User:Yueyue04/sandbox

Abstract
''Cool roofs and green roofs are used to reduce the energy consumption for buildings, as well as mitigate large-scale urban environmental problems like Global Warming and the Urban Heat Island Effect, and storm water runoff events. In this report, I compared the installation cost, temperature reduction, the economic benefits, social benefits, payback period, and disadvantage for both roofs through a real life example at the City College of New York, which analyzed if it is worth the money to convert 47% of Steinman Hall’s rooftop to either an extensive green roof or a cool roof. •Green roofs might be more sustainable since it provides more social benefits: reducing and filtering storm water runoff, absorbing pollutants and CO2, creating a habitat for wildlife, potentially serving as a recreational green space, and having aesthetic value. •A green roof is more expensive to install compare to a cool roof. The installation cost for green roof is $153,191.00 and $22,978.65 for cool roof in this case. •The payback period for a green roof (68 years) is significantly longer than that of a cool roof’s (18 years). •Both the Cool & Green Roofing Manual by NYC Department of Design and Construction and the DOE Cool Roof Calculator prove that green roofs save twice as much energy as cool roofs would. Personally I believe a cool roof may be a more desirable choice for one who are interested in energy saving or reducing peak energy demand, but with limited budgets. For the ones who consider more about environmental benefits, such as improving storm water management, than installation of green roofs would then be a more suitable option.''

There are some serious environmental problems going on in the New York City: increased temperature due to global warming and the urban heat island effect, and storm water runoff. Global warming rises global average temperature due to increasing concentrations of greenhouse gases in the atmosphere. The major greenhouse gases are: water vapor, which causes about 36–70% of the greenhouse effect, carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%, and ozone (O3), which causes 3–7% [1]. CO2 emissions are largely caused by burning of fossil fuels and methane emissions are result from agricultural activities, landfills, and other sources. The term "Urban Heat Island" refers to city regions that are warmer than their rural surroundings due to changes in their landscape, such as buildings, roads, parking lot, and other infrastructure that replaces open land and vegetation. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C) [2]. Global warming and urban heat island effect cause a dramatic increase in the average temperatures of New York City. The average summer temperature in New York City is 7 ºC hotter when compared to Westchester suburbs’ [5], which means higher summer time peak energy consumption, air conditioning cost, and increased emissions of air pollution and greenhouse gases. Increased temperature can also affect human health by contributing to general discomfort. The storm water runoff is another problem in New York City. Storm water runoff is generated when precipitation from rain and snowmelt flow over impervious surfaces such as streets, parking lots, and rooftops, which water cannot infiltrate into the ground. The storm water runoff does more than flood streets. It also carries bacteria and pollutants into lakes, river, and oceans. To solve these problems, we should focus on reducing energy consumption, providing renewable alternatives for cooling needs, and reducing storm water runoff. The green roofs and cool roofs are the most recent strategies for cooling atmosphere temperature for cities.

Methods
Green Roofs Green roofs, also known as living roofs, are vegetative layers grown on an existing roof which involves a high quality water proofing and root repellant system, a drainage system, a filter cloth, a lightweight growing medium and plants. The “green” in green roof actually does not have anything to do with the color green, but rather the environment [13]. Green roofs were developed in Germany in the 1960s and were then spread to many other countries. The vegetation in the green roof has an important role in reducing roof surface temperature and air temperatures, through shade surfaces and process of evapotranspiration. The plants of the green roofs and the growing medium block the sunlight from reaching the underlying roof membrane. Reduced roof surface temperatures help buildings stay cooler because less heat flows through the roof of the building. In the evapotranspiration, the plants absorb water through their roots and release it through their leaves into the air. This process disperses the heat in the atmosphere. There are two types of green roofs: extensive and intensive roofs. They have different purposes and characteristics. Extensive green roofs have a relatively thin layer of soil that supports 13 to 25 pounds of vegetation per square foot. They are designed to grow sedums, grasses, herbs, and moss. They are virtually self-sustaining which means less required maintenance. Intensive roofs can support more than 50 pounds of vegetation per square foot, with almost no limits on the type of plants, including large trees. They are usually designed for human recreation, such as gardening. Intensive green roofs require skilled labor, irrigation, and constant maintenance.

