Cool roof
Encyclopedia
Cool roofs are the roofs that can deliver high solar reflectance (the ability to reflect the visible, infrared
and ultraviolet
wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance
(the ability to radiate absorbed, or non-reflected solar energy). The benefits associated with cool roofs include reduced cooling energy load, reduced air pollution and greenhouse gas emission, and improved human health and comfort. Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters. Therefore, the net energy saving of cool roofs varies depending on climate. Cool roofs may extend the roof service life and help mitigating the urban heat island
effect.
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. They aren't as reflective as whites or light colors, but can still save energy over other paints.
Cool roofs, on the other hand, offer both immediate and long-term benefits including:
If all urban, flat roofs worldwide were whitened, the reduction in carbon emissions would be 24 Gigatonnes, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 1000 square feet (92.9 m²) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime.
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. The use of SRI as a combined measurement of reflectance has been disputed , since it has been shown that two different products with identical SRI numbers can yield significantly different energy savings results depending on what geographic region they are applied in, and the climatic conditions present in this region ].
This tool developed by U.S. Department of Energy's Oak Ridge National Laboratory
estimates cooling and heating savings for low slope roof applications with non-black surfaces.
This tool was developed by 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.
Inherently cool roofs
White vinyl roofs, which are inherently reflective, achieve some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of thermoplastic white vinyl, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the building absorbs. An asphalt
roof only reflects between 6 and 26% of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air conditioning – a strain on both operating costs and the electric power grid.
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 1000 reflective roof products can be found in the CRRC (Cool Roofs Rating Council) website.
Green roofs
Green roofs provide a thermal mass layer which helps reducing the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3-0.5). Because of the lower solar reflectance, green roofs reflect less sunlight and absorb more solar heat than white roofs. The absorbed heat in the green roofs is trapped by the greenhouse effect and then cooled by evapotranspiration.
A green roof typically consists of an insulation layer; a waterproof membrane; a drainage layer, usually made of lightweight gravel, clay, or plastic; a geotextile
or filter mat that allows water to soak through but prevents erosion of fine soil particles; a growing medium; plants; and, sometimes, a wind blanket. Green roofs are classified as either intensive or extensive; some green roof designs incorporate both intensive and extensive elements.
Intensive green roofs require at least one foot of soil and appear as a traditional garden with trees, shrubs and other attractive landscapes. They are multi-layer constructions with elaborate irrigation and drainage systems. These roofs are often designed for recreational purposes and accommodate foot traffic. Intensive green roofs add considerable load to a structure and require intensive maintenance, so they are more common with large businesses or government buildings rather than free-standing homes.
Extensive roofs usually require less maintenance. The soil is shallower (less than 6 inches) and home to smaller, lighter plants such as mosses or wildflowers.
Both types of green roofs offer a variety of benefits including:
. In some climates where there are more heating days than cooling days, white reflective roofs are not typically a worthwhile investment in terms of energy efficiency or savings. The cooling benefits of a highly reflective roof surface do not outweigh the winter month heating benefits of a less reflective, or black, roof surface in cooler climates. Heating accounts for 29% of commercial buildings' yearly energy consumption, while air conditioning only accounts for 6% of that same yearly energy consumption. However, according to the Cool Roofs Rating Council and other sources, "The roof is an insignificant source for heat gain in winter. While cool roof owners may pay slightly more to heat their homes, this amount is usually insignificant compared to the cooling energy savings during the summer". Energy calculators generally show a yearly net savings for dark-colored roof systems in cool climates. The energy trade-off is therefore not clear cut. Oftentimes, reflective roofing materials get dirty, and their reflective benefits diminish, after only a few years. Without a proper maintenance program to keep the material clean, reflective roofing materials seldom provide the energy-saving benefits that could be fully experienced based on their initial SRI.
Additionally, higher R values for insulating materials can lessen the impact of roof surface color. Snow on roofs also provides insulation, but it also adds considerable weight to the roofing assembly, which may not have been accounted for in the initial design. For a medium density of snow the resistance per 25 mm is about 0.110 (m2-°C)/W, 300 mm of snow cover can provide an equivalent of 50 mm of good insulating material. Cool roofs contribute to the retention of snow on roofs in moderate snow fall areas. Dark-colored roofs heat up more quickly and therefore help melt rooftop snow. There can be a 26 °C difference in membrane temperature between areas having 300 mm of snow cover compared to areas having no snow.
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.
