Reflective surfaces (geoengineering) — Wikipedia, the free encyclopedia

Reflective surfaces (geoengineering) - Wikipedia, the free encyclopedia

The albedo of several types of roofs

Reflective surfaces are surfaces that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the surface) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy). [ 1 ] Reflective surfaces are a form of geoengineering .

The most well-known type of reflective surface is the cool roof. While it is true that cool roofs are mostly associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Note that today’s "cool roof" pigments allow metal roofing products to be EnergyStar rated in dark colors, even black.

Solar reflective cars or cool cars reflect more sunlight than dark cars, reducing the amount of heat that is transmitted into the car’s interior. Therefore, it helps decreasing the need for air conditioning, fuel consumption, and emissions of greenhouse gases and urban air pollutants. [ 2 ]

Cool color parking lots are parking lots made with a reflective layer of paint. The project is being undertaken by Jordan Woods of the Berkeley Lab. [ 3 ]

Contents

Benefits of cool roofs [ edit ]

Cool roofs, in hotter climates, can offer both immediate and long-term benefits including:

  • Savings of up to 15% the annual air-conditioning energy use of a single-story building [ 4 ]
  • Help in mitigating the urban heat island effect. [ 5 ]
  • Reduced air pollution and greenhouse gas emissions, as well as a significant offsetting of the warming impact of greenhouse gas emissions. [ 6 ]

Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters. [ 7 ] Therefore, the net energy saving of cool roofs varies depending on climate. However, a 2010 energy efficiency study [ 8 ] looking at this issue for air conditioned commercial buildings across the USA found that the summer cooling savings typically outweigh the winter heating penalty even in cold climates near the Canadian border giving savings in both electricity and emissions. Without a proper maintenance program to keep the material clean, the energy savings of cool roofs can diminish over time due to albedo degradation and soiling. [ 9 ]

Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations, seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes, or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation. [ 10 ]

If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 Gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 93-square-metre (1,000 sq ft) white roof will offset 10 tons of carbon dioxide over its 20-year lifetime. [ 11 ] In a real-world 2008 case study [ 12 ] of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled 1.6°C over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down.

When sunlight falls on a white roof much of it is reflected and passes back through the atmosphere into space. But when sunlight falls on a dark roof most of it is absorbed and converted into much longer wavelengths that we call "heat" which cannot pass back through the atmosphere because they are absorbed by the greenhouse gases. The atmosphere is transparent to sunlight but opaque to heat, which is why white roofs help cool the planet and dark roofs warm the planet. [ 13 ]

A 2012 study by researchers at Concordia University included variables similar to those used in the Stanford study (e.g. cloud responses) and estimated that worldwide deployment of cool roofs and pavements in cities would generate a global cooling effect equivalent to offsetting up to 150 Gigatonnes of carbon dioxide emissions — enough to take every car in the world off the road for 50 years. [ 14 ] [ 15 ]

Disadvantages [ edit ]

A 2011 study by researchers at Stanford University suggested that although reflective roofs decrease temperatures in buildings and mitigate the "urban heat island effect," they may actually increase global temperature. [ 16 ] [ 17 ] The study noted that it did not account for the reduction in greenhouse gas emissions that results from building energy conservation (annual cooling energy savings less annual heating energy penalty) associated with cool roofs (meaning that you will need to use more energy to heat the living space due to reduction in heat from sunlight in winter.) However that apply only to those area with low winter temperature but not the tropical area. Also homes in area receiving snow in winter are unlikely to get more heat due to darker roof since snow will cover it most of the winter. A response paper titled "Cool Roofs and Global Cooling," by researchers in the Heat Island Group at Lawrence Berkeley National Laboratory, raised additional concerns about the validity of these findings, citing the uncertainty acknowledged by the authors, statistically insignificant numerical results, and insufficient granularity in analysis of local contributions to global feedbacks. [ 18 ]

