Vegetated Roofs and Energy Conservation

J U LY 2 3 , 2 0 1 3

Nick Fish, Commissioner

Dean Marriott, Director

C I T Y O F P O R T L A N D l B U R E AU O F E N V I R O N M E N TA L S E R V I C E S

Vegetated Roofs and Energy

Conservation

July 2013

Prepared by: City of Portland

Bureau of Environmental Services

Sustainable Stormwater Division

Contact: Amy Chomowicz

Note: Terms used in this paper, including vegetated roof, ecoroof, and green roof refer to extensive vegetated roofs.

INTRODUCTION

With rising energy costs and concerns about climate change, buildings are receiving greater scrutiny for their potential environmental and financial impacts. Energy use in buildings accounts for approximately 40 percent of all energy use in the United States and a similar fraction of the nation’s carbon dioxide emissions.1 Conserving energy in buildings will play a big role in helping cities and the USA meet climate change and greenhouse gas emission goals. There are many ways to conserve energy in buildings, and there is broad agreement that vegetated roofs conserve energy. According to the US Department of Energy, “because of their many energy-saving and environmental benefits, green roofs are a promising technology for energy-efficient buildings.”2 Using a vegetated roof on a project can garner LEED points, and help to meet the Living Building Challenge, and in some states, a vegetated roof may be eligible for funding from energy conservation programs. Numerous studies have been conducted across North America, Europe, and Asia that examine how vegetated roofs conserve energy. This paper reviews existing literature which reports the energy conservation benefits of vegetated roofs. The purpose of this paper is to summarize findings from existing literature about vegetated roofs and energy conservation, and how vegetated roofs may conserve energy in Portland, Oregon. A secondary purpose is to identify data gaps and to recommend future studies.

Literature Review Almost 50 reports and papers on vegetated roofs and energy conservation were gathered from the Internet, from individual researchers, and from symposia. Recent reports written by known vegetated roof experts and peer-reviewed studies and articles were used for this report. In addition, studies conducted in areas with a climate similar to Portland were also used to prepare this report. Section I introduces vegetated roofs and sets the stage for an assessment of their energy conservation attributes. Section II summarizes findings from eight studies that looked at energy conservation from vegetated roofs. Section III discusses the findings based on the studies reviewed for this report. Section IV recommends future study, and Section V provides conclusions. Appendix A lists all reports found as part of the literature review for this paper.

Background Vegetated roofs use several processes that effect energy use in buildings. They cool roof temperatures and prevent heat from flowing through the roof. This section describes the processes used by vegetated roofs to reduce the temperature of a roof.

1 Sailor, David. “Energy Performance of Green Roofs: the role of the roof in affecting building energy and the urban atmospheric environment.” EPA Local Climate and Energy Program Webcast. June 8, 2010 http://www.epa.gov/heatisland/resources/transcripts/10June2010-Transcript_GreenRoofs.pdf 2 US Department of Energy. Federal Technology Alert. Green Roofs. 2004. http://www1.eere.energy.gov/femp/pdfs/fta_green_roofs.pdf

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Evapotranspiration

Evapotranspiration is the natural process by which plants move water from their roots to their leaves (transpiration) where it is evaporated. The United States Environmental Protection Agency (US EPA) defines evapotranspiration as “cool(ing) the air by using heat from the air to evaporate water.”3 Evapotranspiration is one of the most important ways in which vegetated roofs reduce the amount of heat that enters a building in summer.

Shading

Vegetation directly shades the surface below the plants which results in cooler roof temperatures. Two metrics represent shading: 1) the area of the roof covered by at least one layer of vegetation and 2) the multiple layering of leaves over the growing media. Clearly, both measures can vary greatly from one roof to another.4 The greater the leaf coverage and density, the cooler the roof will be. Greater leaf foliage results in less heat entering the building through the roof and cooler roof surface temperatures.5

Albedo, also called Reflectance

The definition of albedo is “reflective power; specifically: the fraction of incident radiation (as light) that is reflected by a surface.”6 Reflecting the sun’s energy will help to cool the roof temperature. The amount of light reflected varies from material to material, so different roofing materials, including vegetation and soil, will reflect different amounts of the sun’s energy. Researchers at Columbia University reported that “vegetated roofs have a significantly higher albedo (reflectance) than black roofs.” They determined the reflectance of a vegetated roof to be 20 percent whereas black roofs have an albedo of five percent.7 A vegetated roof does an excellent job of cooling the temperature of the roof membrane. To achieve the same cooling effect, a black roof would have to reflect quite a lot of sunlight. The Columbia University researchers found that those non-vegetated roofs would have to reflect 70 to 85 of the sunlight they receive to get the same cooling as a vegetated roof. They termed this the “equivalent albedo.” 8 Thermal Mass

Thermal mass refers to the ability of a material to absorb heat from its surroundings. The plants and soil on a vegetated roof absorb heat during the day and then release it when the air temperature cools, usually at night. The weight of the deck, growing medium, and vegetation

3 US EPA. 2008. Reducing Urban Heat Islands: Compendium of Strategies. Green Roofs. Draft. http://www.epa.gov/heatisld/resources/pdf/GreenRoofsCompendium.pdf 4 Sailor, D.J. A green Roof Model for Building Energy Simulation Programs. Energy and Buildings 40 (2008) 1466-1478 5 Clark, Corrie, Brian Busiek, Peter Adriaens. 2010. Quantifying Thermal Impacts of Green Infrastructure: Review and Gaps. http://www.limno.com/pdfs/2010_Busiek_URRC_CotF.pdf 6 Merriam Webster Dictionary. Accessed October 23, 2012. http://www.merriam-webster.com/dictionary/albedo 7 Gaffin, S.R., C. Rosenzweig, J. Eichenbaum-Pikser, R. Khanbilvardi, T. Susca. 2010. A temperature and

