Cool Roofs
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Introduction
Approximately $40 billion is spent annually in the United States to cool buildings, one-sixth of all energy consumed annually. Black and dark-colored roofing materials can dramatically increase a building's cooling load. Energy-efficient roofing systems, also called "cool roofs," can reduce roof temperature by as much as 100°F during the summer, and thereby reduce the building's energy requirements for air conditioning. Cool roofs reflect the sun's radiant energy before it penetrates into the interior of the building. In fact, reflective materials also help keep the building's neighborhood cool.
Technology Options
Some terms to be aware of when discussing roofing materials and their energy efficiency are: solar reflectance, emittance and the Solar Reflectance Index (SRI).
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There are many types of roofing materials with a varying range of reflectance and emittance levels. Asphalt shingles, the most common type of roofing material, are the least efficient at reflecting the sun's heat energy. The asphalt is composed of asphalt-saturated mats made from organic felts or fiberglass. Roofing granules, one-millimeter sized stones coated with an inorganic silicate material, protect the roof from the sun's ultraviolet light. The coating contains microscopic pigment particles, similar to those used in paint, to provide color.
Asphalt's low solar reflectance can be attributed to several factors. First, there is a limited amount of pigment in the granule coating. Second, the roughness of the shingle contributes to multiple scattering of light and thus to increased absorption. Third, the black asphalt substrate is not 100% covered by the granules, and reflects only about 5% of the light that strikes it.
Reflective Roofing Materials
There are four broad categories of roofing materials that can be used to upgrade a roof's reflectivity to 'ENERGY STAR', levels of efficiency: metal, tile, roofing membranes and reflective coatings.
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The Lawrence Berkeley National Laboratory Pigment Database characterizes the solar spectral radiative properties of conventional and cool pigmented coatings. Click here to see the database.
Efficiency Benefits
Learn from experts how cool roof technology works and its impacts on energy savings in our buildings. Click here to see California Energy Commission Cool Roof Training Videos
Calculators
DOE Cool Roof Calculator
Roof Savings Calculator (ORNL, LBNL, EPA, CEC, DOE)
The Cool Roof Rating Council (CRRC)
The Cool Roof Rating Council (CRRC) is an independent, non-biased organization that has established a rating system for displaying accurate radiative property data on the outermost layer of roof surfaces. Through the CRRC's Product Rating Program, roofing manufacturers and sellers can label various roof surface products with radiative property values measured by accredited, third-party testing laboratories. Interested parties can rely on the rating information provided in the CRRC Product Rating Directory for accurate solar reflectance and thermal emittance values in order to find a product that suits their needs. Many products rated with the CRRC may improve the energy efficiency of buildings while positively impacting our environment. The CRRC does not create its own definition for cool, and as such, does not "certify" products. However, the CRRC's Product Rating Program may be referenced by any code, standard, or program that sets its own requirements for cool roofs.
The California Energy Commmission's (CEC) Building Energy Code Title 24 references the CRRC as the sole supervisory entity from which it will accept ratings for code compliance. Other codes, standards, and programs like the City of Chicago, City of Houston, Green Globes, and LEED have a cool roof component and look to the CRRC for accurate ratings.
ENERGY STAR
Energy Star's Reflective Roof Program can help save you energy and money. In order to qualify for the ENERGY STAR label, roofing products must meet the following specifications:
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Cool Roofs save money and energy by reducing the amount of air conditioning needed to keep a building comfortable. ENERGY STAR qualified reflective roof products can reduce peak cooling demand by 10 to 15% and can reduce building energy use by up to 50%.
Exact energy and money savings will depend on a number of factors, such as the type and efficiency of insulation in the ceilings and exterior walls; the windows; the efficiency of your cooling system; and, most importantly, the climate of the building's location. The Federal Energy Management Program (FEMP) of the U.S. Department of Energy provides the following estimates of energy and cost savings from installing both black and white EPDM membranes on roofs in Washington, D.C., and Phoenix, Ariz.
Performance | Base Model | ENERGY STAR Base Model |
Roof Product Description | Black EPDM | White EPDM |
Initial Solar Reflectance | 6% | 69% |
Three-Year Solar Reflectance | 8% | 58% |
Standard | Reflective | |||
Washington | Phoenix | Washington | Phoenix | |
Annual Heating Energy Use (therms) | 870 | 46 | 930 | 60 |
Annual Cooling Energy Use (kWh) | 20,700 | 40,400 | 19,300 | 37,300 |
Annual Energy Cost | $1,590 | $2,440 | $1,530 | $2,260 |
Lifetime Energy Cost | $17,100 | $25,600 | $16,500 | $23,800 |
Lifetime Energy Cost Savings | - | - | $600 | $1,800 |
Click here for more key ENERGY STAR qualified roof products criteria.
Heat Island Effects
A cool roof not only benefits the building on which it is installed, but also the surrounding neighborhood. In the summer, major urban areas become heat islands, where temperatures can soar 4° to 8°F degrees above the temperature in the surrounding area. This rise in temperature corresponds to a rise in harmful ozone, and therefore smog levels in the urban air. For every degree above 70°F, the incidence of smog increases 3%. A city of buildings with reflective roofs will not experience this effect as strongly. A recent study by the Lawrence Berkeley National Laboratory (LBNL)'s Heat Island Group found that if the buildings in Los Angeles were upgraded with reflective roofs, the city could save $35 million per year in energy costs.
Manufacturers
For a complete list, visit the ENERGY STAR website.
Purchasing Tips
Rebates
- Pacific Gas & Electric's Residential Cool Roof Rebate
- Sacramento Municipality District (SMUD) Residential Coof Roof Program
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