Also, 2012 research at University of California, San Diego ‘s Jacobs School of Engineering into the interaction between reflective pavements and buildings found that, unless the nearby buildings are fitted with reflective glass or other mitigation factors, solar radiation reflected off light-colored pavements can increase the temperature in nearby buildings, increasing air conditioning demands and energy usage. [ 19 ]

In 2014, a team of researchers, led by Matei Georgescu, an assistant professor in Arizona State University ‘s School of Geographical Sciences and Urban Planning and a senior sustainability scientist in the Global Institute of Sustainability explored the reletavive effectiveness of some of the most common adaptation technologies aimed at reducing warming from urban expansion. Results of the new study indicate that the performance of urban adaptation technologies can counteract this increase in temperature, but also varies seasonally and is geographically dependent. [ 20 ]

Specifically, what works in California’s Central Valley, such as cool roofs, does not necessarily provide the same benefits to other regions of the country, like Florida. Assessing consequences that extend beyond near surface temperatures, such as rainfall and energy demand, reveals important tradeoffs that are often unaccounted for. Cool roofs have been found to be particularly effective for certain areas during summertime. However, during winter, these same urban adaptation strategies, when deployed in northerly locations, further cool the environment, and consequently require additional heating to maintain comfort levels. “The energy savings gained during the summer season, for some regions, is nearly entirely lost during the winter season,” Georgescu said. In Florida, and to a lesser extent southwestern states, there is a very different effect caused by cool roofs. “In Florida, our simulations indicate a significant reduction in precipitation," he said. "The deployment of cool roofs results in a 2 to 4 millimeter per day reduction in rainfall, a considerable amount (nearly 50 percent) that will have implications for water availability, reduced stream flow and negative consequences for ecosystems. For Florida, cool roofs may not be the optimal way to battle the urban heat island because of these unintended consequences.” Overall, the researchers suggest that judicious planning and design choices should be considered in trying to counteract rising temperatures caused by urban sprawl and greenhouse gases. They add that “urban-induced climate change depends on specific geographic factors that must be assessed when choosing optimal approaches, as opposed to one-size-fits-all solutions.” [ 21 ]

A series of Advanced Energy Design Guides were developed in cooperation with ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), AIA (The American Institute of Architects ), IESNA (Illuminating Engineering Society of North America), USGBC (United States Green Building Council) and US DOE (United States Department of Energy) in 2011. These guides were aimed at achieving 50% Energy Savings toward a Net Zero Energy Building and covered the building types of Small to Medium Office Buildings, Medium to Big Box Retail Buildings, Large Hospitals and K-12 School Buildings. In Climate Zones 4 and above the recommendation is to follow the ASHRAE 90.1 standard for roof reflectance, which currently does not require roofs to be reflective in these zones. In Climate Zones 4 and above, Cool Roofs are not a recommended Design Strategy. [ 22 ]

A series of Advanced Energy Retrofit Guides for “Practical Ways to Improve Energy Performance” were developed in cooperation with the US DOE (United States Department of Energy) and PNNL (Pacific Northwest National Laboratory) in 2011. These guides were aimed at improvements to existing Retail and Office buildings which could improve their energy efficiency. Cool roofs were not recommended for all locations. “This measure is likely more cost-effective in the hot and humid climate zone, which has a long cooling season, than in the very cold climate zone, for example. For buildings located in warm climates, this measure is worth consideration.” [ 23 ] [ 24 ]

The Copper Development Association has conducted several studies, beginning in 2002, which examined the elevated temperatures of wiring inside conduits at and above various color roof materials. The findings concluded that the temperatures above cool roofs were higher than those of a darker colored roof material. This illustrates the idea in which deflected solar radiation, when impeded by rooftop equipment, piping, or other materials will be subjected to the heat gain of the radiation. [ 25 ]

According to the US DOE ’s Guidelines for Selecting Cool Roofs: “Cool roofs must be considered in the context of your surroundings. It is relatively easy to specify a cool roof and predict energy savings, but some thinking ahead can prevent other headaches. Ask this question before installing a cool roof: Where will the reflected sunlight go? A bright, reflective roof could reflect light and heat into the higher windows of taller neighboring buildings. In sunny conditions, this could cause uncomfortable glare and unwanted heat for you or your neighbors. Excess heat caused by reflections increases air conditioning energy use, negating some of the energy saving benefits of the cool roof.” [ 26 ]