There are some studies that have shown that reflective roofs are not always best in cool climates. Benchmark Inc. did a study in five different cities and used the energy star calculator and the DOE calculator to find the annual savings. Because the DOE calculator includes differences in heating losses, there were significant differences between the savings in all of the cities. However, in Chicago, the annual savings became slightly negative in one of the models because of heating costs. The following graph shows the results:
Calculations performed using the DOE Energy Star Calculator show that high-reflectivity, medium-emissivity roof coatings, such as aluminum roof coatings can yield greater savings in colder regions.
Miller-McCune published a blog article by Robert Reale expressing an opinion that areas where heating is more of a concern than cooling would not benefit, and so
cool roofs are only appropriate in climate zones 1-3.
ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers') position on reflective roofs falls in line with Mr. Reale's article. ASHRAE now promotes the use of reflective roofs only in climate zones 1-3. In zones 4 and above, darker-colored roofing materials are more beneficial.
An article in ecobroker.com also does not recommend reflective roofs in cooler climates. This site is designed to aid real estate agents in finding their clients green homes.
Green roofs are another option to consider for flat roofs in cooler climates.
One issue that is rarely talked about in terms of cool/reflective roofing is "What happens to the heat/UV that is reflective from the roof surface?" Well, if it's coming from a lower building adjacent to taller buildings, the energy is likely transferred into the adjacent building. This negates the energy-saving benefits for the building with the reflective rooftop, however it increases the heat gain, and subsequent energy costs, for the adjacent building. Furthermore, studies show that heat gain through windows has more than 10x the impact on energy costs and consumption that heat gained through the roof assembly. So, the reduction in energy costs (and subsequent carbon emissions) from the building with a reflective roof is multiplied by the adjacent building that picked it up via the windows.
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.
announced a series of initiatives to more broadly implement cool roof technologies on DOE facilities and buildings across the country. 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.
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. 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.
CRRC’s rating program allows manufacturers and sellers to appropriately label their roofing products according to specific CRRC measured properties. The program does not, however, specify minimum requirements for solar reflectance or thermal emittance.
system is used in Canada and the United States. In the U.S., Green Globes is owned and operated by the Green Building Initiative (GBI). In Canada, the version for existing buildings is owned and operated by BOMA Canada under the brand name 'Go Green' (Visez vert).
Green Globes uses performance benchmark criteria to evaluate a building’s likely energy consumption, comparing the building design against data generated by the EPA’s Target Finder, which reflects real building performance. Buildings may earn a rating of between one and four globes. This is an online system; a building’s information is verified by a Green Globes-approved and trained licensed engineer or architect. To qualify for a rating, roofing materials must have a solar reflectance of at least .65 and thermal emittance of at least .90. As many as 10 points may be awarded for 1-100 percent roof coverage with either vegetation or highly reflective materials or both.
(LEED) rating system is a voluntary, continuously evolving national standard for developing high performance sustainable buildings. LEED provides standards for choosing products in designing buildings, but does not certify products.
In the area of roofing, to receive LEED Sustainable Sites Credit 7.2, at least 75% of the surface of a roof must use materials having a Solar Reflective Index (SRI) of at least 78. This criterion may also be met by installing a vegetated roof for at least 50% of the roof area, or installing a high albedo and vegetated roof that, in combination, meets this formula: (Area of SRI Roof/0.75)+(Area of vegetated roof/0.5) = Total Roof Area.
As of August 2008, various LEED initiatives including legislation, executive orders, resolutions, ordinances, policies, and incentives are in place in 98 cities, 29 counties, 25 towns, 31 states, 12 federal agencies or departments, 15 public school jurisdictions and 38 institutions of higher education across the United States.
Examples of LEED-certified buildings with white reflective roofs are:
The aim of the proposed action is to create and implement an Action Plan for the cool roofs in EU. The specific objectives are: to support policy development by transferring experience and improving understanding of the actual and potential contributions by cool roofs to heating and cooling consumption in the EU; to remove market barriers and simplify the procedures for cool roofs integration in construction and building’s stock; to change the behaviour of decision-makers and stakeholders so to improve acceptability of the cool roofs; to disseminate and promote the development of innovative legislation, codes, permits and standards, including application procedures, construction and planning permits concerning cool roofs. The work will be developed in four axes, technical, market, policy and end-users.
In tropical Australia, zinc-galvanized
(silvery) sheeting (usually corrugated) do not reflect heat as well as the truly "cool" color of white, especially as metallic surfaces fail to emit infrared back to the sky. European fashion trends are now using darker-colored aluminium roofing, to pursue consumer fashions.
