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Crerar Benefit-Cost Analysis

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Benefit-Cost Analysis of a Green Roof on Crerar or Prairies Gone to Sedum

 


 

Statement of Purpose

This cost-benefit analysis is part of a broader feasibility study for the installation of a 30,000 sq. ft. green roof on the John Crerar Library at the University of Chicago.

 

The University has made slow but steady progress towards incorporating sustainability as part of its core mission, evidenced by the Sustainability Council and LEED certification for the Searle Laboratory renovation. Still, the Sustainable Endowments Institute gave the University a 'D' grade for the “Green Building” category. More needs to be done to make the University an environmentally responsible institution, and a green roof would be a highly visible and popular indication of the University's determination to become 'green.' A green roof on Crerar would provide environmental, economic, and aesthetic benefits.

 

Background

Mayor Daley wants to make Chicago the greenest city in the nation, with green roofs as a centerpiece of sustainable urban environments. Chicago's City Hall recently installed a 20,000 sq. ft. green roof garden open to the public which showcases green roof technology as one means of transforming the otherwise missed opportunity of gray urban roofscapes into a positive environment for humans, plants, and animals. Green roofs reduce and filter stormwater runoff, reduce temperature fluctuations and energy costs, mitigate the effect of the “urban heat island” which aggravates smog formation, provide natural habitat, and offer major aesthetic improvements over traditional rooftops.

 

Green Roof Types

There are essentially two types of green roofs:

  • Extensive: The cheapest, lowest-maintenance method of green roof. It is characterized by a gravel-based soil layer and suitable for non-native succulents of the Sedum genus. It has the lowest weight per unit-area, averaging between 18-30 lbs/sq. ft. This is within the limits of Chicago building code for roof load-bearing requirements. Requires little to no maintenance.

  • Intensive: The most expensive method of green roof. Characterized by a soil substrate of 8” or more and suitable for native flora. Average weight ranges from 80 (dry) to 150 lbs (saturated) per sq. ft. Beyond most buildings' structural capacity, most likely within Crerar's load -bearing capabilities. Ideal for public access. Requires regular maintenance.

  • Semi-intensive simply indicates anything between extensive and intensive roofing models in terms of initial price, habitat quality, and maintenance costs.

 

Extensive Green Roof. Berlin, Germany                                                               Intensive Green Roof. Chicago, IL

   

 

 

Table 1: Extensive and Intensive Green Roofs

Characteristic Intensive

Green Roof

Extensive

Green Roof

Soil

Requires minimum of one foot of soil depth

Requires only 1 to 5 inches of soil depth

Vegetation

Acccommodates large trees, shrubs, and well-maintained gardens

Capable of including many kinds of vegetative ground cover and grasses

Load

Adds 80-150 pounds per square foot of load to building structure

Adds only 12-50 pounds per square foot depending on soil characteristics and the type of substrate

Access

Regular access accommodated and encouraged

Usually not designed for public accessibility

Maintenance

Significant maintenance required

Annual maintenance walks should be performed until plants fill in

Drainage

Includes complex irrigation and drainage systems

Irrigation and drainage systems are simple

Source: Schloz-Barth, Katrin. 2001. "Green Roofs: Stormwater Management From the Top Down." Environmental Design & Construction. January 15.

 

 

Summary of Benefits and Costs

 

Direct/primary benefits

  1. Increase roof service life, reduce maintenance costs

  2. Reduce energy costs for heating in winter / cooling in summer

  3. Reduce noise transfer from outdoors

  4. Reduce sewage system loads by assimilating rainwater

  5. Mitigate urban heat island effect

 

Direct/primary costs

  1. Installation and materials

  2. Architectural and engineering costs

  3. Maintenance costs

  4. Safety and supplemental irrigation systems

  5. Gardening materials

  6. Long-term monitoring

    1. HOBO weather station

    2. Monitoring equipment

  7. Pest control

 