Seasonal Energy Analysis of Green, White, and Black Roofs. Columbia University, Center for Climate Systems Research. New York. http://ccsr/columbia.edu/cig/greenroofs 8 Gaffin, Stuart, Cynthia Rosenzweig, Lily Parshall, David Beattie, Robert Berghage, Greg O’Keefe, and Dan Braman. Energy Balance Modeling Applied to a Comparison of White and Green Roof Cooling Efficiency. http://www.buildingreen.net/assets/cms/File/GaffinetalPaperDC-0009.pdf

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determine almost all the thermal mass of the roof. Heavier roofs have more thermal mass and will absorb more heat than lighter weight roofs. 9 Insulation

The vegetation and growing medium insulate the building which reduces heat flow through the roof and reduces the energy demand from space conditioning.10 When considering energy conservation, R-value is a useful tool to use. However, a vegetated roof is more complex than synthetic insulation, and the ways a vegetated roof affects energy use in a building are also complex. The insulative property of materials, such as fiberglass batt insulation which is measured by R-value, is not the only way vegetated roofs conserve energy. Therefore, R-value alone is not a good metric to use when evaluating the overall energy conservation capacity of a vegetated roof. An alternate term is being used: equivalent R-value. This concept is similar to equivalent albedo. The equivalent R-value is the additional insulation that would have to be added to, say a black roof, to reduce the heat flows to the amounts measured on the vegetated roof.11 The equivalent R-value depends on ambient air temperature, so the equivalent R-value will vary depending on location, time of day, and time of year.12 II. FINDINGS

To prepare this paper, 48 publications were reviewed. Of these publications, eight reports provide data and findings from original research in settings that most closely match climate and building conditions in Portland, Oregon. A description of these studies is included here. Performance Monitoring of Three Ecoroofs in Portland, Oregon, by Graig Spolek

This study compared the heat flux of a vegetated roof located on the Broadway Building (a student dormitory in Portland, Oregon) to a conventional, ballasted roof. The ecoroof (vegetated roof) was composed of about six inches of soil and was planted with grass and succulents. Both the vegetated roof and the control roof had R-19 rigid insulation. The purpose of the study was to measure vegetated roof energy conservation to help qualify the building for LEED Silver award. This study found the vegetated roof reduced heat flow through the roof by 72 percent in summer and 13 percent in winter. Despite the smaller effect on heat flux in winter, the author notes that because Portland has ten times as many heating degree days as cooling, the greatest energy benefit will occur in winter.13

9 Wark, Christopher. 2011. Cooler than Cool Roofs: How Heat Doesn’t Move Through a Green Roof. http://www.greenroofs.com/pdfs/EnergyEditor-GreenRoofEnergySeries2011.pdf 10 Celik, Serdar, Susan Morgan, and William A. Retzlaff. Energy Conservation Analysis of Various Green Roof

Systems. Southern Illinois University, Edwardsville, IL. http://www.green-siue.com/images/Energy_Conservation_Analysis_of_Various_Green_Roof_Systems.pdf 11 Gaffin, Stuart. E-mail to author, September 1, 2011 12 Wark, Christopher. 2013. Personal communication. January 13, 2013. 13 Spolek, Graig. Performance monitoring of three ecoroofs in Portland, Oregon. Urban Ecosystems, Vol. 11: 349-359. December 2008.

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Green Roof Research in British Columbia—an Overview, by Maureen Connelly and Karen Liu

This study was conducted on the experimental Green Roof Research Facility in Vancouver, BC. The facility has three roofs: a reference roof with a modified bitumen membrane is situated between two vegetated roofs. One roof has six inches of growing media and the other roof has three inches of growing media. Each of the three roof types is 355 square feet and all roofs have R-28 insulation. Data was collected over a 30-day period in the fall of 2004. This period experienced an ambient air temperature range of 48o F to 70o F. During the study period, the vegetated roofs remained cooler than the conventional roof. Despite the relatively mild air temperature, the maximum temperature of the reference roof membrane was 122o F. Average daytime temperature ranges varied greatly between the reference roof and the vegetated roofs. The average temperature range for the vegetated roofs varied very little and was never more than 5o F, whereas the average temperature range for the reference roof was 58o F. Heat flow through the roofs was also measured. During the day, the reference roof had a total heat gain of 1.031 kWh/m2. The vegetated roofs had no heat gain during the day. At night the heat flow out of the building through the reference roof was 1.603 kWh/m2 . The heat flow out of the building through each of the vegetated roofs was .7 kWh/m2. It is interesting to note that the heat flow through the vegetated roofs was similar despite one roof having twice the soil depth as the other. Looking at the total overall heat flow through the roofs, the vegetated roofs reduced heat flow by more than 70 percent compared to the conventional roof.14 Thermal Performance of Vegetated Roofing Systems, by Andre O. Desjarlais, Abdi Zaltash, Jerald A. Atchley, and Michael Ennis