Cool Roof Cool roofs, also known as white roofs, is a technique to achieve cooling effect by painting urban rooftop with a specialized coating material that is white colored and has two unique properties: high solar reflectivity and high infrared emissivity. Solar reflectivity is the ability of the product to reflect sunlight away from the roof. Surfaces with high solar reflectivity reflect more infrared (felt as heat) and ultraviolet rays (type of rays responsible for sunburns). Infrared emissivity refers to the roof’s ability to give off its absorbed heat. Highly emissive surfaces are cooler than non-emissive surfaces since they have the ability to shed more absorbed heat at a faster rate [6]. The white coatings are able to reflect 70% to 90% of the sun's energy when newly installed. There are generally two types of cool roofs: one used on low-slope and the other used on steep-slope. A low-sloped roof is essentially flat, usually 2 inches of vertical rise over 12 inches of horizontal run with only enough incline to provide drainage. These roofs are found on the majority of commercial, industrial, warehouse, office, retail and multi-family buildings, as well as some single-family homes. Most cool roof applications for low-slope buildings have a smooth, bright white surface to reflect solar radiation, reduce heat transfer to the interior, and reduce summertime air conditioning demand. Steep sloped roofs have inclines greater than 2 inch raise over a 12 inch run. These roofs are found most often on residential areas and retail commercial buildings. Cool roof applications for steep-slope buildings come in various colors and can use special pigments to reflect the sun's energy. The cool roofs are best applied to low-sloped roof in good condition.

Project Goals
The purpose of this study is to determine if it is worth the money to convert partial conventional roof in the Grove School of Engineering-Steinman Hall to either extensive a green roof or a cool roof. (The reason to install an extensive green roof instead an intensive green roof is because it requires little to no maintenance and less installation cost.) The analysis for this project includes: area of interest, installation cost, economic benefits, social benefits, disadvantages, and payback period for both roofs. The following figures show the 3-dimensional view, top view, and area that are going to convert to green roof or cool roof for the Steinman Hall.

Discussion
Initial Cost The costs of green roofs depend on the components, such as the growing medium, type of roofing membrane, drainage system, use of fencing or railings, and the types and quantity of plants. According to the a EPA’s report in 2001, the estimated costs of installing a green roof start at $10 per square foot for simpler extensive roofing, and $25 per square foot for intensive roofs [4]. In 2006, EPA also investigated the initial cost ranges for various cool roof products: “Cool roof coatings might cost between $0.75 and $1.50 per square foot for materials and labor, which included routine surface preparation like pressure-washing, but does not include repair of leaks, cracks, or bubbling of the existing roof surface”[5]. The total installation costs for designed area are $153,191.00 and $22978.65 for green roof and cool roof. Note: The government offers some federal/city tax reduction programs to promote and help citizens to install green roofs and cool roofs (See Result section for detail). In this case, public schools are not eligible for the tax reduction programs because they are non-profit organizations and they don’t pay tax.

Reduction in Temperature Green Roofs and cool roofs can reduce the roof temperatures of buildings and building interiors. Lower the air temperature, and reduce the energy needed for building cooling, and the effects of global warming and the urban heat island effect. A study done by Columbia University and the City College of New York, compared the temperature data collected from three different roof surface treatments (black, white and green) at the Con Edison “Learning Center” in Long Island City, Queens, New York [12]. The peak noontime membrane temperature reductions on the white and green roofs compared to the black roof are dramatic. According to Figure 5, the black membrane reached a peak temperature around 170oF (80oC) during the summer season (from 7/26/09 to 9/14/09). The peak white membrane temperature was on average 30oF (17oC) cooler than the black. The peak green membrane temperature was on average 60oF (33oC) cooler than the black.

Extended Roof Life A conventional roof life expectancy is 10–15 years because of physical, chemical and biological stresses. Temperature variations, coupled with UV-radiation and high ozone ratios, accelerate the aging process and lead to material fatigue, shrinking, crack formation and leakage. Both green roofs and cool roofs are able to extend the life time for a conventional roof. Green roofs can last double or triple as long as a conventional roof, and the life expectancy of waterprooﬁng is increased to more than 40 years [11]. Green roofs buffer temperature stresses and variations plus create a protection layer for waterproofing in case of hail, wind or vandalism damage. Compare with green roofs, the life expectancy of cool roofs is 20-25 years [11]. Cool roofs have shorter extend life time for conventional life because cool roofs have a life time about three to five years. According to the Lawrence Berkeley National Laboratory Cool Roofing Material Database, aging reduced initial reflectance values by a standard 35 % from membranes and coatings after three years. Extending the life of the roof membrane may result in cost savings. However, the amount of saving depends on the increase in longevity, which remains an open issue. Longevity engenders further regional benefits that are derived from reducing resource use and sends less material to landfills.