Indirect/secondary benefits

  1. Boost University's public image

  2. Provide research opportunities for the newly constructed Center for Integrative Science

  3. Increase worker productivity

  4. Absorb air pollution, collect airborne particulates, store carbon

  5. Serve as living environments for birds and small animals, increased biodiversity

  6. Serve as attractive alternative to traditional roofs

  7. Increase property values

 

Indirect/secondary costs

  1. Uncertain

 

 

Benefit-Cost Analysis

 

Installation and maintenance costs

Of all the expenses involved in a green roof, installation by far represents the largest portion. Current industry estimates generally range from $10 to $15 per sq. ft. for extensive rooftops; $15 to $25 are more likely for intensive green roofs (Acks 2006; Banting et al. 2005; Kimball 2007; Hoffman 2007)

 

Spread over Crerar's approximately 30,000 sq. ft. unused rooftop area, this results in total installation costs ranging from $300,000 to $750,000. LiveRoof has provided a quote of $13.54 / sq. ft.1[INCLUDE QUOTE LINK]  for Crerar library. TectaGreen, another green roof contractor, gave a rough estimate of $12 to $20 / sq. ft. (Hoffman 2007). For this benefit-cost analysis, I will use $500,000 as an average figure, since $300,000 is a relatively low amount, based on contractor estimates and the ultimate cost of other green roofs.

 

This benefit-cost analysis assumes that green roof maintenance costs are near zero. The costs can be minimized to equipment only by collaborating with science departments and student environmental clubs to oversee the project in the long term and conduct much-needed green roof experiments.

 

Related project costs include pre-installation inspection by a licensed structural engineer and obtaining city permits, which fortunately are designed to expedite green roof construction. [Engineer estimates forthcoming]

 

Roof maintenance

A standard roof waterproof membrane has a life of 10 (MWH 2004) to 20 years (Acks 2006). Both studies cite 40 to 60 years as a reasonable estimate for the membrane with a green roof, though Berlin, Germany has several green roofs which have lasted 90 years or more (Porsche and Köhler, 2003). The most important factors in roof lifespan are UV exposure and expansion/contraction related to temperature fluctuations.

 

Acks (2006) estimates the maintenance costs of an extensive green roof at $0.60 per sq. ft. per year, though project-specific quotes are much lower; LiveRoof maintenance costs1 would be set at $0.15 per sq. ft. per year (Kimball 2007). The LiveRoof estimate would entail annual maintenance costs of $4,500.

 

Acks (2006) calculates that maintenance costs on a 2,397 sq. ft. roof would be reduced by $3,822 per year. Considering both his higher maintenance cost estimate and smaller area, this would equate to approximately $12,000 in saved annual maintenance costs on Crerar. However, a precise number still needs to be obtained from the University of Chicago Facilities Department. [ATTACH EMAILS REQUESTING]

 

An intensive green roof would have much higher maintenance costs, but again, these could be largely negated by working with campus research departments and student environmental organizations. Not only would this be desirable from an economic standpoint, but from a student life and academic standpoint as well. Initial discussions with several individuals around campus who are actively involved in environmental organizations have shown very high interest in pursuing this as a long-term mission. [contact science departments ALSO LINK TO SUSTAINABILITY COUNCIL REPORT DESCRIBING THIS GREEN ROOF PLAN]

 

Energy Usage

A green roof reduces heating costs in the winter and cooling costs in the summer by reducing the amount of heat transfer between the roof surface and the atmosphere. Scientists estimate that strategically planting trees and vegetation reduces cooling energy consumption by up to 25% (EPA 2007).