This study was conducted in eastern Tennessee from June 2008 to June 2009. The study compared four types of vegetated roofs (tray with four inches of soil, vegetated roof with four inches of soil, vegetated roof with eight inches of soil, and bare soil) to a conventional black EPDM roof and a white TPO roof. Each test section was 16 square feet and all had similar insulation under the membrane. Overall, the vegetated roofs had lower peak temperatures and smaller average temperature fluctuations, and they reduced heat flow through the roof. During the summer, the average peak temperature of the membrane on the vegetated roofs was about 94o F cooler than the black roof and 33o F cooler than the white roof. Average day time temperature fluctuations were also much less for the vegetated roofs than the black or white roofs. The average temperature fluctuation of the vegetated roofs was about 10o F, whereas the temperature fluctuation for the black roof was about 125o F, and for the white roof was 64o F. Similar results were found for the winter. The researchers reported the four inch vegetated roof and tray system reduced heat gain by approximately 61 percent and the eight inch vegetated roof reduced heat gain by approximately 67 percent when compared to the white roof. 15

14 Connelly, Maureen, and K. Liu. 2005. Green Roof Research in British Columbia – an Overview. National Research Council of Canada/Vancouver, BC. 15 Desjarlais, Andre O., Abdi Zaltash, Jerald A. Atchley, and Michael Ennis. 2010. “Thermal Performance of Vegetative Roofing Systems.” Proceedings of 25th RCI International Convention.

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A Temperature and Seasonal Energy Analysis of Green, White, and Black Roofs, by S.R. Gaffin, C. Rosenzweig, J. Eichenbaum-Pikser, R. Khanbilvardi, and T. Susca

This study, conducted in Queens, New York on the Consolidated Edison Technical Learning Center building in 2008, studied temperature data from three roofs. The test roofs include a vegetated roof with four inch depth tray system, a roof with black EPDM membrane, and a high-reflectance white EPDM roof. The vegetated roof membrane temperature peaks were on average 60o F cooler than the black roof and 30o F cooler than the white roof in summer. The summer heat gain rate on the vegetated roof was 84 percent lower than under the black roof, and the average winter heat loss rate on the vegetated roof was 34 percent lower than under the black roof. This study also estimated an equivalent R-value for the vegetated roofs studied. The study found the equivalent R-value of a vegetated roof to be R-100 during the warm months of the year and about R-7 during cold months.16

Energy Balance Modeling Applied to a Comparison of White and Green Roof Cooling Efficiency

by Stuart Gaffin, Cynthia Rosenzweig, Lily Parshall, David Beattie, Robert Berghage, Greg O’Keefe, and Dan Braman

This study was conducted at Pennsylvania State University’s Center for Green Roof Research in 2003. Using an energy balance model, the researchers determined the “equivalent” albedo of a white roof to a vegetated roof. Data from six identical, separate experimental structures were used to calibrate the model. The researchers determined the equivalent albedo for a non-vegetated roof needed to reach these temperatures. They found an albedo of .7 - .85 was needed to attain the same cooling effect as the vegetated roof.

Engineering Performance of Rooftop Gardens Through Field Evaluation, by K.K.Y. Liu

This study measured heat flow through an experimental roof on the Field Roof Facility (FRF) in Ottawa, Canada. The FRF has a total roof area of about 800 square feet. The roof is divided into two equal areas by a three-foot tall median divider. One roof has a vegetated roof with six inch soil depth and sedum and the other has a conventional modified bituminous roof assembly. The results of this study showed that the vegetated roof outperformed the reference roof in spring and summer. The average daily energy demand for space conditioning due to heat flow through the reference roof was 6.0 to 7.5 kWh/day (20,500-25,600 BTU/day), and on the vegetated roof it was less than 1.5 kWh/day (5,100 BTU/day). This represents a reduction in energy use of more than 75 percent.

16 Gaffin, S.R., C. Rosenzweig, J. Eichenbaum-Pikser, R. Khanbilvardi, and T. Susca - A Temperature and Seasonal

Energy Analysis of Green, White, and Black Roofs. 2010. Columbia University – Center for Climate Systems Research.

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Average Daily Heat Flow Through Roof Systems

(Nov 22, 2000 - Sep 30, 2002)

0

1

2

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Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

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Figure 1. Average Daily heat flow through roof systems As can be seen in Figure 1, the results of this study also show dramatic peak temperature differences between the reference and vegetated roof. The membrane on the reference roof reached temperatures around 158o F in the afternoon in summer. The membrane on the vegetated roof remained around 77o F. 17 Performance Evaluation of an Extensive Green Roof, by Karen Liu and J. Minor

This study compared data from two vegetated roofs with a reference roof on a recreational facility in Toronto, Canada. The two vegetated roofs were situated side-by-side above the gymnasium and the reference roof was placed over the adjacent mechanical room. One vegetated roof has a soil depth of four inches and has a lighter colored growing medium. The other vegetated roof has a soil depth of three inches. Vegetation covered only about five percent of the vegetated roofs, so results are likely to be greater when the roof is fully vegetated. Compared to the reference roof, both vegetated roofs had lower peak temperatures. Additionally, the peak temperatures of the vegetated roofs occurred later in the day than the reference roof. The peak temperature on the reference roof was 150o F at 2 pm. The vegetated roof with four inches of soil had a peak temperature of 101o F at 6:30 pm, and the vegetated roof with three inches of soil had a peak temperature of 99o F at 7:30 pm.

17 Liu, K.K.Y. - Engineering Performance of Rooftop Gardens Through Field Evaluation. 2003. National Research Council of Canada/Ottawa.