Economic Benefit Both cool and green roofs reduce roof surface temperature and air temperatures. Reduced summertime peak energy consumption means less energy needed to cool a building. Green Roofs for Healthy Cities (GRHC) with support from Tremco Roofing and Building Maintenance and the Athena Institute developed the Green Roof Energy Calculator, which designed to help professionals concerned with sustainability. The Green Roof Energy Calculator compares roofing alternatives to determine which roof has the lowest life-cycle cost. From the calculated result, annual energy saving from green roof compared to a dark roof is $2,260.50. If this area had been a white roof instead, annual energy saving is $2,231.50. The difference annual energy saving between green and cool roof is only $29 (Appendix 2). I double checked above calculated results using the DOE Cool Roof Calculator, which is developed by the U.S. Department of Energy’s Oak Ridge National Laboratory. According to the DOE Cool Roof Calculator, the net annual energy saving for cool roof is $0.087 per square foot (Appendix 3) which is the same assumptions used in Green Roof Energy Calculator. Therefore, the total annual energy saving for cool roof is $1,333.00 per year. Compared to annual energy saving from green roof from Green Roof Energy Calculator, green roofs could save twice as much as cool roofs do per year. In Cool & Green Roofing Manual by DDC, they also confirmed this result [9].

Social Benefits Compared with cool roofs, green roofs provide more benefits: reducing and filtering storm water runoff, absorbing pollutants and CO2, creating a habitat for wildlife, potentially serving as a recreational green space, and having aesthetic value. Green roofs are a new strategy to control storm water runoff due to its ability to absorb and retain water. Water that enters a green roof is stored in the soil medium, then absorbed by plants’ root systems and retained in plant leaves. According to the Green Roof Energy Calculator, an extensive green roof can retain 34% or more of the annual precipitation (Appendix 2). Additional water storage capacity could be provide by installed a drainage layer in a green roof system, which can hold up to 1.5 inches of water depth under the entire green roof system. Even when rainfall leads to saturated conditions, green roofs significantly increased the time to peak prior to producing runoff and are a prime storm water mitigation tool. Green roofs improve the air quality in cities, helps lower disease rates such as asthma. According to Amy Norquist, “One square meter can absorb all the emissions from a car being driven 12,000 miles a year”. Aside from the environmental and economic benefits, green roofs are aesthetically pleasing. It can help in promoting sustainable community gardens and in some instances can serve as additional city green spaces, such as the High Line Park in New York City.

Disadvantages Green roofs are becoming popular because of their environmental benefits. However, their high installation cost and long payback period have been a challenge to the industry. In general, high installation cost for green roofs are due to materials being lifted by cranes to the roof tops, expensive labor cost, and high insurance premiums. In addition, green roofs add weight to the roof which may change the structural requirements where columns, beams, and slabs must be modified, resulting a more expensive structure. Green roof experts discuss the need to introduce materials like plastics into the market because it can reduce the overall weight and improve the performance of waterproofing layers without compromising the benefits of green roofs. The disadvantage for cool roofs is it cannot save energy both in the summer and in the winter. The penalty for cool roofs is higher heating bills in winter season. That’s because cool roofs reflect solar heat that would help warm the building. This phenomenal is also confirmed in a study by Columbia University [12]. According to Figure 5, the average temperature for cool roof during the winter (from 11/18/2008 to 4/17/2009) is around 30ºF. For green roofs, the vegetation layer act as an insulator in the winter, which means less heat from inside the building is lost. The average temperature for green roof is around 45ºF during the winter. The heating penalty of cool roof is considered into the calculation for annual energy saving while using the Green Roof Energy Calculator (Appendix 2) and the DOE Cool Roof Calculator (Appendix 3).

Payback period The economic benefits for green and cool roofs are estimated using the Green Roof Energy Calculator and the DOE Cool Roof Calculator. The return period for an installed extensive green roof system on the top of Steinman hall is about 68 years and 18 years for cool roof. (Detail calculations are shown in Appendix 4).

Conclusions
Cool and green roofs both help to mitigate Global Warming and the Urban Heat Islands effects. The two technologies have different installation cost and performance. Cool roofs generally have lower initial installation cost compare with green roofs. Depending on the type of product, initial costs are in the range from $0.50 to $1.50 per square foot. The initial cost range for a green roof is at least $10 per square foot for an extensive green roof. The installation costs can vary greatly depending on the size of the job, ease of access to the roof, and local market factors. Both cool and green roofs lower roof surface and air temperatures, and reduce summertime peak and overall energy demand. The annual energy saving for both green and cool roof is almost same. The extent of the energy savings varies depending on factors including the local climate, attic ventilation and insulation levels, and—particularly for green roofs—the design and maintenance of the roof. Green roofs also provide additional environmental benefits, including reducing and filtering storm water runoff, absorbing pollutants and CO2, providing natural habitat and a sound barrier, and potentially serving as a recreational green space and having aesthetic value. A cool roof is a better choice for homeowners with limited budgets and interested in energy savings or reducing peak energy demand. For people who consider environmental benefits, such as improving storm water management, they may choose to install green roofs.