Cool roof

In the world of industrial and commercial buildings, a cool roof is a roofing system 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). Most cool roofs are white or other light colors.

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.[1] European fashion trends are now using darker-colored aluminium roofing, to pursue consumer fashions.

Cool roofs enhance roof durability and reduce both building cooling loads and the urban heat island effect.

Also known as albedo, solar reflectance 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 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 quantifies how hot a surface would get relative to standard black and standard white surfaces[citation needed]. It is 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.[2] The use of SRI as a combined measurement of reflectance has been disputed[citation needed], 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[citation needed]].

Cool roofs are an effective alternative to bulk attic insulation under roofs in humid tropical and subtropical climates. Bulk insulation can be entirely replaced by roofing systems that both reflect solar radiation and provide emission to the sky. This dual function is crucial, and relies on the performance of cool roof materials in both the visible spectrum (which needs to be reflected) and far infra-red which needs to be emitted.

Cool roof can also be used as a geoengineering 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.[3] 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.[4] For this reason alone it is still demonstrably worth pursuing as a geoengineering technique.

Contents

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 (122 °F), reaching temperatures of 70 to 90 °C (150-190 °F). This heat increase can contribute to:

  • Increased cooling energy use and higher utility bills;
  • Higher peak electricity demand (the maximum energy load, in megawatts, an electric utility experiences to supply customers instantaneously, generally experienced in summer late afternoons as businesses and residences turn up their air conditioners), raised electricity production costs, and a potentially overburdened power grid;
  • Reduced indoor comfort;
  • Increased air pollution due to the intensification of the "heat island effect"
  • Accelerated deterioration of roofing materials, increased roof maintenance costs, and high levels of roofing waste sent to landfills.

Any building with a dark colored roof, but particularly large buildings, will consume more energy for air conditioning than a “cooler” building – a strain on both operating costs and the electric power grid. Cool roofs offer both immediate and long-term savings in building energy costs. White reflective membranes, metal roofing with "cool roof" pigments, coated roofs and planted or green roofs can:

  • Reduce building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day.
  • Create 15–30%[citation needed] savings on summertime air conditioning expenditures.
  • Enhance the life expectancy of both the roof membrane and the building’s cooling equipment.
  • Improve thermal efficiency of the roof insulation; this is because as temperature increases, the thermal conductivity of the roof’s insulation also increases.
  • Reduce the demand for electric power by as much as 10 percent on hot days.
  • Reduce resulting air pollution and greenhouse gas emissions.[5]
  • 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 1,000-square-foot (93 m2) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime.[6]
  • Provide energy savings, even in northern climates on sunny (not necessarily “hot”) days.

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.

Energy calculators

Calculating cost savings resulting from the use of cool roofs can be done using several tools developed by federal agencies.[7]

  • U.S. Department of Energy (DOE) Cool Roof Calculator[8]

This tool developed by DOE's Oak Ridge National Laboratory estimates cooling and heating savings for low slope roof applications with non-black surfaces.

This tool developed by the U.S. EPA calculates the net savings accruing from installing an ENERGY STAR labeled roof product on an air conditioned building. In addition to cooling savings, the program considers any resulting differences in heating costs.

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 footprint. 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.[citation needed]

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.[10] 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.[11]

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:[12]

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. http://www.energystar.gov/ia/partners/prod_development/revisions/downloads/roofs/RCMA-CommentLetter-081606.pdf

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.[13] 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.[14]

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.

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 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

A green roof typically consist 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:

  • 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

A cool roof case study

In a 2001 federal study,[15] 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.

Programs promoting the use of cool roofs

Across the U.S. Federal Government

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. 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.[16]

Energy Star

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[17] 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.

Certification requirements for different cool roof programs
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[18] 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 Globes 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[19] 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 Design (LEED) rating system[20] 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,[21] 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:[22]

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

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.

See also

Survey On Cool Roofs Europe

The urban heat island effect

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 several degrees Celsius higher than the temperature in the surrounding countryside.

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.[23] 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.[24]

See also


References

  1. ^ H. Suehrcke, E. L. Peterson and N. Selby (2008). "Effect of roof solar reflectance on the building heat gain in a hot climate". Energy and Buildings 40: 2224–35. doi:10.1016/j.enbuild.2008.06.015. 
  2. ^ [1]
  3. ^ Lenton, T. M., Vaughan, N. E. (2009). "The radiative forcing potential of different climate geoengineering options". Atmos. Chem. Phys. Discuss. 9: 2559–2608. doi:10.5194/acpd-9-2559-2009. http://www.atmos-chem-phys-discuss.net/9/2559/2009/acpd-9-2559-2009.pdf. 
  4. ^ Amanda Kimble-Evans (November 10, 2009). "Why a White Roof Is a Cool Roof, for the Planet and Your Pocketbook". Mother Earth News. http://www.motherearthnews.com/Green-Homes/Cool-Roof-White-Roof.aspx. Retrieved 2010-05-10. 
  5. ^ http://www.energy.ca.gov/2008publications/CEC-999-2008-020/CEC-999-2008-020.PDF
  6. ^ Environmental and Energy Study Institute. "Cool Roofs for Cooler Summers". http://www.eesi.org/cool-roofs-cooler-summers-21-jul-2011. Retrieved August 5, 2011. 
  7. ^ Making the case for reflectivity, Environmental Design + Construction
  8. ^ Department of Energy Calculator
  9. ^ ENERGY STAR Calculator
  10. ^ Properties and performance of membranes - NRC-IRC
  11. ^ Maxwell C Baker (1980). Roofs: Design, Application and Maintenance. Polyscience Publications. p. 145. ISBN 0-921317-03-4. 
  12. ^ http://www.benchmark-inc.com/searcharticles/articles/Perspective%20Articles/Volume63a.pdf
  13. ^ http://www.miller-mccune.com/news/white-roof-isnt-always-right-roof-1217
  14. ^ http://www.ecobroker.com/misc/articleview.aspx?ArticleID=40
  15. ^ Konopacki and H. Akbari (June 2001). "Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin". Lawrence Berkeley National Laboratory, Environmental Energy Technologies Division. http://vinylroofs.org/casestudy/lbnl/index.html. 
  16. ^ http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=16175
  17. ^ Energy Star Product Choices
  18. ^ Cool Roof Rating Directory
  19. ^ Green Globes
  20. ^ Leadership in Energy and Environmental Design (LEED)
  21. ^ Comprehensive LEED Program List
  22. ^ [2]
  23. ^ Mitigating the Heat Island Effect
  24. ^ S. Konopacki and H. Akbari (November 2001). "Energy Impacts of Heat Island Reduction Strategies in the Greater Toronto Area, Canada". Lawrence Berkeley National Laboratory, Heat Island Group. http://www.epa.gov/hiri/resources/pdf/toronto_energysavings.pdf. 

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