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Heat flow through the roofs was also markedly different. The vegetated roofs both reduced and delayed heat gain. Heat flow through the reference roof reached a maximum intensity of 15 W/m2, whereas peak heat flow through both vegetated roofs was 2.5 W/m2. The time of day and duration of heat flow through the reference and vegetated roofs was very different. Heat started to enter the reference roof just after sunrise, whereas the vegetated roofs lost heat in the morning and did not start to gain heat until the afternoon. As can be seen in Figure 2, the vegetated roofs consistently reduced the average daily heat flow through the roof throughout the year—more in summer (70 to 90 percent) and less in winter (10 to 30 percent). In the first year of monitoring, the vegetated roofs reduced the total annual heat gain through the roof by 95 percent and reduced heat loss in winter by 23 percent.

Average Daily Heat Flow Through Roof Surfaces

(May 2002 - Jun 2003)

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Figure 2. Comparison of the average daily heat flow through the vegetated roofs and reference roof. Before Installation = before installation of a vegetated roof. After Installation = after installation of a vegetated roof. This bar graph shows the dramatic difference in heat flow when vegetation is added to a roof. Vegetation was added to the gymnasium roof in July 2002 (shown as Before Installation on the graph). The graph shows a dramatic reduction in heat transfer when the vegetation is added. Without vegetation, the reference roof has less heat transfer than the vegetated roofs. However, after the vegetation is added, the heat transfer flips, and the vegetated roofs have dramatically less heat flow than the reference roof (shown as After Installation on the graph). This condition is repeated in May and June 2003 (shown as Condition Repeats on the graph).18

18 Liu, Karen, and John Minor - Performance Evaluation of an Extensive Green Roof. 2005. National Research Council of Canada/Toronto.

Before Installation

After Installation

Condition repeats

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Adaptation to Urban Climates with Green Roofs: A Multi-scale Perspective on Reducing Energy

Consumption, by Brad Bass

This study measured the temperature around roof-top machinery that is used to cool air inside a large Walmart store in Chicago. One part of the large roof is an extensive, vegetated roof and the remainder of the roof is a standard white roof. Monitoring showed that air temperatures at the inlet of HVAC equipment were cooler by as much as 15o F on the vegetated roof compared to the white roof. 19

A Green Roof Model for Building Energy Simulation Programs. Energy Balance Model, by D. J. Sailor

The green roof energy balance model allows a designer to test different parameters on a vegetated roof to determine energy savings. Sailor noted that the model showed significantly varying energy response depending on climate and the parameters used in the model. The model allows the energy designer to specify the thermal properties and depth of the growing media and plant type, height, and leaf area index. Sailor ran simulations of buildings in Chicago and Houston. Results showed electrical consumption in both cities was lower by two percent for the vegetated roof than the conventional roof. The natural gas consumption was also lower for both cities. In Chicago, the natural gas consumption was nine percent lower and in Houston it was 11 percent lower. Sailor reports that soil thickness had the greatest effect on energy consumption.20

III. DISCUSSION

Conserving energy is one of the many benefits that vegetated roofs provide. Natural processes, including evapotranspiration, thermal mass, and reflectance influence roof temperature and heat flow through a vegetated roof and result in lower energy use in the building. Conventional roofs and roofs with high reflectance material do not have the energy conservation benefits that evapotranspiration and additional thermal mass provide. Each vegetated roof is unique so the exact amount of energy conserved will depend on the characteristics of the vegetated roof (soil depth, moisture content, and vegetation type and coverage), the characteristics of the building (height, roof-to-wall ratio, insulation, etc), and local climate. Most of the studies reviewed for this paper compared a vegetated roof to a control roof to isolate the effect of the vegetated roof on roof temperature and energy usage in the building. Most of the studies measured the temperature differential between a vegetated roof, a black roof, and/or a white roof. Several studies measured heat flow through the roof and one study measured the temperature of air drawn into roof-top air handlers. This section discusses how the findings from the studies reviewed relate to Portland, Oregon. Where numerous studies provide data for the discussion, the studies are listed as resources in parentheses. The citations for these resources are found in Appendix A which starts on Page 16.

19 Bass, Brad – Adaptation to Urban Climates with Green Roofs: a Multi-Scale Perspective on Reducing Energy

Consumption. October 2009. World Green Roof Infrastructure Congress - Cities Alive, conference proceedings. 20 Sailor, D.J. A Green Roof Model for Building Energy Simulation Programs. Science Direct. Energy and Buildings Vol. 40 Issue 8, Jan. 2008. 1466-1478.

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Roof Temperature and Heat Flux

According to the US EPA, “reduced surface temperatures help buildings stay cooler because less heat flows through the roof and into the building.”21 A vegetated roof will have a greater overall effect on a lower, uninsulated building. On a tall building, the lower roof temperature will have the greatest direct benefit to the upper floors.22 Day Time Temperature

Based on the literature reviewed, vegetated roofs are consistently cooler than conventional black roofs or high reflectance white roofs. The greatest temperature differential occurs in summer. As cited in the literature, peak temperatures of vegetated roofs are 50o F to 81o F cooler than conventional roof tops. On the warmest days, Portland can expect similar temperature differentials on vegetated roofs. The temperature differential on Portland’s vegetated roofs will most likely resemble results found from a study conducted in Vancouver, British Columbia. In this study, the vegetated roofs were on average 54o F cooler than the conventional roof (see resources #10, 14, 24, 26, 29, 46 in Appendix A on Page 16). Night Time Temperature