 

The Chicago City Hall saves an estimated $3,600 per year in energy costs at todays prices which are unlikely to stay low (9,272 kWhr/yr; 7,372 therms of natural gas/yr) thanks to a mixed extensive/intensive green roof covering 20,300 sq. ft. Studies indicate that energy savings between intensive and extensive green roofs are not substantially different, so I extrapolated the city hall savings to $5,320 per year for Crerar. Though Crerar already has a thick layer of concrete which most likely mitigates temperature fluctuations indoors, the city hall roof is obviously also quite sturdy and can support an intensive green roof. More precise estimates could be calculated using actual energy usage statistics for Crerar. [forthcoming] If all Chicago rooftops were greened, it would save $100 million in annual energy costs and reduce peak demand by 720 megawatts (Roy F. Weston, Inc. 2000). INSTALLING THESE ROOFTOP GARDENS IS A COMMUNITY INVESTMENT IN DELIVERED SAYINGS EQUIVILANT TO A COAL FIRE POWER PLANT.

 

The few studies of green roofs admittedly vary widely in energy savings. Banting et al. (2005) evaluated several green roof studies and found that a green roof in Madrid reduced total energy consumption by 1% with 0.5% reduction in the heating season and a 6% reduction in the cooling season , while a green roof in Ottawa reduced energy use by 75%. This is one example of an opportunity to collect empirical data. By collecting climatic data before and after rooftop installation and comparing those with electricity usage, a more specific energy-green roof profile could be developed for Crerar. This would also help complement the current corpus of data.

 

Temperature

Temperature fluctuations on traditional roofs can easily reach 43 F during the day, and even more over a 24 hour period (50 F) or in a year (MWH 2004). This seriously stresses the rooftop material. Figure 1 illustrates the temperature variation for different types of rooftops, and Table 1 displays more precise numbers. Note that the mean temperatures in the warm period are artificially low due to the exceptional temperature on June 19th, as seen in the graph. The green roof temperatures (green) closely follow actual recorded temperatures (purple). Figure 2 represents daily temperature fluctuations throughout the year.

 

Figure 1: Temperature at Membrane Horizon by Roof Type. July 15-20, 2003. Source: MWH 2004

 

  Table 2: Temperature Fluctuations at Membrane Horizon by Roof Type. July 15-20, 2003. Source: MWH, 2004

Rooftop Warm Period (12:30-16:30) Cool Period (3:00-7:00)

Mean Temp.

(Celsius)

Mean % Warmer than Green Roof Mean Temp.

Mean Temp.

(Celsius)

Mean % Cooler than Green Roof Mean Temp.
White Reflective Paint 31.38 18.8 17.88 15.9
Stone 32.92 24.3 16.72 19
Black Tar 34.56 30.6 18.29 13.9
Green Roof Composite 26.44 - 21.16 -

 

Figure 2: Membrane temperature daily fluctuation (Nov. 22, 2000 – Sept. 31, 2001). Source: Liu, 2002.

 

Stormwater runoff

Many areas use what is called “combined sewer” infrastructure, where wastewater and stormwater combine. By diverting stormwater from the system, we not only reduce the load on treatment facilities, but reduce the risk of combined sewer overflow events, during which untreated sewage returns to surface level. The MWH report postulates that “concentrating green roofs in selected Chicago areas could reduce the frequency of combined sewer overflow events” (MWH 2004). After any rainfall of moderate or greater size it is clear that the Hyde Park sewers are not equipped to handle the capacity, and a green roof could help mitigate that on a local scale. ANY LINKS TO RECENT SEWAGE OUTFLOWS?

 

A series of Chicago test plots found that green roofs halved the amount of stormwater runoff (12.4 gallons vs. 25.4 gallons for a 36 sq. ft. plot). Even when the green roof is saturated, the rate of runoff is much slower and there is a several hour delay before runoff exits the roof. This lightens the immediate load on sewer systems. Furthermore, the green roof acts as a filtration system for the water, reducing incident pollutants.

 

 

Figure 3: Storm Water Runoff Performance for Flashy, 3-Tiered Storm Event. Source: MWH 2004.

 

Another Illinois study confirmed that green roof models retain more stormwater than traditional roofs. Standard rooftops produced approximate runoff of 23cm after a rainfall of 27cm. Green roof test plots produced runoff averaging 16cm (Forrester 2003).