According to the US DOE ’s Guidelines for Selecting Cool Roofs on the subject of Cool Roof Maintenance: "As a cool roof becomes dirty from pollution, foot traffic, wind-deposited debris, ponded water, and mold or algae growth, its reflectance will decrease, leading to higher temperatures. Especially dirty roofs may perform substantially worse than product labels indicate. Dirt from foot traffic may be minimized by specifying designated walkways or by limiting access to the roof. Steep sloped roofs have less of a problem with dirt accumulation because rainwater can more easily wash away dirt and debris. Some cool roof surfaces are “self-cleaning” which means they shed dirt more easily and may better retain their reflectance. Cleaning a cool roof can restore solar reflectance close to its installed condition. Always check with your roof manufacturer for the proper cleaning procedure, as some methods may damage your roof. While it is generally not cost effective to clean a roof just for the energy savings, roof cleaning can be integrated as one component of your roof’s routine maintenance program. It is therefore best to estimate energy savings based on weathered solar reflectance values rather than clean roof values." [ 26 ]

Properties [ edit ]

When the sunlight strikes a dark rooftop, about 15% of it gets reflected back into the sky but most of its energy is absorbed into the roof system in the form of heat. Cool roofs reflect significantly more sunlight and absorb less heat than traditional dark-colored roofs [ 27 ]

There are two properties that are used to measure the effects of cool roofs:

  • Solar reflectance, also known as albedo. is the ability to reflect sunlight. It is expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation. and a value of 1 represents total reflectivity.
  • Thermal emittance is the ability to emit absorbed heat. It is also expressed either as a decimal fraction between 0 and 1, or a percentage.

Another method of evaluating coolness is the solar reflectance index (SRI), which incorporates both solar reflectance and emittance in a single value. SRI measures the roof’s ability to reject solar heat, defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. [ 28 ]

Roof Savings Calculator [ edit ]

The Roof Savings Calculator (RSC) is a tool developed by the U.S. Department of Energy’s Oak Ridge National Laboratory which estimates cooling and heating savings for low slope roof applications with white and black surfaces. [ 29 ]

This tool was the collaboration of both Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory in order to provide industry-consensus roof savings for both residential and commercial buildings. It reports the net annual energy savings (cooling energy savings minus heating penalties) and thus is only applicable to the buildings with a heating and/or cooling system.

Types of cool roofs [ edit ]

Cool roofs fall into one of three categories: roofs made from cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green roofs. [citation needed ]

Cool roofs

White thermoplastic membrane roofs, are inherently reflective, achieving some of the highest reflectance and emittance measurements of which roofing materials are capable. [citation needed ] A roof made of white thermoplastic, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the roof absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation.

Coated roofs

An existing (or new) roof can be made reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 500 reflective coatings can be found in the Cool Roofs Rating Council. [ 30 ]

Green roofs

Green roofs provide a thermal mass layer which helps reduce the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3-0.5). [ 31 ] Green roofs may not reflect as much as a cool roof but do have other benefits such as evapotranspiration which cools the plants and the immediate area around the plants, aiding in lowering rooftop temperatures, naturally.

Cool climates [ edit ]

In some climates where there are more heating days than cooling days, white reflective roofs may not be effective in terms of energy efficiency or savings because the savings on cooling energy use can be outweighed by heating penalties during winter. According to the U.S. Energy Information Administration, 2003 Commercial Buildings Energy Consumption Survey, heating accounts for 36% of commercial buildings’ annual energy consumption, while air conditioning only accounts for 8% in United States. [ 32 ] Energy calculators generally show a yearly net savings for dark-colored roof systems in cool climates.

