Green Roof Research
A Temperature and Seasonal Energy Analysis of Green, White, and Black Roofs
S.R. Gaffin, C. Rosenzweig, J. Eichenbaum-Pikser,, R. Khanbilvardi, and T. Susca
Center for Climate Systems Research, Columbia University
A comparison study which examines temperature and energy efficiency of various roofing membranes including a modular 4” green roof, high-reflectance white EPDM roofing membrane, and nearly black EPDM roofing membrane to examine their impact on the urban heat-island effect. Major findings from this study indicate that that white EPDM roofing membranes were associated with temperatures 30oF cooler than black EPDM roofing membranes. Green roofs were on average 60oF cooler than black EPDM roofs in the summer months supplying evidence of prolonged roof service life for both white and green roofs. During winter months the average heat loss rate for the green roof was 34% lower than under the black roof. The summer heat gain rate on the green roof was 84% lower than under the black roof. Evidence supplied by this study was extrapolated to investigate expected energy savings for each roofing strategy. White EPDM roofing membranes could expect a savings of $200 per year. Green roof implementation could expect savings of $400 per year. Despite the marked discrepancies in thermal performance of the various roofing systems authors attribute low energy cost savings to the moderate temperature ranges in the study area, application of other roofing practices such as insulation board, and low energy costs stemming from low fossil fuel prices. Authors encourage longer studies to investigate how these thermal differences persist over time due to the inevitable darkening of white EPDM membranes. Green roof benefits are likely to persist over longer time periods compared to high-reflective roofing membranes.
Stormwater Monitoring Two Ecoroofs in Portland, Oregon, USA
Doug Hutchinson, Peter Abrams, Ryan Retzlaff, and Tom Liptan
City of Portland, Bureau of Environmental Services
This study examines the rainfall retention rates and water quality for the Pacific-Northwest region of the United States. The Pacific Northwest is characterized by dry hot summers and mild and wet winters. Green roof stormwater retention and quality was examined for these unique climatic variables. Average precipitation retention was calculated to be 69% for the 4-5 inch green roof systems. During the dry summer months green roofs often retained 100% of a rainfall event. Authors also documented an increase in stormwater retention performance over time. Authors attribute this pattern to vegetation maturity and their corresponding increased evapotranspiration rates.
Rooftop to Rivers: Green Strategies for Controlling Stormwater and Combined Sewer Overflows
Nancy Stoner, Christopher Kloss, and Crystal Calarusse
This is a comprehensive literature review of stormwater environmental concerns, practices, and best management practices. Associated case studies conducted by the Natural Resource Defense Council identify an average annual rainfall retention rate of 75% for the Chicago City Hall green roof. The green roof averages 10oF to 15oF cooler than the traditional black tar roof. During particularly hot months this temperature difference may be as great as 50oF.
Re-Greening Washington, DC: A Green Roof Vision Based on Quantifying Storm Water and Air Quality Benefits
Casey Trees Endowment Fund & Lunimno-Tech, Inc.
This paper offers a quantitative assessment of the storm water and air quality benefits provided by green roofs at various coverage scenarios in Washington DC. Planned expansion of the city’ impervious surfaces and population requires the implementation of novel storm water and air quality management practices to achieve federal standards in respect to storm water mitigation and air quality standards. The goals of this research include 1) quantify the contribution green roofs can make towards improving air and water quality in the District of Columbia, 2)access benefits at difference coverage scenarios and propose a green roof coverage objective to form the basis of a Green Roof Vision for DC, and 3) identify nest steps and areas for further study. Coverage scenarios included 20, 40, 60, 80, and 100 percent of DC’s roof space for buildings exceeding 10,000 sf. Results from this study indicate that extensive green roof systems can reduce runoff volumes by 65 percent, while intensive green roof systems can reduce runoff volume by as much as 85 percent. Assuming an 80 percent use of extensive and 20 percent use of intensive green roof systems on buildings exceeding 10,000 sf the District of Columbia could reasonably expect a runoff volume decrease of 69 percent. Although 10,000 sf buildings comprise only 6 percent of the total DC area buildings authors note that in highly developed areas green roof implementation could have a dramatic impact on storm water runoff. Green roof impacts on air quality indicate that green roof systems are most efficient at removing high levels of ozone, particulate matter, sulfur dioxide, nitrogen dioxide, and carbon monoxide. Authors noted significant storm water and air quality benefits for green roof deployment on at least 20 percent of DC’s existing 10,000 sf buildings. Additionally this is a reasonable target when compared to other cities and countries with green roof incentive programs.
Hydrologic and Water Quality Performance From Green Roofs In Goldsboro and Raleigh, North Carolina.
Amy Moran, EI, Bill Hunt, Ph.D., PE, and Jonathan Smith, PE.
Authors identify that storm water management practices including bio-retention areas, detention ponds, constructed wetlands, and sand filters often require large amounts of surface areas and may not be feasible in highly developed areas. Green roof offer an alternative best management practice to address storm water runoff concerns for large cities with limited surface area in which to deploy conventional storm water management strategies. Study objectives included 1) estimating percent precipitation retained by the green roof, 2) estimating the percent peak flow reduction, and 3) determining whether green roofs can be used as nutrient reduction BMP. Results from this study indicate that green roofs were effective for water retention and abated peak flow rates retaining on average the first 15 mm of rainfall per event. Rates of peak flow reduction were dependent on rain event characteristics with green roofs exhibiting greater reduction rates for smaller rain events. Results from nutrient analysis indicate a general increase in both total nitrogen and total phosphorous concentrations in runoff exiting the green roofs. Authors attribute this pattern to the growth media composition particularly related to the organic matter and fertilizers present. A subsequent column study of growth media demonstrated that total nitrogen concentration in green roof runoff may decrease over time as it is leeched from the growth media.
Integration of Green Roof and Solar Photovoltaic Systems
Dr. Sam C. M. Hui and Miss S. C. Chan
University of Hong Kong
This paper explores the potential benefits of combining green roof implementation with solar photovoltaic panels. Authors explore the individual performance of each system and compare efficiency with combined deployment. Theoretical models, experiments, and field studies in this paper analyze thermal and energy effects of integrating these systems. Major findings from this paper indicate that a positive influence exists for combining these strategies. Green roof surface and soil temperatures were found to be lower when used in conjunction with solar PV panels likely attributed to the shading effect of the solar system. Energy consumption was also shown to decrease with combined implementation of the systems due to the increased efficiency of the solar PV panels under lower ambient air temperatures produced by green roofs. Solar PV panels were found to produce 8.3% more energy when combined with green roofs. Authors note that the extent of the benefits depends on factors such as system design and building site conditions.
Interaction Between PV-Systems and Extensive Green Roofs
Manfred Kohler, Werner Wiartalla, Rene Feige
University of Applied Sciences Neubrandenburg, Neuubrandenburg
In this study voltage characteristics of a solar PV-panel systems were compared between systems on traditional Bitumen roofs and systems deployed on extensive green roofs. Examining over five years of data authors indicate an average increase in PV panel efficiency when used in conjunction with a green roof to be nearly 6 percent. Authors attribute differences in PV panel performance to the combined shading and cooling effects of green roof and solar PV systems operating in conjunction with each other. Authors also make note of complications arising from project specific constraints as well as type of PV system deployed citing the fact that PV systems are much more complicated than extensive green roof systems requiring additional planning and design considerations for maximizing energy production efficiency. Unique challenges also exist when combining these green systems such as plant selection requirements in shaded zones created by solar panel “umbrellas” and restricted green roof access leading to heavily traveled pathways for PV panel accessibility which could impact plant coverage and ultimately PV energy production efficiency.