 

A study of the potential of putting green roofs on every roof in Toronto, which has a similar precipitation pattern to Chicago, estimated that net benefits resulting from reduced stormwater runoff1 would range from $41.8 to $118 million, assuming 4,984 hectares of green roofs (Banting et al. 2005). While the extrapolation to a single building is admittedly variable, it is useful to give a sense of scale, and squares with other single-building estimates in the literature. A 30,000 ft.2 Crerar green roof, according to these numbers, would reduce system-wide costs by approximately $4,473 per year (based on a mean value of $80 million; range between $2,773 and $6,599). It is not unreasonable to assume a linear relationship between stormwater runoff and costs (Acks 2006). More importantly, this value is very similar to individual building estimates in a green roof study in Georgia (Carter and Keeler 2007).

 

It is difficult to estimate the actual costs and benefits of reduced sewer load, particularly because the system is so complex and spread out. For accurate city-wide estimates, it would be necessary to establish the relationship between the quantity of water entering a wastewater treatment plant and capital expenditures, and then how much the treatment plant would benefit from the reduction of immediate stormwater runoff. Also important would be any expenditures associated with overloaded sewer systems, including maintenance and repair.

 

Sound reduction

A green roof with a 12 cm (4.7 inches) substrate layer can reduce sound by 40 decibels; a 20 cm (7.9 inches) substrate layer can reduce sound from the outdoors by anywhere from 8 to 50 decibels (Green Roofs for Healthy Cities 2005;  Scholz-Barth and Tanner, 2004). The higher end is more indicative of the reductions in traffic noise. However, such high reductions are unlikely in Crerar, which is already quite insulated from outside noise.

 

Several studies (in Europe) have found a willingness-to-pay for sound reduction between 2€ and 9€ per decibel per year ($2.70 – $12.17). Considering that Crerar is already fairly insulated from external noise and that the amount of noise reduction is therefore not as “beneficial,” I have reduced estimated sound reduction to 5dB, pricing it at $2.00 per person per decibel per year, over an estimated 5,000 users, equaling $10,000. However, even this very conservative estimate seems extraordinarily high, and ends up being the second largest 'benefit.' Therefore, I will not include this in the final calculation unless I can find more appropriate estimates. [Number of unique patrons forthcoming]

 

Pollution

The American Lung Association's 2007 State of the Air report gave Chicago and Cook County an 'F' for particulate and ozone pollution, and indicated that over 2 million people in the area are at heightened risk for health problems resulting from acute exposure to these pollutants. Ozone is a key component of smog, and the two terms are often used interchangeably (ALA, 2007).

 

Green Roofs for Healthy Cities estimates that 1 m2 (10.76 ft2) of grass roof can remove between 0.2 kg of airborne particulates from the air every year. For Crerar, this equates to 558 kg (1,230 lbs.) of particulates annually.

 

In a benefit-cost analysis for 'greening' rooftops in Washington D.C., researchers found that green roofs were particularly effective at removing ozone (35% of total pollution removal) and particulates (34% of total) from the atmosphere (Deutsch et al. 2005). For DC, high concentrations of these two pollutants are responsible for the federal air quality standard violations in the DC metro area. With a green roof, there are observable reductions in NO2 (13%), CO (13%), and SO2 (5%).

 

Based on Acks' (2006) calculations for the public benefits resulting from a scenario in which 50% of New York City's rooftops would be greened, it is estimated that Crerar alone would provide a benefit (annualized medium) of $4191.99. This is still a somewhat rough calculation, since Acks uses a slightly different discount rate (5%) and time horizon (55 years). This corresponds to health-related costs, such as hospital visits and medicine, lost wages, etc.

 

In order to better estimate these in monetary terms, it would first be necessary to calculate the causal relationship between particulate matter and ozone/smog pollution on health costs. An even more useful analysis would determine local air quality and how/whether a green roof on Crerar would actually help mitigate those area-specific factors.

