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).

  • Solar Reflectance is the fraction of the solar energy that is reflected by a roof, expressed as a number between zero and one. The higher the value, the better the roof reflects solar energy. For example, white reflective coating or membrane has a reflectance value of 0.85 (reflects 85% of solar energy hitting it and absorbs the remaining 15%), while asphalt has a value of 0.09 (reflects 9%).
  • Emittance is the amount of absorbed heat that is radiated from a roof, expressed as a number between zero and one. The higher the value, the better the roof radiates heat.
  • Solar Reflectance Index (SRI) indicates the roof's ability to reject solar heat, and is the combined value of reflectivity and emittance. It is defined so that a standard black is zero (reflectance 0.05, emittance 0.90) and a standard white is 100 (reflectance 0.80, emittance 0.90). Because of the way SRI is defined, very hot materials can have slightly negative SRI values, and very cool materials can have SRI values exceeding 100.


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.

  • Metal Roofs are durable, fire retardant and almost maintenance-free. They reflects heat and block its transfer into the building's top floor. Research by the Florida Solar Energy Center in 1985 showed that metal absorbed 34% less heat than asphalt shingles, and homeowners switching to metal roofing reported saving up to 20% on their energy bills. The one drawback to metal roofs is that they have a low emittance, which means they trap solar radiation and don't emit the heat. Metal roofs typically have solar reflectance values between 0.50 and 0.70 but their overall efficiency is reduced by their low emittance levels. They perform better when combined with a polymeric coating that helps to offset the low emittance of the metal. These coatings, which are similar to paint, can be factory-applied.
  • Roofing Tiles can be ceramic or fabricated from cement concrete. Some of the lighter-weight types contain fibers (e.g., cellulose) for added strength. The color of a tile may be applied as a coating or dispersed throughout. Perhaps the most common type of roof tile is the Spanish-style red barrel tile made from fired clay. The modern version of this tile is sometimes a cement tile with a suitable coating. They can have respectable reflectance ratings if they are of a light color. They also have enhanced air circulation compared to other roofing types, because ambient air can circulate below as well as above the tile. Enhanced air circulation helps the roof shed solar heat more readily. Solar reflectance levels for tiles range from a low of 0.18 for red concrete tile to a high of 0.74 for white concrete tile. Tile is more appropriate for steeper-sloping roofs.
  • Roofing Membranes are fabricated from strong, flexible waterproof materials. They may be applied in multiple layers, as in the venerable built-up roof (BUR), or they may consist of a single-ply membrane. Membranes usually contain a fabric made from felt, fiberglass or polyester for strength, which is laminated to or impregnated with a flexible polymeric material. The polymeric material may be asphalt, synthetic rubber known as EPDM, or synthetic polymers such as PVC. The color of the polymer ranges from black to white, often depending on the amount of carbon present. The upper surface of the membrane may be coated with a pigmented material which determines the color and solar reflectance, or it may simply be ballasted with roofing gravel. When a dark membrane is surfaced with roofing granules, the membrane has the appearance (and solar reflectance) of asphalt shingles and is less effective in reducing cooling loads. Roofing membranes generally have high reflectance and emittance levels (above 0.60), as long as it is of a light color.
  • White Reflective Coatings contain transparent polymeric materials, such as acrylic, and a white pigment, such as titantium dioxide (rutile), to make them opaque and reflective. These coatings typically reflect 70 to 80% of the sun's energy. Despite the white appearance, these pigments strongly absorb the 5% or so of the sun's energy that falls in the ultraviolet (UV). Thus, the pigments help protect the polymer material and the substrate underneath from UV damage. As long as the coating is white or light-colored, the roof will have high reflectance and emittance levels.

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:

  • For low-slope roofs (surfaces with a slope of 2:12 inches or less), the initial solar reflectance must be at least 0.65. The material must maintain a level of at least 0.50 after three years of installation under normal conditions.
  • For high-slope roofs (surfaces with a slope of 2:12 inches or greater), the initial solar reflectance must be at least 0.25. The material must maintain a level of at least 0.15 after three years of installation under normal conditions.


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

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  • Avoid non-solar-reflective asphalt shingles or coatings, if your budget allows.
  • Evaluate your climate. Do heating or cooling loads dominate your energy usage? If cooling dominates, you may be a good candidate for a new energy-efficient roof.
  • Look for these important properties when selecting a roof material: high solar reflectance, endurance of high reflectance over time and high emittance. High emittance lowers the roof temperature by increasing the release of heat by thermal radiation. For a high-emittance roof, avoid unpainted metal roofs and aluminum coatings. If installing a metal roof, make sure it is painted a light color (not a clear coating).
  • Visit the Cool Roof Rating Council's (CRRC) Rated Products Directory for credible and accurate roof performance data on nearly 1000 different products. Search for product information by product type, manufacturer, brand or model name, slope application, minimum solar reflectance and thermal emittance, or CRRC Product ID. The CRRC is the only source of independent, third-party verified ratings, and products must be rated with the CRRC to comply with the CEC's Building Energy Code, Title 24.
  • Find a good contractor who knows about energy-efficient roofing technologies. This is the most important step you can take in obtaining the right roof. A good place to start is the 'National Roofing Contractors Association.' Check out the "Consumer" section of their website for help selecting a good contractor in your area.
  • Visit Lawrence Berkeley National Laboratory (LBNL)'s Cool Roof Materials Database for reflectance, emittance and SRI levels of different roofing materials.
  • Improve the efficiency of your insulation and air conditioning system, which will help you realize maximum savings from your investment in a cool roof.