At night, the temperature of a conventional roof tends to be the same as a vegetated roof or slightly cooler. It is possible that slowing the loss of heat in the evening could result in heat building up within a building. However, the insulative value of a vegetated roof is relatively low when compared to building insulation. It is likely that insulation would have a greater effect than a vegetated roof at trapping heat in a building. Also, in Portland evenings tend to be cool even in summer. The relative small amount of heat remaining in a building at night due to the vegetated roof may make for a more comfortable indoor environment (see resources #14, 24, 26, 29, 46 in Appendix A on Page 16). Heat Flux

Heat flux refers to the movement of heat through the roof. A vegetated roof keeps buildings cooler in the summer by reducing the quantity of heat entering the building through the roof. This effect reduces the energy required to cool the building in summer. A vegetated roof also keeps buildings warmer in the winter by reducing the heat leaving the building through the roof during cooler months. This effect reduces the energy required to heat the building. Nearly all studies reviewed indicate that vegetated roofs are best at reducing summer heat gain, and the majority of studies reported summer heat gain was reduced by 70 to 90 percent (see resources #10, 14, 24, 25, 26, 43 in Appendix A on Page 16). Energy usage in a building is complex and depends on many factors so it is difficult to predict how much energy a vegetated roof will save in a specific building. Energy savings calculations are especially difficult for tall buildings where the roof area may be relatively small compared to

21 U.S. Environmental Protection Agency, Office of Atmospheric Programs, Climate Protection Partnership Division - Reducing Urban Heat Islands: Compendium of Strategies, Green Roofs. October 2008. 22 Saiz, Susana, Christopher Kennedy, Brad Bass, and Kim Pressnail. Comparative Life Cycle Assessment of

Standard and Green Roofs. American Chemical Society/Environmental Science & Technology, Vol. 40, No. 13. 2006.

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the surface area of the walls. Because vegetated roofs are more effective at reducing heat gain than heat loss, they produce the greatest energy savings in summer than in the winter. However, Portland has far more cool days when heating is required, so small energy gains in winter can add up over the course of the year. One study points out that “in Portland, this could be significant yearly savings because Portland has many more heating days than cooling.”23 From the studies reviewed, the winter heat loss reduction is estimated to be between 10 and 30 percent (see resources #14, 24, 26, 43 in Appendix A on Page 16). Actual financial savings in Portland are relatively low because the current cost of energy is low. Potential rising energy costs would result in greater financial savings from vegetated roofs.

Peak energy pricing is another consideration when determining cost savings from energy conservation. As in many other cities, Portland electric utilities price energy higher during the times of the day when energy use is greatest. For example, Portland General Electric’s peak pricing is between noon and 6 pm.24 In summer, conventional black roofs reach their peak temperature when the overall demand for air conditioning is highest and the price of electricity is greatest. Conversely, vegetated roofs have their greatest cooling effect at this time. A vegetated roof will reduce air conditioning usage during the time of day when electricity costs the most. Although little monitoring has been conducted on the impact a vegetated roof has on actual energy use inside the building, according to one study, every decrease in internal building air temperature of 0.5o C may reduce electricity use for air conditioning up to eight percent.25 One

23 Spolek, Graig - Performance monitoring of three ecoroofs in Portland, Oregon. Urban Ecosystems, Vol 11, 349-359. December 2008. 24 Portland General Electric web page. http://www.pge.com/mybusiness/energysavingsrebates/timevaryingpricing/ Accessed August 8, 2012. 25 Getter, Kristin L., and D. Bradley Rowe. 2006. The Role of Extensive Green Roofs in Sustainable Development. Hortscience 41(5): 1276-1285. http://hortsci.ashspublications.org/content/41/5/1276.full.pdf+html

Figure 3. This chart shows how a vegetated roof is cooler during a time of day when a conventional black roof will be at its peak temperature. This roof in Pleasanton, CA, which is east of San Francisco, reached a

peak temperature of over 170oF at 1:00 pm in July. At this time,

air temperature was 90oF, and the green roof membrane was

about 79oF.

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study, by Stephen Peck and Associates, reported that indoor temperatures under a vegetated roof were 5.4o F to 7.2o F cooler than outdoor temperatures between 77o F and 86o F.26

R-value and Equivalent R-value

Although R-value solely is not a good measure of a vegetated roof’s energy conservation potential, an often asked question is: what is the R-value of a vegetated roof? Several researchers are evaluating the R-value of a vegetated roof and others have attempted to determine the equivalent R-value of a vegetated roof. From the literature reviewed, it appears that a vegetated roof will contribute a relatively small amount to the overall insulation of a roof when compared to conventional insulation materials such as rigid foam or fiberglass batt insulation. The R-value of a vegetated roof with four inches of soil is reported as being between R-2 and R-8. Fiberglass batt insulation has an R-value of 3.14 per inch, or R-13 for a four-inch thick insulation. When assessing the R-value of a vegetated roof, it is important to consider all the ways a vegetated roof affects energy usage in a building in addition to this one metric. Researchers at Columbia University studied a vegetated roof on a Consolidated Edison building in Queens, NY to determine the equivalent R-value of a vegetated roof. This R-value takes into account not only the thermal resistance to heat flow, but the roof’s albedo and its ability to keep the roof membrane cool through evapotranspiration and shading. 27 Clearly, these are not processes that synthetic building insulation uses to insulate a building. These processes make it difficult to determine an R-value that would apply to all vegetated roofs. The depth of the soil and moisture content, the type and thickness of vegetation, and climate are all factors that form a vegetated roof’s equivalent R-value. Heating Ventilation and Air Conditioning Use Reduction

When a roof is cooler, the air conditioning requirement in summer will be lower because cooler air is pulled into the air conditioning system. As previously discussed, studies have shown that the temperature differential between a vegetated roof and a conventional roof is dramatic. The peak temperature of a vegetated roof can be 50o F – 80o F cooler than a conventional roof.