Green building
programs advocate the use of cool roofing to mitigate the urban heat island effect and the resulting poorer air quality (in the form of smog) the effect causes. By reflecting sunlight, light-colored roofs minimize the temperature rise and reduce smog formation. In some densely populated areas, a quarter of the land cover may be roof surface alone.
To best combat the urban heat island effect, a combined strategy that maximizes the amount of vegetation by planting trees along streets and in open spaces, as well as by building green roofs and painting buildings with solar reflective coatings, offers more potential cooling than any individual strategy. Abating the urban heat island effect even has worthwhile effects in cooler climates. An LBNL study showed that, if strategies to mitigate this effect, including cool roofs, were widely adopted, the Greater Toronto metropolitan area could save more than $11 million annually on energy costs.
technique to tackle global warming
based on the principle of solar radiation management
, provided that the materials used not only reflect
solar energy, but also emit infra-red radiation to cool the planet. This technique can give between 0.01-0.19 W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated. This is generally small when compared to the 3.7 W/m2 of positive forcing from a doubling of CO2. However, in many cases it can be achieved at little or no cost by simply selecting different materials. Further, it can reduce the need for air conditioning
, which causes CO2 emissions which studies have shown worsen global warming. For this reason alone it is still demonstrably worth pursuing as a geoengineering technique.
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
and ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance
Thermal emittance
Thermal emittance is radiant emittance of heat. This should not be confused with thermal emissivity, which is a spectrum-dependent ability of a material to release absorbed heat, although the terms are related...
(the ability to radiate absorbed, or non-reflected solar energy). The benefits associated with cool roofs include reduced cooling energy load, reduced air pollution and greenhouse gas emission, and improved human health and comfort. Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters. Therefore, the net energy saving of cool roofs varies depending on climate. Cool roofs may extend the roof service life and help mitigating the urban heat island
Urban heat island
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...
effect.
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. They aren't as reflective as whites or light colors, but can still save energy over other paints.
Benefits of cool roofs
Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (90 °F), reaching temperatures of 70 to 90 °C (150-190 °F). This heat increase can cause negative effects on cooling energy use and environments.Cool roofs, on the other hand, offer both immediate and long-term benefits including:
- Reduced building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day;
- Savings of up to 15% the annual air-conditioning energy use of a single-story building;
- Extended service life of roofs;
- Improved energy efficiency of roofs, especially when there isn't adequate insulation provided in the roof envelope;
- Improved thermal comfort in buildings that do not have air conditioning; and
- Reduced air pollution and greenhouse gasGreenhouse gasA greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
emissions.
If all urban, flat roofs worldwide were whitened, the reduction in carbon emissions would be 24 Gigatonnes, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 1000 square feet (92.9 m²) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime.
Cool roof properties
When the sunlight strikes a rooftop, most of it gets reflected back into the sky but some of its energy is absorbed into the roof system in the form of heat. Cool roofs reflect more sunlight and absorb less heat than traditional dark-colored roofs. There are two properties that are used to measure the effects of cool roofs.- Solar reflectance, also known as albedoAlbedoAlbedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
, 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. The use of SRI as a combined measurement of reflectance has been disputed , since it has been shown that two different products with identical SRI numbers can yield significantly different energy savings results depending on what geographic region they are applied in, and the climatic conditions present in this region ].
Cool roof savings calculators
Calculating cost savings resulting from the use of cool roofs can be done using several tools developed by federal agencies.This tool developed by U.S. Department of Energy's Oak Ridge National Laboratory
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
estimates cooling and heating savings for low slope roof applications with non-black surfaces.
This tool was developed by Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
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
Cool roofs for commercial and industrial buildings fall into one of three categories: roofs made from inherently cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green planted roofs.Inherently cool roofs
White vinyl roofs, which are inherently reflective, achieve some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of thermoplastic white vinyl, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the building absorbs. An asphalt
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
roof only reflects between 6 and 26% of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air conditioning – a strain on both operating costs and the electric power grid.
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 1000 reflective roof products can be found in the CRRC (Cool Roofs Rating Council) website.
Green roofs
Green roofs provide a thermal mass layer which helps reducing the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3-0.5). Because of the lower solar reflectance, green roofs reflect less sunlight and absorb more solar heat than white roofs. The absorbed heat in the green roofs is trapped by the greenhouse effect and then cooled by evapotranspiration.
