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In the world of industrial and commercial buildings, 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 release a large percentage of absorbed, or non-reflected, solar energy) is a cool roof.

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. A newer 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. 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.

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 incident solar radiation and provide emission to the sky.

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 90 degrees F, reaching temperatures of 150-190 degrees F (66 to 88 degrees C). 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"; and
Accelerated deterioration of roofing materials, increased roof maintenance costs, and high levels of roofing waste sent to landfills.

A large commercial, industrial or multifamily residential building with a dark colored roof 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, coated roofs and planted or green roofs can:

reduced building heat-gain, as as a white reflective roof typically increases only 10-25 degrees F above ambient temperature during the day
create 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
reduce resulting air pollution and greenhouse gas emissions
provide energy savings, even in northern climates

Energy Calculators

Calculating cost savings resulting from the use of cool roofs can be done using several tools developed by federal agencies.^[1]

U.S. Department of Energy (DOE) Cool Roof Calculator^[2]

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

ENERGY STAR® Roofing Comparison Calculator^[3]

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 and lower a building’s carbon footprint. In climates where there are more heating days than cooling days, white reflective roofs are a worthwhile investment for many reasons.^[4] The cooling benefits of a highly reflective roof surface far outweigh the potential winter month heating benefits of a less reflective, or black, roof surface. Energy calculators generally show a yearly net savings. This is true because the sun is lower to the horizon in winter and not hitting the roof as directly or as intensely as it would in summer, it shines fewer hours and there are more cloudy days, and snow cover reflects the sun’s energy. Another reason: because cool roofs cut peak use during the summer when rates are the highest, they can help reduce the demand charge that a building pays all year on the basis of its greatest energy use.

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, 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

To make a roof reflective, present technology is best obtained with white paint, but U.S. energy-star awards may soon be available for "cool colors" in addition to white painted roofs. Cool roof coatings function across a broader spectrum of solar radiation than white paint. Highly emissive and reflective cool roof painting systems are available in Australia for a material cost, epoxy primer and highbuild coolroof, so this alternative solution should not be ignored in tropical northern Australia.

The recent boom in "green" construction products has led to several new cool roof coatings and systems^[5]. Two such coatings are a white paint based products called, "Hyperglass® Rubber Roof Coating" and "Hyperglass® Rubber Roof Finishing Top Coat", both products were created by noted, retired inventor Col.Ronald Savin and are marketed by Hyperseal, Inc.

The innovation of these cool roof coatings is the addition of a substantial amount of "hollow glass microspheres" to the formulation. The addition of glass to paint is not new and has been known to increase its insulating and reflective properties as well as its scrubability. It has previously been marketed separately as an additive to normal white paint Potters SPHERIGLASS®. Hyperglass® is the first coating on the market to claim the glass will stay in solution, rather than traditional methods of adding glass which require immediate application and constant stirring to keep the glass from separating out of the paint Insuladd. This formulation is patented and has been nominated for a "2008 Precision Craft Green Log Home and Lifestyle" award. Green Log Awards

Green Roofs

Planted roofs 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.

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

A Cool Roof Case Study

In a 2001 federal study^[6], 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 43 degrees 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 168 degrees F, but once retrofitted with a white reflective surface, it measured 125 degrees 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

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^[7] 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 by for comparable non-reflective roof products, either by a given company or relative to industry standards.

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^[8] 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^[9] 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) certification ^[10] 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, ^[11] 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:^[12]

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

The Urban Heat Island Effect

For millions of Americans 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 ambient air temperature as much as 8-10 degrees 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, offers more potential cooling than any individual strategy^[13]. 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.^[14]

References

External links

^ [1]
^ Department of Energy Calculator, http://www.ornl.gov/sci/roofs+walls/facts/CoolCalcEnergy.htm
^ ENERGY STAR Calculator
^ S. Konopacki and H. Akbari, “Energy Impacts of Heat Island Reduction Strategies in the Greater Toronto Area, Canada,” Lawrence Berkeley National Laboratory, Heat Island Group, November 2001.
^ Cool Roof Coatings Market Heating Up by Cynthia Challener, JCT Coatings Tech
^ http://www.vinylroofs.org/Links/sustainability/LBNL_study2.pdfS. Konopacki and H. Akbari, “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, June 2001.
^ http://www.energystar.gov/ia/products/prod_lists/roofs_prod_list.pdf ENERGY STAR product choices
^ www.coolroofs.org Cool Roof Rating Directory
^ http://www.thegbi.org/commercial/about-green-globes Green Globes
^ http://www.usgbc.org/DisplayPage.aspx?CMSPageID=222
^ https://www.usgbc.org/ShowFile.aspx?DocumentID=691
^ http://www.vinylroofs.org/cool_greenprograms.html
^ http://www.vinylroofs.org/Links/library/apr_nyserda_report.pdf
^ http://www.epa.gov/hiri/resources/pdf/toronto_energysavings.pdf S. Konopacki and H. Akbari, “Energy Impacts of Heat Island Reduction Strategies in the Greater Toronto Area, Canada,” Lawrence Berkeley National Laboratory, Heat Island Group, November 2001.

[1] [1]

[2] Department of Energy Calculator, http://www.ornl.gov/sci/roofs+walls/facts/CoolCalcEnergy.htm

[3] ENERGY STAR Calculator

[4] S. Konopacki and H. Akbari, “Energy Impacts of Heat Island Reduction Strategies in the Greater Toronto Area, Canada,” Lawrence Berkeley National Laboratory, Heat Island Group, November 2001.

[JCT-5] Cool Roof Coatings Market Heating Up by Cynthia Challener, JCT Coatings Tech

[6] ttp://www.vinylroofs.org/Links/sustainability/LBNL_study2.pdfS. Konopacki and H. Akbari, “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, June 2001.

[7] ttp://www.energystar.gov/ia/products/prod_lists/roofs_prod_list.pdf ENERGY STAR product choices

[8] www.coolroofs.org Cool Roof Rating Directory

[9] ttp://www.thegbi.org/commercial/about-green-globes Green Globes

[10] ttp://www.usgbc.org/DisplayPage.aspx?CMSPageID=222

[11] ttps://www.usgbc.org/ShowFile.aspx?DocumentID=691

[12] ttp://www.vinylroofs.org/cool_greenprograms.html

[13] ttp://www.vinylroofs.org/Links/library/apr_nyserda_report.pdf

[14] ttp://www.epa.gov/hiri/resources/pdf/toronto_energysavings.pdf S. Konopacki and H. Akbari, “Energy Impacts of Heat Island Reduction Strategies in the Greater Toronto Area, Canada,” Lawrence Berkeley National Laboratory, Heat Island Group, November 2001.

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