Urban Heat Island

The US EPA estimates that annual US energy demand for air conditioning accounts for almost one-sixth of the energy generated per year. This also represents an expenditure of approximately $40 billion to fight heat gain in buildings.28 Scientists at the US Department of Energy Lawrence Berkeley National Laboratory studying the urban heat island effect estimate that using alternative surfaces to reduce the temperature of

26 Peck Steven W., Chris Callaghan, Monica E. Kuhn, and Brad Bass. 1999. Greenbacks from Green Roofs: Forging

a New Industry in Canada. Status report on benefits, barriers and opportunities for green roof and vertical garden

technology diffusion. http://greenroofs.org/pdf/Greenbacks.pdf 27 Gaffin, S.R., C. Rosenzweig, J. Eichenbaum-Pikser, R. Khanbilvardi, and T. Susca - A Temperature and Seasonal

Energy Analysis of Green, White, and Black Roofs. 2010. Columbia University – Center for Climate Systems Research. 28 U.S. Department of Energy, Federal Energy Management Program/Technology Alert - Green Roofs. August 2004.

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ambient air in cities by just 5.4o F would save up to six billion dollars per year in energy costs, nationwide.29 According to a Department of Energy report, the Pacific Northwest will be hotter in summer months, and the use of air conditioning systems is likely to increase in the future.30 Greater use of vegetated roofs can help to mitigate higher temperatures and the associated increased air conditioning. Vegetated roofs and white roofing material reflect more sunlight and are cooler than a conventional black roof. However, the reflectance of a white roof declines fairly quickly, as much as 15 percent, over one year if the roof is not washed regularly and maintained.31 Other Benefits

On a conventional roof the extreme temperatures cause the membrane to expand and contract which damages the membrane. Reducing temperature fluctuations on a roof lead to a longer-lasting membrane.

IV. RECOMMENDED FUTURE STUDIES Portland-specific studies are needed to characterize the energy savings from vegetated roofs in Portland, Oregon. The studies should be conducted on buildings that best characterize the current building stock in Portland. The following section describes several studies that would help in understanding how vegetated roofs in Portland can save energy. Before-and-After Study

Anecdotal evidence from building owners indicates that the interior of a building is cooler in summer and warmer in winter with a vegetated roof. Data is needed to confirm this feedback. The study should measure indoor temperature before and after a vegetated roof is installed. This study can also look at energy consumption before and after a vegetated roof is installed. This type of study can also be used to determine energy savings in a building during time of peak energy pricing. Determine Heat Flux

Several studies in North America have tested the heat flux reduction due to a vegetated roof. This approach can be replicated in Portland, Oregon to determine summer and winter heat flow through a vegetated roof.

Assess Urban Heat Island Reduction Value

Evaluating the effect a vegetated roof has on urban heat Island can be measured by comparing temperature data from a conventional roof membrane and a vegetated roof membrane.

29 Ibid 30 Pacific Northwest National Laboratory. Climate Change Impacts on Residential and Commercial Loads in the

Western U.S. Grid. Richland WA. September 2008. 31 Gaffin, Stuart, Cynthia Rosenzweig, Lily Parshall, David Beattie, Robert Berghage, Greg O’Keefe, and Dan Braman. Energy Balance Modeling Applied to a Comparison of White and Green Roof Cooling Efficiency. http://www.buildingreen.net/assets/cms/File/GaffinetalPaperDC-0009.pdf

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Measuring air temperature at various elevations above the vegetated and conventional roof surface will give additional information about the effect of a vegetated roof on air temperature. Comparing daytime and nighttime temperatures will help to determine if heat radiating from a vegetated roof at night in summer affects the overall urban heat island benefits. Determine the Effect of a Vegetated Roof on Heating, Ventilation, and Air Conditioning

(HVAC) Use and Equipment Sizing

Very little information exists about the effect vegetated roofs have on heating and air conditioning use and HVAC equipment sizing. A Portland-specific study that accounts for the effect of a vegetated roof on heating and cooling loads and the effect on HVAC intake air temperatures will give an indication of energy savings in commercial and institutional buildings. Also, this data may be useful in other areas of North America.

Determine Equivalent R-value for Portland

Simple R-value does not tell the whole story behind the energy conservation benefits vegetated roofs provide. Determining an equivalent R-value would make it possible to compare a vegetated roof to other roofing materials and insulation. Conduct Whole Building Energy Modeling

Climate affects heat gain in summer and heat loss in winter, but inhabitants and equipment inside the building also affect indoor temperatures. Whole building energy modeling will indicate the heat contribution from the roof and associated energy conservation a vegetated roof may provide.

V. CONCLUSION

As reported in the studies reviewed, vegetated roofs use several processes to conserve energy in buildings.

• Vegetated roofs reduce energy use in buildings by cooling the temperature of the roof and by reducing heat flow through the roof.

• Vegetated roofs provide the greatest energy conservation benefit on a shorter (one to three story), uninsulated commercial building, but all buildings will benefit from reduced roof temperatures.