Case studies [ edit ]

In a 2001 federal study, the Lawrence Berkeley National Laboratory (LBNL) measured and calculated the reduction in peak energy demand associated with a cool roof’s surface reflectance. [ 33 ] LBNL found that, compared to the original black rubber roofing membrane on the Texas retail building studied, a retrofitted vinyl membrane delivered an average decrease of 24 °C (75.2 °F) in surface temperature, an 11% decrease in aggregate air conditioning energy consumption, and a corresponding 14% drop in peak hour demand. The average daily summertime temperature of the black roof surface was 75 °C (167 °F), but once retrofitted with a white reflective surface, it measured 52 °C (126 °F). Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or $0.07/square foot.(This figure is for energy charges as well as peak demand charges).

Instruments measured weather conditions on the roof, temperatures inside the building and throughout the roof layers, and air conditioning and total building power consumption. Measurements were taken with the original black rubber roofing membrane and then after replacement with a white vinyl roof with the same insulation and HVAC systems in place.

Though a full year of actual data was collected, due to aberrations in the data, one month of data was excluded along with several other days which didn’t meet the parameters of the study. Only 36 continuous pre-retrofit days were used and only 28 non-continuous operating days were used for the post-retrofit period. [ 34 ]

Another case study, conducted in 2009 and published in 2011, was completed by Ashley-McGraw Architects and CDH Energy Corp for Onondaga County Dept. of Corrections, in Jamesville, New York, evaluated energy performance of a green/vegetative roof, a dark EPDM roof and a white reflective TPO [disambiguation needed ] roof. The measured results showed that the TPO and vegetative roof systems had much lower roof temperatures than the conventional EPDM surface. The reduction in solar absorption reduced solar gains in the summer but also increased heat losses during the heating season. Compared to the EPDM membrane, the TPO roof had 30% higher heating losses and the vegetative roof had 23% higher losses. [ 35 ]

Promotional programs [ edit ]

Across the U.S. Federal Government [ edit ]

In July 2010, the United States Department of Energy announced a series of initiatives to more broadly implement cool roof technologies on DOE facilities and buildings across the country. [ 36 ] As part of the new efforts, DOE will install a cool roof, whenever cost effective over the lifetime of the roof, during construction of a new roof or the replacement of an old one at a DOE facility.

In October 2013, the United States Department of Energy ranked Cool Roofs as a 53 out of 100 (0 to 100 weighted average) for a cost effective energy strategy. [ 37 ] "Climate issues can affect cool roof performance. Cool roofs are more beneficial in warmer climates and may cause energy consumption for heating applications to rise in colder climates. Cool roofs have a lower impact the more insulation is used. The Secretary of Energy directed all U.S. Department of Energy (DOE) offices to install cool roofs, when life-cycle cost effectiveness is demonstrated, when constructing new roofs, or when replacing old roofs at DOE facilities. Other Federal agencies were also encouraged to do the same." [ 37 ]

Energy Star [ edit ]

Energy Star is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to reduce greenhouse gas emissions and help businesses and consumers save money by making energy-efficient product choices.

For low slope roof applications, a roof product qualifying for the Energy Star label under its Roof Products Program must have an initial solar reflectivity of at least 0.65, and weathered reflectance of at least 0.50, in accordance with EPA testing procedures. [ 38 ] Warranties for reflective roof products must be equal in all material respects to warranties offered for comparable non-reflective roof products, either by a given company or relative to industry standards.

Unlike other Energy Star rated products, such as appliances, this rating system does not look at the entire roof assembly, but only the exterior surface. Consumers (i.e. building owners) may believe that the Energy Star label means their roof is energy efficient, however, the testing is not as stringent as their appliance standard and does not include the additional components of a roof (i.e. roof structure, fire rated barriers, insulation, adhesives, fasteners, etc.). [ 39 ] A disclaimer is posted on their website "Although there are inherent benefits in the use of reflective roofing, before selecting a roofing product based on expected energy savings consumers should explore the expected calculated results that can be found on the Department of Energy’s "Roof Savings Calculator" website at www.roofcalc.com. Please remember the Energy Savings that can be achieved with reflective roofing is highly dependent on facility design, insulation used, climatic conditions, building location, and building envelope efficiency." [ 39 ]

Certification requirements for different cool roof programs

The albedo of several types of roofs

Reflective surfaces are surfaces that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the surface) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy). [ 1 ] Reflective surfaces are a form of geoengineering .

