Green Roof Valuation: A Probabilistic Economic Analysis Of .
Environ. Sci. Technol. 2008, 42, 2155–2161Green Roof Valuation: AProbabilistic Economic Analysis ofEnvironmental BenefitsC O R R I E C L A R K , † P E T E R A D R I A E N S , * ,†AND F. BRIAN TALBOT‡Department of Civil and Environmental Engineering, Collegeof Engineering, and Department of Operations andManagement Sciences, Ross School of Business, University ofMichigan, Ann Arbor, MI 48109-2125Received March 16, 2007. Revised manuscript receivedDecember 3, 2007. Accepted December 5, 2007.Green (vegetated) roofs have gained global acceptance as atechnology that has the potential to help mitigate the multifaceted,complex environmental problems of urban centers. Whilepolicies that encourage green roofs exist at the local and regionallevel, installation costs remain at a premium and deterinvestment in this technology. The objective of this paper is toquantitatively integrate the range of stormwater, energy, andair pollution benefits of green roofs into an economic model thatcaptures the building-specific scale. Currently, green roofsare primarily valued on increased roof longevity, reducedstormwater runoff, and decreased building energy consumption.Proper valuation of these benefits can reduce the presentvalue of a green roof if investors look beyond the upfront capitalcosts. Net present value (NPV) analysis comparing aconventional roof system to an extensive green roof systemdemonstrates that at the end of the green roof lifetime the NPVfor the green roof is between 20.3 and 25.2% less than theNPV for the conventional roof over 40 years. The additionalupfront investment is recovered at the time when a conventionalroof would be replaced. Increasing evidence suggests thatgreen roofs may play a significant role in urban air qualityimprovement. For example, uptake of NOX is estimated to rangefrom 1683 to 6383 per metric ton of NOX reduction. Thesebenefits were included in this study, and results translate to anannual benefit of 895-3392 for a 2000 square meter vegetatedroof. Improved air quality leads to a mean NPV for thegreen roof that is 24.5-40.2% less than the mean conventionalroof NPV. Through innovative policies, the inclusion of airpollution mitigation and the reduction of municipal stormwaterinfrastructure costs in economic valuation of environmentalbenefits of green roofs can reduce the cost gap that currentlyhinders U.S. investment in green roof technology.IntroductionUrbanization increases stress on private and public utilitiesresulting increases the demand for energy, water and sewerservices, and transportation (1). To meet increased energydemand, more than 150 new coal-fired power plants areproposed in the U.S. alone by 2030 with residential and* Corresponding author phone: 734-763-8032; fax: 734-763-2275;e-mail: adriaens@umich.edu.†College of Engineering.‡Ross School of Business.10.1021/es0706652 CCC: 40.75Published on Web 02/09/2008 2008 American Chemical Societycommercial buildings currently contributing to 39% of energyconsumption (2, 3). Converting green space into neighborhoods, shopping malls, and other developments increasesthe need for infrastructure investment in storm sewer systems(4). New road infrastructure leads to increased vehicleemissions and, along with parking lots and rooftops, roadscontribute to elevated urban surface temperatures by reducing a city’s albedo. Increased urban temperature, commonlyreferred to as the urban heat island effect (UHIE), incombination with emissions from the electric utility industry,impact local and regional air quality (5). As growth isinevitable, a multifaceted and scalable solution is needed totemper the environmental impacts of growing cities. Increasingly, developers, architects, and city planners recognizethat green (vegetated) roofs may be part of the solution.Composed of a drainage layer, a solid matrix “soil” layer, andvegetation, green roofs reduce the thermal gain directlybeneath the roof (6) and improve the water balance betweenevapotranspiration and runoff (7).Much of the research on green roofs focuses on theinsulation capability during summer months, which reducesthe flux of solar radiation in a building (8). A study byTakebayashi and Moriyama (2007) on the surface heat budgetof a green roof and a high reflectivity (white) roof revealedthat both systems have a small sensible heat flux comparedto a concrete roof surface (9). The small heat flux on thewhite roof is due to the low net radiation, whereas that ofthe green roof was attributed to the large latent heat flux byevaporation (9).There are two main parameters that influence the solarradiation reaching the roof deck, leaf foliage and soil media.The more extensive the foliage density of a particular plant,the more the heat flux through the roof decreases (8, 10)and the greater the decrease in surface temperatures (11).Thick soil layers reduced cooling needs during summermonths while thin substrate layers resulted in little to nocooling benefit (10). Additionally, a dry environment andwind speed increase the rate of evapotranspiration, therebyaiding the absorbance of solar radiation by plants (10).Generally, heat transfer is greater on roof surfaces that arenot vegetated (11).Green roofs retain as much as seventy percent of annualrainfall precipitation depending on regional climate (12).Rainfall retention is also affected by slope and substrate depth:in general, the flatter the roof, the greater the retention andpeak flow reduction (12). While increased thickness providesincreased storage capacity, moisture is also retained for alonger period of time limiting the effectiveness of retentionfor subsequent storm events. Villarreal and Bengtsson (2005)found that the moisture content of the media had a greateraffect on peak flow and total stormwater volume reductionthan slope (13).Green roofs exhibit the capacity to reduce pollution inurban environments from ground level ozone (14). Vegetationplays a role in lowering surface temperatures through latentheat removal from soils via evaporation and transpiration inthe presence of high moisture levels (15). The absorption ofincoming solar radiation by impervious surfaces creates anurban heat island where temperatures are elevated. Anthropogenic heat and pollution can further intensify the UHIEby creating an inversion layer, resulting in increased airconditioning demand (16), and heat-stressed related mortality and illness (17).With vehicular and power plant emissions, the reactivechemistry in urban areas can be greatly affected by nitrogenoxides. Nitrogen oxides (NOX) alone or in combination withVOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY92155
other air pollutants such as ozone, sulfur oxides, andparticulate materials (PM) can cause respiratory diseases andincrease the risk of heart attacks (18). Damage from NOX canextend to plants as well reducing growth, respiration,photosynthesis, stomatal conductance, and enzyme activities(19). While no studies modeling the effects or removal of airpollutants by green roofs have been reported in the peerreviewed literature, there is extensive work on the uptake ofreactive nitrogen species by vegetation (20).Although green roofs have been shown to mitigatestormwater runoff volume and to reduce the heating andcooling loads of buildings, the challenges for widespreadintegration of green roofs include the premium cost overconventional roofs, and widely diverging municipal management practices for stormwater and air pollution control.For example, in the USA, the financial burden of managingstormwater is rarely applied to property owners accordingto area and intensity of impervious area. Reducing theuncertainty in the quantification of economic benefits ofgreen roofs is a necessary first step to develop policies aimedat stimulating widespread acceptance of the technology inthe United States.The objective of this paper is to quantitatively integrateprobabilistic ranges of stormwater, energy, and air pollutionbenefits in an economic model capturing the buildingspecific scale. A secondary goal is to assess the impact andopportunities of market-based air credit valuation as a policytool for green roof diffusion.Materials and MethodsThe first step describes a cost-benefit analysis that can beapplied to a range of green roof projects through a probabilistic evaluation procedure. This analysis provides information relevant to building owners, developers, or designersregarding the costs and environmental benefits (stormwaterreduction, energy savings, and air quality) of green rooftechnology. This section summarizes the steps for the costbenefit analysis at the building scale.Installation Costs for Conventional and Green Roofs.To determine how the environmental benefits reduce theinstallation cost gap between green and conventional roofs,the magnitude of the gap was first determined. Cost and sizedata were obtained from reroofing cost and time estimatesprovided by plant operations for seventy-five campus roofsfrom the University of Michigan in Ann Arbor, Michigan.Within this sample, the mean cost of a conventional flat roofwas 167 per m2 (standard deviation: 28 per m2). The meancampus roof is 1870 m2 and the mean building floor area is9730 m2.The distribution of green roof installation costs was basedon available green roof case data (21). As the price of greenroofs can vary according to design and function (e.g., intensivegreen roof can serve as a garden), the cases used in the dataanalysis were limited to extensive roofs with a depth between5 and 15 cm. The collected data represent the additional costof the green roof components. The distributions of theconventional roof and green roof were summed to obtainthe total cost of installation for a new green roof with a newconventional roof. The mean difference between the cost ofthe green roof and the conventional roof is defined as thecost gap. The internal rate of return was then determined foreach environmental benefit.Stormwater Fees and Reductions. The reduction ofstormwater volume by green roofs benefits municipalities;however, not all local water authorities pass the economicsavings on to the owner of the green roof. Traditionally, thebudget for stormwater management is provided throughproperty taxes or potable water use fees. In recent years,municipalities have been moving toward stormwater fees21569ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 6, 2008based upon total impervious surface on