• Vegetated roofs provide many benefits: they conserve energy while also cooling ambient air temperatures and helping cities meet urban heat island reduction goals; they filter air pollutants; they reduce stormwater runoff that can cause combined sewer overflows, localized flooding, and stream degradation; they provide open space for humans and habitat for terrestrial species; and they are visually beautiful.

While it may seem easier and less expensive in the short-term to use white roofing materials, these roofs are not without their drawbacks. If not properly maintained, they lose reflectance due to the build-up of particulate matter on the roof. Some building codes and policies require white roofing material, but do not require that the roofs be maintained. Also, white roofs offer only one benefit—solar reflectance—whereas vegetated roofs provide a broad range of environmental and human health benefits. Conventional black roofs contribute to environmental problems, but vegetated roofs help solve them.

14

Different roofing materials and techniques are needed, and these new roofing approaches will require changes in design, construction, and maintenance. Continuing the construction approaches that have caused urban environmental problems will not result in the changes needed to address these issues. Greater use of sustainable construction approaches, such as vegetated roofs, will help to ameliorate urban human and environmental health problems especially in urban areas. During this period in the United States - when the building trades and building owners are beginning to get comfortable with this technology - policies, codes and incentives will encourage the use of vegetated roofs. Direct subsidies, grants, building bonuses, and tax reductions are all ways to encourage the use of vegetated roofs.

* Indicates the resource is referenced in this report.

APPENDIX A

VEGETATED ROOFS AND ENERGY CONSERVATION RESOURCES

1. Bass, Brad. Adaptation to Urban Climates with Green Roofs: a Multi-Scale Perspective

on Reducing Energy Consumption. October 2009. World Green Roof Infrastructure Congress - Cities Alive, conference proceedings.*

2. Bass, Brad, David Sailor, Graig Spolek and Steven Peck. Introduction to the New Green

Roof Energy Calculator. 2010. Green Roofs for Healthy Cities - Cities Alive/Vancouver, BC.

3. Bell, Harriet, and Graig Spolek. Measured Energy Performance of Greenroofs. June

2009. Greening Rooftops for Sustainable Communities, conference proceedings. 4. Blackhurst, Michael, Chris Hendrickson, and H. Scott Matthews. “Cost Effectiveness of

Green Roofs.” December 2010. Journal of Architectural Engineering. 5. British Columbia Institute of Technology, Centre for the Advancement of Green Roof

Technology. An Introduction to Green Roofs. Vancouver, BC. 6. Castleton, HF, V. Stovin, S.B.M. Beck, and J.B. Davison. Green Roofs; Building Energy

Savings and the Potential for Retrofit. 2010. Elsevier/ScienceDirect.

7. Celik, Serdar, Susan Morgan, and William A. Retzlaff. Energy Conservation Analysis of

Various Green Roof Systems. Southern Illinois University, Edwardsville, IL.*

8. Clark, Corrie, Brian Busiek, and Peter Adriaens. Quantifying Thermal Impacts of Green

Infrastructure: Review and Gaps. 2010. Water Environment Federation. Cities of the Future/Urban River Restoration.*

9. ClimateSmart Loan Program. Commercial Eligible Measures List. Boulder County,

Colorado. 10. Connelly, Maureen, and K. Liu. Green Roof Research in British Columbia – an

Overview. 2005. National Research Council of Canada/Vancouver, BC.* 11. Desjarlais, Andre O., Abdi Zaltash, Jerald A. Atchley, and Michael Ennis. Thermal

Performance of Vegetative Roofing Systems. March 2010. Proceedings of 25th RCI International Convention.*

12. Entrix, Inc. Energy and Greenhouse Gases. January 2010. EcoBenefits of Grey to Green

Program, sections 4.1.2.2 – 4.2.

* Indicates the resource is referenced in this report.

13. Facilities Management Resources.com/Benchmarking, FM Link webpage. Green Roofs:

A Sustainable Solution for Energy Savings. 14. Gaffin, S.R., C. Rosenzweig, J. Eichenbaum-Pikser, R. Khanbilvardi, and T. Susca. A

Temperature and Seasonal Energy Analysis of Green, White, and Black Roofs. 2010. Columbia University – Center for Climate Systems Research.*

15. Gaffin, S.R., C. Rosenzweig, R. Khanbilvardi, J. Eichenbaum-Pikser, D. Hillel, P.

Culligan, W. McGillis, and M. Odlin. Stormwater Retention for a Modular Green Roof

Using Energy Balance Data. January 2011. Columbia University – Center for Climate Systems Research.

16. Gaffin, Stuart, Cynthia Rosenzweig, Lily Parshall, David Beattie, Robert Berghage, Greg

O’Keefe, and Dan Braman. Energy Balance Modeling Applied to a Comparison of White

and Green Roof Cooling Efficiency.* 17. Getter, Kristin L., and D. Bradley Rowe. “The Role of Extensive Green Roofs in

Sustainable Development.” August 2006. HortScience Journal, Vol. 41(5), pages 1276-1285.*

18. GSA Green Roof Benefits and Challenges. Green Roofs and Urban Heat Islands. 19. Halstead, Maria Cristina. Thermal Performance of Green Roof Variations. 2010.

American Solar Energy Society. 20. Koomey, Jonathan G. Trends in Carbon Emissions from U.S. Residential and

Commercial Buildings: Implications for Policy Priorities. June 1996. Climate Change Analysis Workshop, proceedings. Springfield, VA.

