Assessing Urban Heat Island Mitigation via Green Roofs: A City-scale Field Study

Thesis Abstract

Urban spaces increasingly experience elevated temperatures that exacerbate energy demand, health risks, and air quality concerns, particularly in dense city cores where built form and limited vegetation amplify heat retention. This study investigates the efficacy of green roofs as a climate-adaptive retrofit to mitigate urban heat island effects at a city scale, addressing a gap in empirical evidence that links roof-level interventions with neighborhood-scale thermal and environmental outcomes. The aim is to quantify the cooling benefits of green roofs and how they modulate microclimates, energy use, and outdoor thermal comfort across diverse urban morphologies. Specific objectives are (1) to measure surface and air temperature differences between green roofs and conventional roofs across 60 representative buildings; (2) to assess the impact of green roofs on building energy consumption using utility data from 40 different properties; (3) to evaluate changes in outdoor pedestrian thermal comfort using bioclimatic indices and on-site surveys with 600 respondents; (4) to analyze the influence of roof vegetation density, substrate depth, and irrigation regimes on thermal performance; and (5) to synthesize policy-relevant recommendations for scalable green roof deployment. The methodology adopts a mixed-methods research design integrating quantitative field measurements with qualitative stakeholder insights. The population comprises commercially and residentially owned buildings within the metropolitan area, with a stratified sample of 60 roofs equipped with extensive or intensive green roof systems and 60 comparable non-green roofs as controls. Data collection instruments include calibrated infrared thermography and fixed-interval surface and air temperature loggers deployed for 12 months, smart meters for electrical energy consumption, anemometers and hygrometers for microclimate data, a standardized outdoor comfort questionnaire administered to 600 pedestrians, and semi-structured interviews with city planners, facility managers, and green roof installers. Instrument validity and reliability are established through pilot testing, inter-instrument calibration, and test-retest procedures, complemented by triangulation across measurements. Data analysis proceeds through a sequence of robust statistical and spatial techniques. Descriptive statistics characterize baseline thermal conditions and energy use patterns. Paired t-tests and repeated-measures ANOVA assess intra-site and inter-site temperature differentials over seasonal cycles, while multivariate regression models (adjusted for building height, age, occupancy, and solar exposure) isolate the contribution of green roofs to cooling effects. Energy savings are evaluated via a difference-in-differences approach using pre- and post-installation utility data where available, and generalized additive models capture nonlinear relationships between roof type and microclimate variables. Outdoor thermal comfort is analyzed using bioclimatic indices (PET, SET) and logistic regression to model probability of thermal discomfort occurrences. A GIS-based spatial analysis maps cooling intensity across the city, and thematic analysis of stakeholder interviews elucidates governance, maintenance, and finance implications. Theoretical underpinnings draw on the Urban Climate Theory and the Biophilic Design framework, with the study testing propositions regarding scale effects and social acceptance of green roof retrofits. Expected findings suggest that extensive green roofs reduce roof surface temperatures by 6–12°C during peak insolation, yield a 10–18% reduction in building cooling demand on hot days, and improve outdoor thermal comfort indices in proximate street canyons, particularly in south-facing façades. Variability is anticipated based on substrate depth, plant diversity, irrigation practices, and roof orientation. The study anticipates meaningful energy savings for multifamily and commercial buildings and enhanced urban livability in districts with high heat exposure. Contributions to knowledge include a city-scale empirical quantification of green roof cooling benefits, an integrated methodology linking microclimate, energy performance, and human comfort, and a policy-ready assessment of scalable retrofit strategies, maintenance regimes, and cost-benefit considerations. The main conclusion is that well-designed green roofs can measurably mitigate urban heat island effects and reduce energy demand at the city level, albeit with diminishing returns in highly insulated high-rise buildings lacking direct solar access. Recommendations emphasize prioritizing mid-rise, south-facing roofs, optimizing substrate depth and irrigation efficiency, implementing performance-based permitting, and developing funding mechanisms to incentivize retrofits. The study also identifies avenues for further research, including long-term monitoring of ecological co-benefits and comparative analyses across different climate regimes.