A cool roof has to do more than look good. It needs to reflect the sun’s heat and emit absorbed radiation back into the atmosphere, literally staying cooler and reducing the amount of heat transferred to the building below.

According to the Cool Roof Rating Council, cool roofs consist of materials that very effectively reflect the sun’s energy from the roof surface. Cool materials for low-slope roofs are mainly bright white in color, although non-white colors are starting to become available for sloped roof applications.

A cool roof also must have high emissivity, allowing it to emit infrared energy.

Cool roofs can reduce the roof surface temperature by up to 100º F, thereby reducing the heat transferred into the building below. This helps to reduce energy costs, improve occupant comfort, cut maintenance costs, increase the life cycle of the roof, and reduce urban heat islands along with associated smog.

Cool Color Technologies

There are many “cool color” products that use darker pigments, which are highly reflective in the near infrared (non-visible) portion of the solar spectrum. With “cool color” technologies, there are roofs that come in a wide variety of colors and still maintain a high solar reflectance.

The two basic characteristics that determine the ‘coolness’ of a roof are solar reflectance (SR) and thermal emittance (TE). Both properties are rated on a scale from 0 to 1, where one is the most reflective or emissive.

The Cool Roof Rating Council measures these two properties for roofing products, both for the product’s initial values and after three years of weather exposure.

What are the Benefits of a Cool Roof? According to the CRRC, there are numerous benefits to a cool roof:
• Increased ecological sustainability;
• Reducing your utility bills associated with air conditioning;
• Increasing occupant comfort and avoid installing an air conditioner where there isn’t already one;
• Decreasing the size and prolong the life of your air conditioning system;
• Lowering roof maintenance costs and extends roof life, avoiding reroofing costs and reducing solid waste;
• Assist in meeting building codes;
• Mitigate your community’s Urban Heat Island Effect;
• Maintain aesthetics with a roof that performs and looks good; and
• Receive utility rebates (in some locations). A cool roof can significantly reduce your cooling energy costs and increase your comfort level by reducing temperature fluctuations inside your building. Average energy savings range from 7 percent to 15 percent of total cooling costs.

What is the right roofing material?
Although the CRRC cannot recommend specific roofing products, below is a description of some things to consider in selecting a roofing product for your building, including a description of the types of roofing materials currently rated.

Slope

All roofs essentially fall into one of two general categories: low-slope (less than 2 inches of rise over 12 inches of run) and steep-slope (2:12 or greater). CRRC's directory indicates which slope(s) a product is intended for. In addition, check with a local roofing contractor, roof consultant or architect to determine the most appropriate roofing product type for your home.

Climate

In most climate zones, worldwide, cool roofs can significantly reduce a building’s cooling load. However, cool roofs can also increase heating costs in winter months. This increase is usually greatly outweighed by the cooling energy savings achieved during summer months because the sun is low in the sky during winter months, the weather tends to be overcast, and snow often covers the roof, usually making the heat gain through the roof negligible in all but the most northern climate zones in the US.

In selecting your roof, it is important to understand your climate zone and the actual energy your home or building will save. There are two cool roof calculators available online that can give good estimates of how much energy can be saved.

You can find them at: DOE Cool Roof Calculator http://www.ornl.gov/sci/roofs% 2Bwalls/ facts/CoolCalcEnergy.htm or EPA Cool Roof Calculator http://www.roofcalc.com

Product Types

Below is a list of definitions of roof product types. Built-up Roofing (includes asphalt and coal tar pitch):

Built-up Roofing (BUR) consists of built-up layers of coated asphalt and insulation applied on site and can be covered with a capsheet (or surfacing material).

Foam Roof Systems: Foam systems can also be divided into the field-applied and factory- applied categories. Field-applied foam systems are similar to field-applied coatings, as they are sprayed on in liquid form and harden as they set on top of the roof. Factory- applied foam systems are formed into rigid panels and coated with a reflective coating. The foam usually gives the roof system additional insulation properties.

Metal: Metal roofing products can be shaped to look like shingles, or shakes, or to fit unique curvatures, in addition to a typical “standing seam” configuration. They come in a variety of textures and colors, including some darker “cool” colors with special additives that allow these dark colors to achieve significantly greater reflectance than previous versions of the same product.

Modified Bitumen: Modified bitumen is bitumen (asphalt or tar) modified with plastic and layered with reinforcing materials then topped with a surfacing material. Like BURs, the radiative properties of modified bitumen (mod bit for short) are determined by the surfacing material.

Roof Coatings: Roof coatings can be divided into two categories: field-applied and factory-applied. Field-applied coatings are applied directly onto the roof surface, either on a new roof assembly or over an existing roof surface (and can be applied over top of just about anything, so long as the right coating is selected). Factory-applied coatings are applied at the factory prior to distribution. Examples of factory-applied coatings include coatings applied to metal and glazes that are applied to tiles.

Shingles, Slate, or Tile: All of these products types use the same concept, where pieces fit together to form a roof. Asphalt shingles are fairly common for residential roofing applications, probably because they are relatively inexpensive and simple to install. Slate and tile products also come in a wide variety of colors, shapes and textures, and, because of their heavy mass, they have thermal properties that may yield additional energy savings beyond their reflectance and emittance properties.

Single-Ply: Single-ply roofing is a prefabricated sheet of rubber polymers. Single- ply roofing is laid down in a single layer over a low or steep-sloped roof. The single-ply membrane can be loose-laid and weighted down with ballast or pavers or firmly set on the roof and attached with mechanical fasteners or adhesives. There are two main types of single-ply materials:

Single-Ply-Thermoset (includes EPDM, Hypalon): Thermosets are materials that can not be hot-air welded because it changes their physical characteristics. Instead, tape or a contact cement to seal the seams.

Single-Ply–Thermoplastic (includes TPO, PVC, etc.): Single-Ply-Thermoplastic is a flexible sheet membrane which consists of compounded plastic polymers. When heat is applied onto the surface, the single ply-thermoplastic seams are melded together making the material seamless and effective. Most thermoplastics are manufactured to include a reinforcement layer (usually polyester or fiberglass) for extra durability and strength.