21. Lee, Allen - Life Cycle Cost Analysis. Green Roofs From an Investment Perspective.

Quantec. 22. Leonard, Timothy, and James Leonard. The Green Roof and Energy Performance–

Rooftop Data Analyzed. May 2005. Greening Rooftops for Sustainable Communities, conference proceedings.

23. Liu, Karen, and Brad Bass. Performance of Green Roof Systems. 2005. National

Research Council of Canada/Vancouver, BC. 24. Liu, Karen, and John Minor. Performance Evaluation of an Extensive Green Roof. 2005.

National Research Council of Canada/Toronto.* 25. Liu, K.K.Y. Engineering Performance of Rooftop Gardens Through Field Evaluation.

2003. National Research Council of Canada/Ottawa.*

* Indicates the resource is referenced in this report.

26. Loftness, Vivian. Improving Building Energy Efficiency in the U.S.: Technologies and

Policies for 2010 to 2050. Carnegie Mellon University. 27. Miller, Charlie. Role of Green Roofs in Managing Thermal Energy. Roofscapes.

28. NASA, Earth Observatory. Beating the Heat in the World’s Big Cities. Whites Versus

Greens. http://earthobeservatory.nasa.gov/Features/GreenRoof/greenroof3.php. 29. Niu, Hao, Corrie Clark, Jiti Zhou, and Peter Adriaens. Scaling of Economic Benefits

from Green Roof Implementation in Washington, DC. 2010. American Chemical Society/ Environmental Science & Technology Journal.

30. Oberndorfer, Erica, Jeremy Lundholm, Brad Bass, Reid R. Coffman, Hitesh Doshi, Nigel

Dunnett, Stuart Gaffin, Manfred Köhler, Karen K.Y. Liu, and Bradley Rowe. “Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services.” November 2007. BioScience Journal, Vol. 57, No. 10., pages 823-833.

31. Pacific Northwest National Laboratory. Climate Change Impacts on Residential and

Commercial Loads in the Western U.S. Grid. Richland WA. September 2008.* 32. Peck, Steven W., C. Callaghan, M. E. Kuhn, and B. Bass. 1999. Greenbacks from green

roofs: forging a new industry in Canada. Canada Mortgage and Housing Corporation. March 1999. Ottawa, Canada.*

33. Pérez, Gabriel, Lídia Rincón, Anna Vila, Josep M. González, and Luisa F. Cabeza -

Energy Efficiency of Green Roofs and Green Facades in Mediterranean Continental

Climate. Edifici CREA, Universitat de Lleida, and Universitat Politècnica de Catalunya, Spain.

34. Sailor, David. “Energy Performance of Green Roofs: the Role of the Roof in Affecting

Building Energy and the Urban Atmospheric Environment.” June 2010. U.S. EPA, Local Climate and Energy Program Webcast - presentation.*

35. Sailor, D.J. “A Green Roof Model for Building Energy Simulation Programs.” Energy

and Buildings 40 (2008) 1466-1478. Elsevier/ ScienceDirect.*

36. Sailor, D.J., and M. Hagos. “An Updated and Expanded Set of Thermal Property Data for Green Roof Growing Media.” 2011. Energy and Buildings. Elsevier/ScienceDirect.

37. Sailor, D.J., D. Hutchinson, and L. Bokovoy. “Thermal Property Measurements for

Ecorooof Soils Common in the Western U.S.” Energy and Buildings 40 (2008) 1466-1478. Elsevier/ ScienceDirect.

38. Saiz, Susana, Christopher Kennedy, Brad Bass, and Kim Pressnail. “Comparative Life

Cycle Assessment of Standard and Green Roofs.” 2006. American Chemical

Society/Environmental Science & Technology, Vol. 40, No. 13.*

* Indicates the resource is referenced in this report.

39. Shepard, Nora. Green Roof Incentives: A 2010 Resource Guide. February 1, 2010. DC

Greenworks. 40. Sonne, Jeffrey. Evaluating Green Roof Energy Performance. Florida Solar Energy

Center, University of Central Florida. 41. Spolek, Graig. “Performance Monitoring of Three Ecoroofs in Portland, Oregon.”

December 2008. Urban Ecosystems, Vol 11, Number 4.* 42. Stamats Business Media/Buildings.com webpage - Green Roofs and the Urban Heat

Island Effect. July 2009. 43. U.S. Department of Energy, Federal Energy Management Program/Technology Alert -

Green Roofs. August 2004.* 44. U.S. Environmental Protection Agency, Office of Atmospheric Programs, Climate

Protection Partnership Division - Reducing Urban Heat Islands: Compendium of

Strategies, Green Roofs. October 2008.* 45. Velasco, Paulo César Tabares - A New Model to Calculate Energy Savings of Green

Roofs to be Used in Building Energy Simulation Programs. 46. Wark, Christopher - Cooler than Cool Roofs: How Heat Doesn’t Move Through a Green

Roof. 2011. Greenroofs.com publication April 2010 – April 2011.* http://www.greenroofs.com/archives/energy_editor.htm

47. Wise, S., J. Braden, D. Ghalayini, J. Grant, C. Kloss, E. MacMullan, S. Morse, F.

Montalto, D. Nees, D. Nowak, S. Peck, S. Shaikh, and C. Yu - Integrating Valuation

Methods to Recognize Green Infrastructure’s Multiple Benefits. CNT. 48. Wolfe, Paul D., and Katie Felver - Blue and Green: A Study of Urban Green Roof

Performance at the Indigo and Cyan Buildings in Portland Oregon. 2010. American Solar Energy Society.

Printed on recycled paper.