 

Aesthetics

“There is significant evidence springing from multiple research projects to support the theory that people’s exposure to natural elements increases their ability to focus, cope with stress, generate creative ideas, reduce volatility and promote the perception of self as part of a meaningful greater whole. In short, exposure to natural elements enhances an individual’s mental well being” (Banting et al. 2005)

 

Acks (2006) calculated the net aesthetic benefits of a private green roof at $3,149. This is for a 2,349 sq. ft. rooftop, with an estimated six people receiving aesthetic benefits, each willing to pay an average of $28.33 per year. This roof will likely provide aesthetic benefits to several hundred individuals. Until more precise numbers are available, I will assume 500 individuals will regularly benefit from the roof's aesthetics. I conservatively estimate this figure at to $14,166.67 per year. Over 40 years at a 6% discount rate, this equals $213,155.87. Assuming, however, that aesthetic benefits do not begin to “accrue” until the 2nd year, the 40-year figure is slightly lower, at $187,182.85. This remains a very conservative estimate because is does not factor in the 12-fold increase in green roof surface area and corresponding opportunity for planting a much wider variety of plants can be grown. The increased number of individuals will play an important role in determining net aesthetic benefits, and it is important to recognize that the green roof would be a very unique addition to the landscape in the Crerar quadrangle.

 

In the future, we will conduct contingent valuation studies to determine individuals' willingness-to-pay for the green roof - this will hopefully allow us to incorporate non-market values such as well-being and happiness. This will also give us a more site-specific valuation of aesthetic benefits.

 

Biodiversity / Habitat

Due to the shortage of green space in cities, and particularly the low biodiversity of the green spaces available, green roofs offers safe and desirable habitat to several species, such as beneficial beetles and butterflies. Elevated urban ecosystems offer “unique protection from grade [ground] level predators, traffic noise and human intervention” (US DOE, 2004). While I do not quantify this in monetary terms, it is an important aspect of any green roof project, and needs to be recognized when considering the total benefits.

 

Basel, Switzerland has a relatively extensive set of regulations surrounding green roofs, with an emphasis on biodiversity. On one Basel green roof, a dense combination of micro-habitats supports 79 beetle and 40 spider species; 13 of the beetles and 7 of the spiders are endangered (Marinelli, 2006).

 

Germany witnessed the first wave of modern green roof construction during the 19th century, and Manfred Köhler concludes that a relatively diverse flora is possible on extensive green roofs in inner cities as well as rural areas. He also suggests that plant diversity can be even higher if varied micro-climates, especially sunny and shady areas, are created, initial plantings are enhanced, and a minimal amount of irrigation and maintenance is provided (Marinelli, 2006). The Crerar rooftop offers the opportunity for such micro-climates, due to selective shading from adjacent buildings.

 

One study in London found several thousand examples of hundreds of different invertebrate species on an extensive green roof, of which approximately 10% were “nationally rare or scarce, or have limited range of distribution” (Kadas, 2006). These numbers are higher than many brownfield sites, often known as hotspots of biodiversity. Figures 4 and 5 illustrate this.

Figure 4: Total number of invertebrates collected at each study site in 2004. Source: Kadas, 2006.

Figure 5: Total number of taxonomic arachnid (Araneae), aculeate Hymenoptera, Coleoptera, and notable species in the sample, 2004. Source: Kadas, 2006.

 

Intensive green roofs are usually simply not feasible – the increased load per square foot exceeds most roofs' load-bearing capacity. Intensive roofs have much deeper soil substrates and can support native flora and fauna whereas extensive rooftops are only feasible using a limited set of plant species, primarily non-native Sedum varieties. One of the clearest benefits is biodiversity – the Chicago area was formerly a rich prairie-savanna ecosystem, and there are several sites in or near Hyde Park showcasing some of this biodiversity, such as Nichols Park, Washington Park, and Burnham Park. Native plants, once established, require relatively little maintenance since they are well-adapted to local wind, precipitation, and temperature patterns. They also offer habitat and food for a number of insect and bird species.