A green roof typically consists of an insulation layer; a waterproof membrane; a drainage layer, usually made of lightweight gravel, clay, or plastic; a geotextile
Geotextile
Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain...
or filter mat that allows water to soak through but prevents erosion of fine soil particles; a growing medium; plants; and, sometimes, a wind blanket. Green roofs are classified as either intensive or extensive; some green roof designs incorporate both intensive and extensive elements.
Intensive green roofs require at least one foot of soil and appear as a traditional garden with trees, shrubs and other attractive landscapes. They are multi-layer constructions with elaborate irrigation and drainage systems. These roofs are often designed for recreational purposes and accommodate foot traffic. Intensive green roofs add considerable load to a structure and require intensive maintenance, so they are more common with large businesses or government buildings rather than free-standing homes.
Extensive roofs usually require less maintenance. The soil is shallower (less than 6 inches) and home to smaller, lighter plants such as mosses or wildflowers.
Both types of green roofs offer a variety of benefits including:
- Improved air quality as the plants absorb and convert carbon dioxide to oxygen
- Long lifespan - some green roofs in Europe have lasted more than 40 years
- Excellent insulation
- Cooled surrounding environment
- Potentially increases the area of habitat for wildlife such as birds and insects
Cool roofs in cool climates
No matter where cool roofs are installed, they cut down on the urban heat island effect, however they do not always lower a building’s carbon footprintCarbon footprint
A carbon footprint has historically been defined as "the total set of greenhouse gas emissions caused by an organization, event, product or person.". However, calculating a carbon footprint which conforms to this definition is often impracticable due to the large amount of data required, which is...
. In some climates where there are more heating days than cooling days, white reflective roofs are not typically a worthwhile investment in terms of energy efficiency or savings. The cooling benefits of a highly reflective roof surface do not outweigh the winter month heating benefits of a less reflective, or black, roof surface in cooler climates. Heating accounts for 29% of commercial buildings' yearly energy consumption, while air conditioning only accounts for 6% of that same yearly energy consumption. However, according to the Cool Roofs Rating Council and other sources, "The roof is an insignificant source for heat gain in winter. While cool roof owners may pay slightly more to heat their homes, this amount is usually insignificant compared to the cooling energy savings during the summer". Energy calculators generally show a yearly net savings for dark-colored roof systems in cool climates. The energy trade-off is therefore not clear cut. Oftentimes, reflective roofing materials get dirty, and their reflective benefits diminish, after only a few years. Without a proper maintenance program to keep the material clean, reflective roofing materials seldom provide the energy-saving benefits that could be fully experienced based on their initial SRI.
Additionally, higher R values for insulating materials can lessen the impact of roof surface color. Snow on roofs also provides insulation, but it also adds considerable weight to the roofing assembly, which may not have been accounted for in the initial design. For a medium density of snow the resistance per 25 mm is about 0.110 (m2-°C)/W, 300 mm of snow cover can provide an equivalent of 50 mm of good insulating material. Cool roofs contribute to the retention of snow on roofs in moderate snow fall areas. Dark-colored roofs heat up more quickly and therefore help melt rooftop snow. There can be a 26 °C difference in membrane temperature between areas having 300 mm of snow cover compared to areas having no snow.
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.
There are some studies that have shown that reflective roofs are not always best in cool climates. Benchmark Inc. did a study in five different cities and used the energy star calculator and the DOE calculator to find the annual savings. Because the DOE calculator includes differences in heating losses, there were significant differences between the savings in all of the cities. However, in Chicago, the annual savings became slightly negative in one of the models because of heating costs. The following graph shows the results:
Calculations performed using the DOE Energy Star Calculator show that high-reflectivity, medium-emissivity roof coatings, such as aluminum roof coatings can yield greater savings in colder regions.
Miller-McCune published a blog article by Robert Reale expressing an opinion that areas where heating is more of a concern than cooling would not benefit, and so
cool roofs are only appropriate in climate zones 1-3.
ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers') position on reflective roofs falls in line with Mr. Reale's article. ASHRAE now promotes the use of reflective roofs only in climate zones 1-3. In zones 4 and above, darker-colored roofing materials are more beneficial.
An article in ecobroker.com also does not recommend reflective roofs in cooler climates. This site is designed to aid real estate agents in finding their clients green homes.
Green roofs are another option to consider for flat roofs in cooler climates.