The most well-known type of reflective surface is the cool roof. While it is true that cool roofs are mostly associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Note that today’s "cool roof" pigments allow metal roofing products to be EnergyStar rated in dark colors, even black.

Solar reflective cars or cool cars reflect more sunlight than dark cars, reducing the amount of heat that is transmitted into the car’s interior. Therefore, it helps decreasing the need for air conditioning, fuel consumption, and emissions of greenhouse gases and urban air pollutants. [ 2 ]

Cool color parking lots are parking lots made with a reflective layer of paint. The project is being undertaken by Jordan Woods of the Berkeley Lab. [ 3 ]

Contents

Benefits of cool roofs [ edit ]

Cool roofs, in hotter climates, can offer both immediate and long-term benefits including:

  • Savings of up to 15% the annual air-conditioning energy use of a single-story building [ 4 ]
  • Help in mitigating the urban heat island effect. [ 5 ]
  • Reflective surfaces (geoengineering) - Wikipedia, the free encyclopedia
  • Reduced air pollution and greenhouse gas emissions, as well as a significant offsetting of the warming impact of greenhouse gas emissions. [ 6 ]

Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters. [ 7 ] Therefore, the net energy saving of cool roofs varies depending on climate. However, a 2010 energy efficiency study [ 8 ] looking at this issue for air conditioned commercial buildings across the USA found that the summer cooling savings typically outweigh the winter heating penalty even in cold climates near the Canadian border giving savings in both electricity and emissions. Without a proper maintenance program to keep the material clean, the energy savings of cool roofs can diminish over time due to albedo degradation and soiling. [ 9 ]

Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations, seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes, or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation. [ 10 ]

If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 Gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 93-square-metre (1,000 sq ft) white roof will offset 10 tons of carbon dioxide over its 20-year lifetime. [ 11 ] In a real-world 2008 case study [ 12 ] of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled 1.6°C over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down.

When sunlight falls on a white roof much of it is reflected and passes back through the atmosphere into space. But when sunlight falls on a dark roof most of it is absorbed and converted into much longer wavelengths that we call "heat" which cannot pass back through the atmosphere because they are absorbed by the greenhouse gases. The atmosphere is transparent to sunlight but opaque to heat, which is why white roofs help cool the planet and dark roofs warm the planet. [ 13 ]

A 2012 study by researchers at Concordia University included variables similar to those used in the Stanford study (e.g. cloud responses) and estimated that worldwide deployment of cool roofs and pavements in cities would generate a global cooling effect equivalent to offsetting up to 150 Gigatonnes of carbon dioxide emissions — enough to take every car in the world off the road for 50 years. [ 14 ] [ 15 ]

Disadvantages [ edit ]

A 2011 study by researchers at Stanford University suggested that although reflective roofs decrease temperatures in buildings and mitigate the "urban heat island effect," they may actually increase global temperature. [ 16 ] [ 17 ] The study noted that it did not account for the reduction in greenhouse gas emissions that results from building energy conservation (annual cooling energy savings less annual heating energy penalty) associated with cool roofs (meaning that you will need to use more energy to heat the living space due to reduction in heat from sunlight in winter.) However that apply only to those area with low winter temperature but not the tropical area. Also homes in area receiving snow in winter are unlikely to get more heat due to darker roof since snow will cover it most of the winter. A response paper titled "Cool Roofs and Global Cooling," by researchers in the Heat Island Group at Lawrence Berkeley National Laboratory, raised additional concerns about the validity of these findings, citing the uncertainty acknowledged by the authors, statistically insignificant numerical results, and insufficient granularity in analysis of local contributions to global feedbacks. [ 18 ]

Also, 2012 research at University of California, San Diego ‘s Jacobs School of Engineering into the interaction between reflective pavements and buildings found that, unless the nearby buildings are fitted with reflective glass or other mitigation factors, solar radiation reflected off light-colored pavements can increase the temperature in nearby buildings, increasing air conditioning demands and energy usage. [ 19 ]