 

Intensive green roofs are also more aesthetically pleasing; Sedum varieties are essentially ground cover, and do not grow above a few inches. Prairie grasses and wildflowers, however, are much more attractive and visible, and are well-adapted to this particularly climate. Considering the high visibility of the Crerar roof (adjacent to other, taller buildings), this is an important factor to take into consideration when making a green roof plan. To estimate the monetary value of the biodiversity, it would be possible to conduct a willingness-to-pay study of students and faculty in adjacent buildings.

 

Maintenance costs are also much higher, again, but this could be worked out with research departments and student environmental organizations.

 

Another additional cost that would result from an intensive green roof would be access. Currently there is only access through the mechanical room; for public access, the access would need to be fire-code and ADA compliant, and slight rooftop modifications (e.g. railings) would need to be made. These have not yet been estimated.

 

Benefit-Cost Conclusions

The following two tables are based on a discount rate of 6% over 20 years and 6% over 40 years, respectively. While the net balance after 20 years remains negative, after 40 years, it does become positive, and this is without factoring in a number of sizable benefits, such as noise reduction, increased biodiversity, improved public image, and higher levels of donorship.

 

Several alumni from the Graduate School of Business have expressed serious interest in making the University a more environmentally responsible and sustainable institution. It should be expected that such a high-visibility environmental project would increase donorship to the University, particularly since environmental and sustainability issues have become a keystone of public consciousness, public policy, and private investment decisions.

 

Table 3: Net Benefits and Costs of a green roof on John Crerar Library over 20 years at a 6% discount rate. Aesthetic benefits accrue starting in the second year.

Benefits/Costs Value (6% discount) Time
Installation -$500,000.00 N/A
Maintenance $86,024.41 20 years
Energy usage $61,019.98 20 years
Stormwater Runoff $51,304.96 20 years
Pollution/Health $48,081.80 20 years
Aesthetics $136,517.52 20 years
Biodiversity ??? 20 years
Total -$117,051.33

 

 

Table 4: Net Benefits and Costs of a green roof on John Crerar Library over 40 years at a 6% discount rate. Aesthetic benefits accrue starting in the second year.

Benefits/Costs Value (6% discount) Time
Installation -$500,000.00 N/A
Maintenance $112,847.23 40 years
Energy usage $80,046.30 40 years
Stormwater Runoff $67,302.09 40 years
Pollution/Health $63,073.93 40 years
Aesthetics $187,182.85 40 years
Biodiversity ??? 40 years
Total $10,452.40

 

 

The University recently received a D+ grade on the Sustainable Endowments Institute report card, particularly in Green Building. Undertaking a large-scale, high-profile project centered around the notion of sustainability and environmental responsibility could provide a major boost to the University's public image, attracting additional donors and press. This is nearly impossible to quantify this ex ante, and still difficult ex post, unless donors explicitly cite the project as a reason for increased gifts. Nevertheless, there is definitely alumni interest in "green" projects on campus, and a green roof would likely attract additional donations.

 

The University's motto, Crescat scientia; vita excolatur, expresses a profound sense of responsibility to expand the breadth and depth of human knowledge and thereby enrich society. Environmental concerns are making headlines and public awareness of the importance of environmentally responsible living is growing. Green roofs are a small part of an overall movement towards environmental sustainability, and are by no means a cure-all to our problems, but they are incredibly important, particularly for cities. Green roofs address a number of particularly urban problems in ways which other solutions cannot. Green roofs help sustain biodiversity by creating habitats and habitat corridors, reduce the negative impacts of urban heat islands, such as air pollution (smog is produced by temperature-sensitive processes). They also provide aesthetic benefits and increase people's overall sense of well-being. Unfortunately, green roofs suffer from a lack of empirical data and experimentation. Conveniently, this is one of the University's true strengths. A green roof on the Crerar Library would be an ideal opportunity to help provide society with the information necessary to make green roofs more extensible, enjoyable, and economical.