a property, creatingan opportunity to “credit” green roofs for stormwaterreduction. Two methods were used for determining stormwater fees and the reduced fee for a green roof. The firstmethod is limited to the City of Ann Arbor, Michigan and itsnew stormwater ordinance. The commercial stormwater feeis 279.10 per acre per quarter ( 0.28 per square meter peryear) (22). The second method takes an average fee based onavailable data from eleven municipalities with establishedstormwater management fees (Supporting Information TableS1). It was assumed that the reduction in stormwater feesdue to a green roof is normally distributed at fifty percentof the stormwater fee for the building footprint according todata on fee reduction policies in Portland, Oregon; Minneapolis, Minnesota; and Ann Arbor, Michigan (23–25).Energy Savings Determination and Valuation. Theenergy savings were based on mixed-use administrative/laboratory buildings at the University of Michigan campusin Ann Arbor, Michigan. Total expenditures for energy(natural gas and electricity) consumption (mean 225 00),total energy consumption (mean 4050 MWh), and energyconsumption by fuel source (mean 2370 MWh from electricityand 1670 MWh from natural gas) were obtained for 75university buildings for fiscal year 2003. National commercialbuilding energy consumption statistics provided additionaldata (e.g., average commercial conductance, system loadfactors) (26). To determine the roof’s contribution to theHVAC energy requirement, the heat flux through the roofwas determined according to two methods.The first method is based on EnergyPlus v2.0.0, a buildingenergy simulation software program supported and madeavailable by the US Department of Energy (27). It can modelbuilding heating, cooling, lighting, ventilating, and otherenergy flows, based on climate and building use, material,and size inputs. Version 2.0.0, released in April 2007, containsthe capability to include a green roof (referred to as ecoroof)on a building. The ecoroof component accounts for heat fluxthrough a 1-dimensional heat transfer model. The modelaccounts for heat transfer processes within the soil and plantcanopy, but it does not account for the soil moisturedependent thermal properties of the green roof (28).The second method is a simplified 1-dimensional heatflux equation that assumes an R-value of 1.2 ft2 F h/Btu(conductance of 4.7 W/m2/K) per centimeter depth for a10.2 cm soil media of a green roof.·Q ) h A T )A TRwhere Q is the heat flux through the roof (W), A is the areaof the roof (m2), T is the temperature difference betweenthe building interior and the ambient temperatures (K), andh is the heat transfer coefficient (W/m2/K). This coefficientis a function of the thermal conductivity of a material andthe material thickness. The inverse of h is the R-value, whichrepresents a material’s resistance to heat flow. The larger theR, the less heat flux Q. In the construction industry, R-value(ft2 F h/Btu) is commonly used to compare theeffectiveness of insulation in building materials. For thismethod, an average R-value of 11.34 ft2 F h/Btu(conductance of 0.50 W/m2/K) was assumed for the conventional roof according to national commercial buildingdata (26). The total combined R-value for a conventionalroof with a green roof is 23.4 ft2 F h/Btu (totalconductance of 0.24 W/m2/K). The requisite energy consumption by the HVAC system to compensate for the lossthrough the roof was then determined. Annual totals for heatloss and cooling loss were multiplied by a system factor assuggested by Huang and Franconi (26).Energy costs due to the heat flux were determinedassuming natural gas for heating and electricity for cooling.
Pricing for energy was based upon available university energyexpenditure information, 0.08/kWh for electricity and 0.02/kWh of natural gas. Heating and cooling degree-days wereused for the R-value analysis, while hourly weather data wassupplied for the EnergyPlus model (29).Air Quality Improvement and Valuation. Impact on airquality was limited to the mitigation of nitrogen oxide (NOx).Nitrogen oxide emission allowances are currently traded inthe U.S.; market-based economic valuations for 2005–2006ranged from 900 per ton ( 992 per Mg) to 4282 per ton( 4721 per Mg) (30, 31). To quantify nitrogen oxide uptakeby plants (per unit area), data from Morikawa, et al. (1998)were used (32). That study evaluated the NOX uptake potentialof 217 plant taxa under controlled conditions in a greenhouseenvironment. Although sedums, the traditional vegetated roofplants of choice, were not evaluated, the study included amember of the same family, Crassulaceae. Published resultswere in terms of mg N g-1 dry weight per 8 h of daylightexposure. The following assumptions were made to obtainthe uptake capacity per unit area (kgNO2 m-2 y-1): (i) Ninetypercent of plant mass is water; (ii) Leaf thickness is 2 mm;(iii) Leaf area index (LAI) is 5 (m2 leaf area per m2 surfacearea) according to a global mean (33); (iv) Average hours ofdaylight per day (12) (34). Calculations were performed tocapture the potential impact of all 217-plant taxa on NOXuptake. The distribution of uptake potentials (SupportingInformation Figure S1) is assumed to be log-normal with