One issue that is rarely talked about in terms of cool/reflective roofing is "What happens to the heat/UV that is reflective from the roof surface?" Well, if it's coming from a lower building adjacent to taller buildings, the energy is likely transferred into the adjacent building. This negates the energy-saving benefits for the building with the reflective rooftop, however it increases the heat gain, and subsequent energy costs, for the adjacent building. Furthermore, studies show that heat gain through windows has more than 10x the impact on energy costs and consumption that heat gained through the roof assembly. So, the reduction in energy costs (and subsequent carbon emissions) from the building with a reflective roof is multiplied by the adjacent building that picked it up via the windows.
A cool roof case study
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 reflectivity. 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 (43 °F) in surface temperature, an 11 percent decrease in aggregate air conditioning energy consumption, and a corresponding 14 percent drop in peak hour demand. The average daily summertime temperature of the black roof surface was 75 °C (168 °F), but once retrofitted with a white reflective surface, it measured 52 °C (125 °F). Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or $0.07/sq. ft.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.
Across the U.S. Federal Government
In July 2010, the United States Department of EnergyUnited States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...
announced a series of initiatives to more broadly implement cool roof technologies on DOE facilities and buildings across the country. 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.
Energy Star
Energy Star is a joint program of the U.S. Environmental Protection AgencyUnited States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
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. 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.
| Slope | Solar Reflectance | Emittance | Solar Reflectance Index |
|---|---|---|---|
| ENERGY STAR | |||
| Low, initial | 0.65 | ||
| Low, aged | 0.50 | ||
| Steep, initial | 0.25 | ||
| Steep, aged | 0.15 | ||
| Green Globes | |||
| 0.65 | 0.90 | ||
| USGBC LEED | |||
| Low Slope | 78 | ||
| Steep Slope | 29 | ||
Cool Roof Rating Council (CRRC)
CRRC has created a rating system for measuring and reporting the solar reflectance and thermal emittance of roofing products. This system has been put into an online directory of more than 850 roofing products and is available for energy service providers, building code bodies, architects and specifiers, property owners and community planners. CRRC conducts random testing each year to ensure the credibility of its rating directory.CRRC’s rating program allows manufacturers and sellers to appropriately label their roofing products according to specific CRRC measured properties. The program does not, however, specify minimum requirements for solar reflectance or thermal emittance.
Green Globes
The Green GlobesGreen Globes
Green Globes is an environmental assessment, education and rating system that is promoted in the United States by the Green Building Initiative, a Portland, Oregon-based non-profit....
system is used in Canada and the United States. In the U.S., Green Globes is owned and operated by the Green Building Initiative (GBI). In Canada, the version for existing buildings is owned and operated by BOMA Canada under the brand name 'Go Green' (Visez vert).
Green Globes uses performance benchmark criteria to evaluate a building’s likely energy consumption, comparing the building design against data generated by the EPA’s Target Finder, which reflects real building performance. Buildings may earn a rating of between one and four globes. This is an online system; a building’s information is verified by a Green Globes-approved and trained licensed engineer or architect. To qualify for a rating, roofing materials must have a solar reflectance of at least .65 and thermal emittance of at least .90. As many as 10 points may be awarded for 1-100 percent roof coverage with either vegetation or highly reflective materials or both.
LEED
The U.S. Green Building Council’s Leadership in Energy and Environmental DesignLeadership in Energy and Environmental Design
Leadership in Energy and Environmental Design consists of a suite of rating systems for the design, construction and operation of high performance green buildings, homes and neighborhoods....
(LEED) rating system is a voluntary, continuously evolving national standard for developing high performance sustainable buildings. LEED provides standards for choosing products in designing buildings, but does not certify products.
In the area of roofing, to receive LEED Sustainable Sites Credit 7.2, at least 75% of the surface of a roof must use materials having a Solar Reflective Index (SRI) of at least 78. This criterion may also be met by installing a vegetated roof for at least 50% of the roof area, or installing a high albedo and vegetated roof that, in combination, meets this formula: (Area of SRI Roof/0.75)+(Area of vegetated roof/0.5) = Total Roof Area.
As of August 2008, various LEED initiatives including legislation, executive orders, resolutions, ordinances, policies, and incentives are in place in 98 cities, 29 counties, 25 towns, 31 states, 12 federal agencies or departments, 15 public school jurisdictions and 38 institutions of higher education across the United States.