In 2014, a team of researchers, led by Matei Georgescu, an assistant professor in Arizona State University ‘s School of Geographical Sciences and Urban Planning and a senior sustainability scientist in the Global Institute of Sustainability explored the reletavive effectiveness of some of the most common adaptation technologies aimed at reducing warming from urban expansion. Results of the new study indicate that the performance of urban adaptation technologies can counteract this increase in temperature, but also varies seasonally and is geographically dependent. [ 20 ]

Specifically, what works in California’s Central Valley, such as cool roofs, does not necessarily provide the same benefits to other regions of the country, like Florida. Assessing consequences that extend beyond near surface temperatures, such as rainfall and energy demand, reveals important tradeoffs that are often unaccounted for. Cool roofs have been found to be particularly effective for certain areas during summertime. However, during winter, these same urban adaptation strategies, when deployed in northerly locations, further cool the environment, and consequently require additional heating to maintain comfort levels. “The energy savings gained during the summer season, for some regions, is nearly entirely lost during the winter season,” Georgescu said. In Florida, and to a lesser extent southwestern states, there is a very different effect caused by cool roofs. “In Florida, our simulations indicate a significant reduction in precipitation," he said. "The deployment of cool roofs results in a 2 to 4 millimeter per day reduction in rainfall, a considerable amount (nearly 50 percent) that will have implications for water availability, reduced stream flow and negative consequences for ecosystems. For Florida, cool roofs may not be the optimal way to battle the urban heat island because of these unintended consequences.” Overall, the researchers suggest that judicious planning and design choices should be considered in trying to counteract rising temperatures caused by urban sprawl and greenhouse gases. They add that “urban-induced climate change depends on specific geographic factors that must be assessed when choosing optimal approaches, as opposed to one-size-fits-all solutions.” [ 21 ]

A series of Advanced Energy Design Guides were developed in cooperation with ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), AIA (The American Institute of Architects ), IESNA (Illuminating Engineering Society of North America), USGBC (United States Green Building Council) and US DOE (United States Department of Energy) in 2011. These guides were aimed at achieving 50% Energy Savings toward a Net Zero Energy Building and covered the building types of Small to Medium Office Buildings, Medium to Big Box Retail Buildings, Large Hospitals and K-12 School Buildings. In Climate Zones 4 and above the recommendation is to follow the ASHRAE 90.1 standard for roof reflectance, which currently does not require roofs to be reflective in these zones. In Climate Zones 4 and above, Cool Roofs are not a recommended Design Strategy. [ 22 ]

A series of Advanced Energy Retrofit Guides for “Practical Ways to Improve Energy Performance” were developed in cooperation with the US DOE (United States Department of Energy) and PNNL (Pacific Northwest National Laboratory) in 2011. These guides were aimed at improvements to existing Retail and Office buildings which could improve their energy efficiency. Cool roofs were not recommended for all locations. “This measure is likely more cost-effective in the hot and humid climate zone, which has a long cooling season, than in the very cold climate zone, for example. For buildings located in warm climates, this measure is worth consideration.” [ 23 ] [ 24 ]

The Copper Development Association has conducted several studies, beginning in 2002, which examined the elevated temperatures of wiring inside conduits at and above various color roof materials. The findings concluded that the temperatures above cool roofs were higher than those of a darker colored roof material. This illustrates the idea in which deflected solar radiation, when impeded by rooftop equipment, piping, or other materials will be subjected to the heat gain of the radiation. [ 25 ]

According to the US DOE ’s Guidelines for Selecting Cool Roofs: “Cool roofs must be considered in the context of your surroundings. It is relatively easy to specify a cool roof and predict energy savings, but some thinking ahead can prevent other headaches. Ask this question before installing a cool roof: Where will the reflected sunlight go? A bright, reflective roof could reflect light and heat into the higher windows of taller neighboring buildings. In sunny conditions, this could cause uncomfortable glare and unwanted heat for you or your neighbors. Excess heat caused by reflections increases air conditioning energy use, negating some of the energy saving benefits of the cool roof.” [ 26 ]

