Energy Saving Cool Roofs

Energy Star® labeled roof products are reflective roof products that lower the roof surface temperature by up to 100°F, thereby decreasing the amount of heat transferred into a building's interior. Energy Star qualified roof products save money and energy by reducing the amount of air conditioning needed to keep a building comfortable. An Energy Star compliant high slope roof must have an initial solar reflectance of 25% and 15% after 3 years.

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. 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.

• Gerard USA manufactures a Energy Star rated metal roof tiles (See our product section)

Roofing Tiles fabricated from cement or 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.

Flat Roof 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.

• IB Roof Systems manufactures a Energy Star PVC roof system (See our product section)

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.