A cool roof is one that has been designed to reflect more sunlight and absorb less heat than a standard roof. Cool roofs can be made of a highly reflective type of paint, a sheet covering, or highly reflective tiles or shingles. Nearly any type of building can benefit from a cool roof, but consider the climate and other factors before deciding to install one.
Just as wearing light-colored clothing can help keep you cool on a sunny day, cool roofs use solar-reflective surfaces to maintain lower roof temperatures. Standard or dark roofs can reach temperatures of 150°F or more in the summer sun. A cool roof under the same conditions could stay more than 50°F cooler
BENEFITS OF COOL ROOFS
A cool roof can benefit a building and its occupants by:
Reducing energy bills by decreasing air conditioning needs
Improving indoor comfort for spaces that are not air conditioned
Decreasing roof temperature, which may extend roof service life.
Beyond the building itself, cool roofs can also benefit the environment, especially when many buildings in a community have them. Cool roofs can:
Reduce local air temperatures (sometimes referred to as the urban heat island effect)
Lower peak electricity demand, which can help prevent power outages
Reduce power plant emissions, including carbon dioxide, sulfur dioxide, nitrous oxides, and mercury, by reducing cooling energy use in buildings.
TYPES OF ROOFS AND HOW THEY CAN BE MADE COOL
There are many types of roof systems available, but the surface exposed to the sun is the one that determines if a roof is cool or not. You can usually make a new or existing roof cool by selecting the appropriate surface.
Cool roof coatings are white or special reflective pigments that reflect sunlight. Coatings are like very thick paints that can protect the roof surface from ultra-violet (UV) light and chemical damage, and some offer water protection and restorative features. Products are available for most roof types.
LOW SLOPED ROOFS
Single-ply membranes are pre-fabricated sheets rolled onto the roof and attached with mechanical fasteners, adhered with chemical adhesives, or held in place with ballast (gravel, stones, or pavers).
How they can be made cool: Reformulate or coat black membranes to make them reflective.
Built-up roofs consist of a base sheet, fabric reinforcement layers, and (usually) a dark protective surface layer.
How they can be made cool: The surface layer can be made different ways, and each has cool options:
Substitute reflective marble chips or gray slag for dark gravel in a flood coat of asphalt
Use reflective mineral granules or a factory-applied coating rather than a dark coating on a mineral surfaced sheet
Apply a cool coating directly on top of a dark asphaltic emulsion coating.
Modified bitumen sheet membranes have one or more layers of plastic or rubber material with reinforcing fabrics, and are surfaced with mineral granules or a smooth finish. These can also be used to surface a built-up roof—known as a "hybrid" roof.
How they can be made cool: Pre-coat with a cool roof coating at the factory.
Spray polyurethane foam roofs are constructed by mixing two liquid chemicals together that react and expand to form one solid piece that adheres to the roof. Foams are highly susceptible to mechanical, moisture, and UV damage, and rely on a protective coating.
How they can be made cool: The protective coatings are usually already reflective, and offer cool roof performance.
STEEP SLOPED ROOFS
Shingle roofs consist of overlapping panels made from a variety of materials such as fiberglass asphalt, wood, polymers, or metals.
How they can be made cool: Buy cool asphalt shingles, which use specially coated granules that provide better solar reflectance. (Coating existing asphalt shingles to make them cool, however, is not normally recommended or approved by shingle manufacturers.) Other roof shingles can be coated at the factory or in the field to make them more reflective.
Tile roofs can be made of clay, slate, or concrete. Tiles can be glazed to provide waterproofing or coated to provide customized colors and surface properties.
How they can be made cool: Some are naturally reflective enough to achieve cool roof standards, and surface treatments can transform tiles with low solar reflectance into cool roof tiles.
LOW AND STEEP SLOPED ROOFS
Metal roofs are available with natural metallic finishes, oven-baked paint finishes, or granular coated surfaces.
How they can be made cool: Unpainted metals are typically good solar reflectors but poor thermal emitters, so they rarely satisfy low slope cool roof requirements. Painting a metal roof can increase its solar reflectance and thermal emittance, allowing it to achieve cool roof status. Alternatively, you can apply cool reflective coatings.
DECIDING WHETHER TO INSTALL A COOL ROOF
When deciding whether to install a cool roof, you’ll need to determine whether the cost will justify the energy savings. How much energy you will save depends on several factors such as your home's climate and environment, how well insulated your current roof is, the type of roof you have, and the efficiency of your heating and cooling system.
If you are building a new home, you can decide during the planning phase what type of roof to install and whether it should be a cool roof. If you want to convert an existing roof into a cool roof, you have three basic options:
Coat the roof
Re-cover it with a new waterproofing surface
Tear off the existing roof and replace it with a new one.
If your roof is in poor condition or near the end of its life, it is usually best to re-cover, replace, or retrofit the roof.
COST AND ENERGY SAVINGS
A cool roof does not necessarily cost more than a non-cool roof, especially if you are installing a new roof or replacing an existing one. However, converting a standard roof that's in good condition into a cool roof can be expensive. Major roof costs include upfront installation (materials and labor) and ongoing maintenance (repair, recoating, and cleaning). Additional cool roof costs include specialized materials and labor.
Cool roofs can save money several ways, including energy savings, rebates and incentives, HVAC equipment downsizing, and extended roof lifetime. One way to estimate how much energy you would save by installing a cool roof is by using a cool roof calculator, available at DOE Cool Roof Calculator.
CLIMATE AND ENVIRONMENT
Your climate is an important consideration when deciding whether to install a cool roof. Cool roofs achieve the greatest cooling savings in hot climates, but can increase energy costs in colder climates due to reduced beneficial wintertime heat gains.
MOISTURE CONTROL
In warm, moist locations, cool roof surfaces can be more susceptible to algae or mold growth than hot roofs. Some roof coatings include special chemicals that prevent mold or algae growth for a few years.
In cold climates, roofs can accumulate moisture through condensation, and it is possible that cool roofs might be more susceptible to accumulating moisture than dark roofs of the same design. Condensation can be avoided using proper design techniques.
sources:http://energy.gov/energysaver/cool-roofs
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