According to the US DOE ’s Guidelines for Selecting Cool Roofs on the subject of Cool Roof Maintenance: "As a cool roof becomes dirty from pollution, foot traffic, wind-deposited debris, ponded water, and mold or algae growth, its reflectance will decrease, leading to higher temperatures. Especially dirty roofs may perform substantially worse than product labels indicate. Dirt from foot traffic may be minimized by specifying designated walkways or by limiting access to the roof. Steep sloped roofs have less of a problem with dirt accumulation because rainwater can more easily wash away dirt and debris. Some cool roof surfaces are “self-cleaning” which means they shed dirt more easily and may better retain their reflectance. Cleaning a cool roof can restore solar reflectance close to its installed condition. Always check with your roof manufacturer for the proper cleaning procedure, as some methods may damage your roof. While it is generally not cost effective to clean a roof just for the energy savings, roof cleaning can be integrated as one component of your roof’s routine maintenance program. It is therefore best to estimate energy savings based on weathered solar reflectance values rather than clean roof values." [ 26 ]

Properties [ edit ]

When the sunlight strikes a dark rooftop, about 15% of it gets reflected back into the sky but most of its energy is absorbed into the roof system in the form of heat. Cool roofs reflect significantly more sunlight and absorb less heat than traditional dark-colored roofs [ 27 ]

There are two properties that are used to measure the effects of cool roofs:

  • Solar reflectance, also known as albedo. is the ability to reflect sunlight. It is expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation. and a value of 1 represents total reflectivity.
  • Thermal emittance is the ability to emit absorbed heat. It is also expressed either as a decimal fraction between 0 and 1, or a percentage.

Another method of evaluating coolness is the solar reflectance index (SRI), which incorporates both solar reflectance and emittance in a single value. SRI measures the roof’s ability to reject solar heat, defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. [ 28 ]

Roof Savings Calculator [ edit ]

The Roof Savings Calculator (RSC) is a tool developed by the U.S. Department of Energy’s Oak Ridge National Laboratory which estimates cooling and heating savings for low slope roof applications with white and black surfaces. [ 29 ]

This tool was the collaboration of both Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory in order to provide industry-consensus roof savings for both residential and commercial buildings. It reports the net annual energy savings (cooling energy savings minus heating penalties) and thus is only applicable to the buildings with a heating and/or cooling system.

Types of cool roofs [ edit ]

Cool roofs fall into one of three categories: roofs made from cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green roofs. [citation needed ]

Cool roofs

White thermoplastic membrane roofs, are inherently reflective, achieving some of the highest reflectance and emittance measurements of which roofing materials are capable. [citation needed ] A roof made of white thermoplastic, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the roof absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation.

Coated roofs

An existing (or new) roof can be made reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 500 reflective coatings can be found in the Cool Roofs Rating Council. [ 30 ]

Green roofs

Green roofs provide a thermal mass layer which helps reduce the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3-0.5). [ 31 ] Green roofs may not reflect as much as a cool roof but do have other benefits such as evapotranspiration which cools the plants and the immediate area around the plants, aiding in lowering rooftop temperatures, naturally.

Cool climates [ edit ]

In some climates where there are more heating days than cooling days, white reflective roofs may not be effective in terms of energy efficiency or savings because the savings on cooling energy use can be outweighed by heating penalties during winter. According to the U.S. Energy Information Administration, 2003 Commercial Buildings Energy Consumption Survey, heating accounts for 36% of commercial buildings’ annual energy consumption, while air conditioning only accounts for 8% in United States. [ 32 ] Energy calculators generally show a yearly net savings for dark-colored roof systems in cool climates.