 

Sources

Acks, Kenneth, Cost Benefit Group, LLC (2006). A Framework for Cost-Benefit Analysis of Green Roofs, Initial Estimates. Retrieved Apr. 26 2007 from http://ccsr.columbia.edu/cig/greenroofs/Green_Roof_Cost_Benefit_Analysis.pdf.

 

American Lung Association. State of the Air: 2007. Retrieved 10 May 2007 from http://lungusa.kintera.org/sota07pdf.

 

Banting, Doug, Hitesh Doshi, James Li, Paul Missios, Angela Au, and Beth Anne Curie (2005). Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto. Retrieved Apr. 26 2007 from http://www.toronto.ca/greenroofs/pdf/fullreport103105.pdf.

 

Carter, T., Keeler, A. (2007). Life-cycle cost–benefit analysis of extensive vegetated roof systems, Journal of Environmental Management (2007), doi:10.1016/j.jenvman.2007.01.024 .

 

Deutsch, B. Heather Whitlow, Michael Sullivan, and Anouk Savineau (2005). Re-Greening Washington, D.C.: A green roof vision based on quantifying storm water and air quality benefits. Retrieved Apr. 27 2007 from http://www.greenroofs.org/resources/greenroofvisionfordc.pdf.

 

EPA (2007). Heat Island Effect: Green Roofs. Retrieved Apr. 20 2007 from http://www.epa.gov/hiri/strategies/greenroofs.html.

 

Forrester, K., V. Jost, K. Luckett, S. Morgan, T. Yan, and Retzlaff. Evaluating Performance of a Green Roof System with Different Growing Mediums, Sedum Species and Fertilizer Treatments .

 

Hoffman, Jamie (2007). Personal communication. TectaGreen.

 

Kadas, Gyongyver (2006). “Rare Invertebrates Colonizing Green Roofs in London .” URBANhabitats, 4(1). Retrieved 23 May 2007 from http://www.urbanhabitats.org/v04n01/invertebrates_full.html.

 

Kimball, Rob (2007). Personal communication. LiveRoof.

 

Liu, K.K.Y. (2002). “Energy efficiency and environmental benefits of rooftop gardens.” Construction Canada, 44(2), 17,20-23. Retrieved 23 May 2007 from http://www.eltgreenroofs.com/PDFs/nrcc45345.pdf.

 

Marinelli, Janet (2006). “Introduction: Green Roofs and Biodiversity.” URBANhabitats, 4(1). Retrieved 23 May 2007 from http://www.urbanhabitats.org/v04n01/introduction.html.

 

MWH (2004). Green roof test plot: 2003 End of Year Project Summary Report. Prepared for the City of Chicago Department of Environment. Retrieved Apr. 20 2007 from http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/2003GreenRoofReport.pdf.

 

Porsche, Ulrich, and Manfred Köhler (2003). “Life Cycle Costs of Green Roofs: A comparison of Germany, USA, and Brazil.” World Climate and Energy Event, 1-5 Dec. 2003.R Retrieved May 18 2007 from http://www.gruendach-mv.de/en/RIO3_461_U_Porsche.pdf.

 

Scholz-Barth, K. (2001). "Green Roofs: Stormwater Management From the Top Down." Environmental Design & Construction. January 15.

 

Scholz-Barth, K., and S. Tanner (2004). Federal Technology Alert: Green Roofs. DOE Energy Efficiency and Renewable Energy (EERE). Retrieved 15 May 2007 from http://www.osti.gov/energycitations/servlets/purl/15009602-KD2isR/native/15009602.pdf.

 

Weston, Roy F. et al (2000). Urban Heat Island Initiative Pilot Project: Final Report. Prepared for City of Chicago 

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