amean of 0.27 ( 0.44 kgNO2 m-2 y-1. An implicit assumptionwas that the uptake capacity is constant on a year-to-yearbasis.Once the annual uptake of NOX was determined, the resultwas translated to health benefits. These calculations werebased upon two estimation methods developed by the U.S.Environmental Protection Agency (EPA) as part of a regulatoryimpact analysis of NOX reductions in 1998 (35). The conclusion of the analysis for the Eastern U.S., was that fewerpremature deaths and fewer cases of chronic bronchitistranslated into an economic benefit between 1680 and 6380per Mg adjusted to 2006 dollars (35). The two estimates werebased upon the results of several atmospheric models thatprovided estimates for secondary ozone, nitrogen deposition,and particulate formation (35). The range of economic benefitaccounts for uncertainty in atmospheric acid sulfate concentration, which affects ammonium nitrate particulateformation (35). For the purposes of this study, the estimatesare referred to as the low estimate ( 1680 per Mg) and thehigh estimate ( 6380 per Mg). It should be noted that thesevalues are in a similar range of emission allowance values.Economic Analysis and Sensitivity Analysis. Once thecosts and benefits were determined on a per unit area basis,the results were integrated into an economic model todetermine the length of time required for a return oninvestment in a 2,000 m2 green roof using a net present value(NPV) analysis (Supporting Information Table S2). An interestrate of five percent (based upon the 2006, 20 year U.S.government bond interest rate) and inflation rate of threepercent (based upon the 2005-2006 Consumer’s Price Index)were used (36, 37).It was assumed that the conventional roof would bereplaced after twenty years (38, 39). Maintenance costs havenot been included in this analysis. A sensitivity analysisevaluated model sensitivity to economic parameters, climatefactors, and variability in air pollution uptake.Results and DiscussionThe following summarizes the NPV analysis. The implicationsof the benefits on city environmental policy are also discussed.Stormwater Benefits. For the Ann Arbor assessment, aper square meter area cost was assumed (instead of the fullcost for one acre). The stormwater fee for a conventionalTABLE 1. Roof Conductance According to Different EnergyModelsroof Conductance (W/m2/K)roof nalgreen0.50.240.380.360.59 (45)0.42 (45)roof of 2000 m2 is then 520 per year (22). As Ann Arborconsiders a green roof to be a pervious surface, then thegreen roof fee would be 0 per year. The mean stormwaterfee was found to be 0.17/m2 (standard deviation: 0.12/m2)(40–49). Potential fee reductions for green roofs resulted ina mean stormwater fee of 0.08/m2 (standard deviation: 0.06/m2). For the 2000 m2 roof, conventional roof fees wouldbe 340, whereas the green roof scenario would have fees of 160 per year. A few municipalities offer fee reductions togreen roof projects (assuming reduced impervious area andadequate storm capture) to pass the value of the public benefitof stormwater reduction to the building owner (e.g., Minneapolis, Minnesota) (24).Energy Assessment. The heat flux was based on a 2000m2 roof utilizing hourly climate data from nearby Detroit,Michigan for the EnergyPlus simulation and heating andcooling degree days for Ann Arbor, Michigan for the R-valueanalysis. Roof conductance values and energy savingsbetween conventional and green roof systems were differentaccording to model method, and are summarized in Table1. A study by Saiz et al. (2006) compared several roof systemsfor a roof in Madrid, and the conductance of the roofs areprovided in Table 1 (50). The conductivity estimates for theconventional roof and green roof by Saiz et al. is larger thanthe results from both models presented here. This may bedue to their use of an existing building in Madrid, Spain forthe analysis (age, different insulation requirements) and theassumption of pine bark and compost as the primaryconstituents of the soil media for the green roof, which wouldaffect soil moisture properties. For the EnergyPlus analysis,the difference in consumption for a one floor commercialfacility with a green r
of the green roof components. The distributions of the conventional roof and green roof were summed to obtain the total cost of installation for a new green roof with a new conventional roof. The mean difference between the cost of the green roof and the conventional roof is defined
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Roof Layout Plan. Roof Plan A roof plan is commonly drawn to a scale of 1/8 inch per foot or 1/16 inch per foot. A roof plan shows the shape of the roof, as well as roofing materials, underlayment and location of vents. Roof Plan A roof framing plan shows size and direction of the
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there is no reason for not installing a green roof on this type of roof if the load-bearing capacity of the second skin allows for it and it is possible to ensure that the green roof does not prevent ventilation. With an "inverted roof", the thermal insulation is installed on top of the roof waterproofing membrane, i. e. in the damp area.
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