Examples of LEED-certified buildings with white reflective roofs are:
| Building Name | Owner | Location | LEED Level |
|---|---|---|---|
| Donald Bren School of Environmental Science & Management | University of California, Santa Barbara | Santa Barbara, California | Platinum |
| Frito-Lay Jim Rich Service Center | Frito-Lay, Inc. | Rochester, New York | Gold |
| Edifice Multifunction | Travaux Public et Services Gouvernementaux Canada | Montreal, Quebec | Gold |
| Seattle Central Library | City of Seattle | Seattle, Wash. | Silver |
| National Geography Society Headquarters Complex | National Geographic Society | Washington, D.C. | Silver |
| Utah Olympic Oval | Salt Lake City Olympic Winter Games 2002 Organizing Committee | Salt Lake City, Utah | Certified |
| Premier Automotive Group North American Headquarters | Ford Motor Company | Irvine, California | Certified |
Cool Roofs Europe and other countries
This project is co-financed by the European Union in the framework of the Intelligent Energy Europe Programme.The aim of the proposed action is to create and implement an Action Plan for the cool roofs in EU. The specific objectives are: to support policy development by transferring experience and improving understanding of the actual and potential contributions by cool roofs to heating and cooling consumption in the EU; to remove market barriers and simplify the procedures for cool roofs integration in construction and building’s stock; to change the behaviour of decision-makers and stakeholders so to improve acceptability of the cool roofs; to disseminate and promote the development of innovative legislation, codes, permits and standards, including application procedures, construction and planning permits concerning cool roofs. The work will be developed in four axes, technical, market, policy and end-users.
In tropical Australia, zinc-galvanized
Hot-dip galvanizing
Hot-dip galvanizing is a form of galvanization. It is the process of coating iron, steel, or aluminum with a thin zinc layer, by passing the metal through a molten bath of zinc at a temperature of around 860 °F...
(silvery) sheeting (usually corrugated) do not reflect heat as well as the truly "cool" color of white, especially as metallic surfaces fail to emit infrared back to the sky. European fashion trends are now using darker-colored aluminium roofing, to pursue consumer fashions.
Urban heat island effect and cool roofs
For hundreds of millions to perhaps billions of people living in and near cities, urban heat islands are a growing concern. An urban heat island occurs where the combination of heat-absorbing infrastructure such as dark asphalt parking lots and road pavement and expanses of black rooftops, coupled with sparse vegetation, raises air temperature by 1 to 3°C higher than the temperature in the surrounding countryside.Green building
Green building
Green building refers to a structure and using process that is environmentally responsible and resource-efficient throughout a building's life-cycle: from siting to design, construction, operation, maintenance, renovation, and demolition...
programs advocate the use of cool roofing to mitigate the urban heat island effect and the resulting poorer air quality (in the form of smog) the effect causes. By reflecting sunlight, light-colored roofs minimize the temperature rise and reduce smog formation. In some densely populated areas, a quarter of the land cover may be roof surface alone.
To best combat the urban heat island effect, a combined strategy that maximizes the amount of vegetation by planting trees along streets and in open spaces, as well as by building green roofs and painting buildings with solar reflective coatings, offers more potential cooling than any individual strategy. Abating the urban heat island effect even has worthwhile effects in cooler climates. An LBNL study showed that, if strategies to mitigate this effect, including cool roofs, were widely adopted, the Greater Toronto metropolitan area could save more than $11 million annually on energy costs.
Cool roofs and geoengineering
Cool roof can also be used as a geoengineeringGeoengineering
The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...
technique to tackle global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
based on the principle of solar radiation management
Solar radiation management
Solar radiation management projects are a type of geoengineering which seek to reflect sunlight and thus reduce global warming. Examples include the creation of stratospheric sulfur aerosols. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems...
, provided that the materials used not only reflect
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
solar energy, but also emit infra-red radiation to cool the planet. This technique can give between 0.01-0.19 W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated. This is generally small when compared to the 3.7 W/m2 of positive forcing from a doubling of CO2. However, in many cases it can be achieved at little or no cost by simply selecting different materials. Further, it can reduce the need for air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
, which causes CO2 emissions which studies have shown worsen global warming. For this reason alone it is still demonstrably worth pursuing as a geoengineering technique.
See also
- Green roofGreen roofA green roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems...
- List of low-energy building techniques
External links
- Cool Roof Rating Council
- Lawrence Berkeley National Laboratory, Heat Island Group
- U.S. Department of Energy's Cool Roofs Project
- U.S. Environment Protection Agency's Heat Island Program
- Cool Roofs Europe
- Comprehensive Cool Roof Guide from the Vinyl Roofing Division of the Chemical Fabrics and Film Association
- Green Log Awards
- SimRoof roof thermal simulator