Case studies [ edit ]

In a 2001 federal study, the Lawrence Berkeley National Laboratory (LBNL) measured and calculated the reduction in peak energy demand associated with a cool roof’s surface reflectance. [ 33 ] LBNL found that, compared to the original black rubber roofing membrane on the Texas retail building studied, a retrofitted vinyl membrane delivered an average decrease of 24 °C (75.2 °F) in surface temperature, an 11% decrease in aggregate air conditioning energy consumption, and a corresponding 14% drop in peak hour demand. The average daily summertime temperature of the black roof surface was 75 °C (167 °F), but once retrofitted with a white reflective surface, it measured 52 °C (126 °F). Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or $0.07/square foot.(This figure is for energy charges as well as peak demand charges).

Instruments measured weather conditions on the roof, temperatures inside the building and throughout the roof layers, and air conditioning and total building power consumption. Measurements were taken with the original black rubber roofing membrane and then after replacement with a white vinyl roof with the same insulation and HVAC systems in place.

Though a full year of actual data was collected, due to aberrations in the data, one month of data was excluded along with several other days which didn’t meet the parameters of the study. Only 36 continuous pre-retrofit days were used and only 28 non-continuous operating days were used for the post-retrofit period. [ 34 ]

Another case study, conducted in 2009 and published in 2011, was completed by Ashley-McGraw Architects and CDH Energy Corp for Onondaga County Dept. of Corrections, in Jamesville, New York, evaluated energy performance of a green/vegetative roof, a dark EPDM roof and a white reflective TPO [disambiguation needed ] roof. The measured results showed that the TPO and vegetative roof systems had much lower roof temperatures than the conventional EPDM surface. The reduction in solar absorption reduced solar gains in the summer but also increased heat losses during the heating season. Compared to the EPDM membrane, the TPO roof had 30% higher heating losses and the vegetative roof had 23% higher losses. [ 35 ]

Promotional programs [ edit ]

Across the U.S. Federal Government [ edit ]

In July 2010, the United States Department of Energy announced a series of initiatives to more broadly implement cool roof technologies on DOE facilities and buildings across the country. [ 36 ] As part of the new efforts, DOE will install a cool roof, whenever cost effective over the lifetime of the roof, during construction of a new roof or the replacement of an old one at a DOE facility.

In October 2013, the United States Department of Energy ranked Cool Roofs as a 53 out of 100 (0 to 100 weighted average) for a cost effective energy strategy. [ 37 ] "Climate issues can affect cool roof performance. Cool roofs are more beneficial in warmer climates and may cause energy consumption for heating applications to rise in colder climates. Cool roofs have a lower impact the more insulation is used. The Secretary of Energy directed all U.S. Department of Energy (DOE) offices to install cool roofs, when life-cycle cost effectiveness is demonstrated, when constructing new roofs, or when replacing old roofs at DOE facilities. Other Federal agencies were also encouraged to do the same." [ 37 ]

Energy Star [ edit ]

Energy Star is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to reduce greenhouse gas emissions and help businesses and consumers save money by making energy-efficient product choices.

For low slope roof applications, a roof product qualifying for the Energy Star label under its Roof Products Program must have an initial solar reflectivity of at least 0.65, and weathered reflectance of at least 0.50, in accordance with EPA testing procedures. [ 38 ] Warranties for reflective roof products must be equal in all material respects to warranties offered for comparable non-reflective roof products, either by a given company or relative to industry standards.

Unlike other Energy Star rated products, such as appliances, this rating system does not look at the entire roof assembly, but only the exterior surface. Consumers (i.e. building owners) may believe that the Energy Star label means their roof is energy efficient, however, the testing is not as stringent as their appliance standard and does not include the additional components of a roof (i.e. roof structure, fire rated barriers, insulation, adhesives, fasteners, etc.). [ 39 ] A disclaimer is posted on their website "Although there are inherent benefits in the use of reflective roofing, before selecting a roofing product based on expected energy savings consumers should explore the expected calculated results that can be found on the Department of Energy’s "Roof Savings Calculator" website at www.roofcalc.com. Please remember the Energy Savings that can be achieved with reflective roofing is highly dependent on facility design, insulation used, climatic conditions, building location, and building envelope efficiency." [ 39 ]

Certification requirements for different cool roof programs


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