristics. Multilevel mixed-effects regression models assess the association between green roof attributes and heat mitigation outcomes while accounting for city-level random effects and repeated measurements. ANOVA and post-hoc tests examine cross-city differences in peak cooling performance across roof configurations. Interaction terms test whether climate zone moderates the relationship between substrate depth, irrigation, and cooling effect. Energy use implications are evaluated through partial least squares structural equation modeling to link thermal performance with envelope energy savings and occupant comfort proxies. For qualitative data, thematic analysis identifies contextual factors influencing performance outcomes and stakeholder perceptions of feasibility and maintenance burdens. A validated conceptual model informed by urban climate theory and the Sustainable Urban Transport and Heat Island literature underpins the interpretation of results. Expected findings anticipate that green roofs yield statistically significant reductions in surface temperatures, with greater effects in hotter climates and for deeper substrate configurations, while irrigation enhances performance in arid regions but incurs trade-offs in water-scarce settings. Cross-city comparisons are expected to reveal that structural and operational factors—such as roof orientation, planter density, substrate thermal conductivity, and maintenance regimes—modulate cooling effectiveness more than climatic differences alone. The study contributes to knowledge by providing robust, comparable evidence across diverse urban contexts, clarifying which roof design parameters deliver consistent heat mitigation gains and how these gains translate into potential energy savings and ventilation benefits. It offers transferable design guidelines, including substrate depth ranges, plant assemblages, and irrigation strategies, tailored to climate zone and building type, alongside policy implications for urban heat management and green retrofit incentives. The conclusion emphasizes scalable implementation pathways, identifies data gaps for long-term monitoring, and recommends standardized metrics for cross-city benchmarking to advance the adoption of green roofs as a climate-resilient urban infrastructure strategy.