CHOOSING AND SPECIFYING A COOL ROOF:

PRODUCT RATING RESOURCES, PRODUCT TYPES, BUILDING PROGRAMS & CODES

A cool roof should be chosen based on the slope of the roof, energy savings goals, the project location and climate, local code requirement or green building credits, as well as aesthetic preferences. Designers who are seeking sustainable design credits may also want to consider the cradle-to-cradle aspects of their materials choices, including recycled content, end of life recyclability and use of toxic materials.

Once the project parameters have been established, an appropriate roofing product must be selected. Because of the environmental benefits of cool roofs, a growing number of building codes include cool roof measures. Hence, it is prudent to first check with the local or state building code to see if there are specific cool roofing requirements that must be satisfied. Such is the case with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1, California's Building Energy Efficiency Standards, Title 24, as well as a number of city building codes, including the Cities of Chicago, Houston, and Dallas.^v In addition, voluntary green building programs, such as The U.S. Green Building Council's LEED® program and the GreenGlobesâ„¢ environmental rating system, provide credit for cool roofs and several electric utility companies provide rebates.

PRODUCT RATING RESOURCES

Rated product databases can assist the designer in selecting an appropriate cool roof product because they list pertinent product information that can be easily compared. The designer can search roofing products by the initial and aged solar reflectance and thermal emittance values as well as the slope application and type of roofing material. A low-slope roof has a pitch less than or equal to 2:12. A steep-sloped roof has a pitch greater than 2:12.

While existing rating systems are complementary to one another, they do have slight differences in their requirements. ENERGY STAR, for example, aims to capture the most efficient products and set minimum requirements for both initial and aged solar reflectance.  In order for a product to be listed by ENERGY STAR, it must meet their minimum requirements (initial solar reflectance of 0.65 and three-year aged value of 0.50 for low-slope products and an initial reflectance of 0.25 and aged value of 0.15 for steep-sloped products). The Cool Roof Rating Council (CRRC), on the other hand, does not set minimum requirements, but does require that all testing be conducted by a licensed CRRC Accredited Independent Testing Laboratory (AITL). The primary values of independent ratings for cool roofs are standardized and consistent test methods,
credible test results, equal subjection of products to weatherization in key climates, aged testing, and reliable product comparisons.

Both rating systems include aged testing, where products are exposed to natural weather conditions for a three-year period of time. The CRRC uses three specific locations representing three key climate zones (hot/dry, hot/humid, and cold/temperate) to determine aged product performance. ENERGY STAR accepts products that have been rated by the CRRC so long as the ratings meet ENERGY STAR's minimum requirements for both initial and aged SR/TE values. The most reliable source for solar reflectance and thermal emittance data for cool roofs is independent roofing product ratings.

PRODUCT TYPES

This composite image shows only a few of the many "cool" roofing product types that are currently available in the marketplace. Clockwise from left: Photo A courtesy of GAF Materials Corporation, Photo B courtesy of Hydro-Stop, LLC., Photo C courtesy of Custom-Bilt Metals, Photo D courtesy of Decra Roofing

 

Materials for roofs can vary from asphalt to acrylics, and many of these products have cool roof alternatives. The following list provides examples of some common roofing product types, but does not account for every single type of roofing material in the marketplace:

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 field-applied coating (surfacing materials). The "cool" part of this particular roof type refers to the properties of the capsheet, typically a white mineral fiberglass surface, or coating, which are UV-resistant.

Foam Roof Systems: Field-applied foam systems 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 in the factory. The foam usually gives the roof system additional insulation properties and the coatings provide the "cool" rating.

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 factory-applied textures and colors, including darker "cool" colors with infrared reflective pigments. Metal products can also be coated in "cool" custom colors to meet a variety of client preferences.

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 are determined by the surfacing material, so a "cool" modified bitumen product will be finished off with a capsheet or coating to achieve a high solar reflectance.

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 may require an appropriate primer. Factory-applied coatings are applied directly to products at the factory prior to distribution. Examples of factory-applied coatings include coatings applied to metal, and glazes that are applied to tiles. Once applied, the coating is what determines the reflective properties of the roofing product.

Shingles, Slate, or Tile: These roofing products are commonly used for residential buildings, or steeper-sloped buildings, including some commercial buildings. For "cool" colored shingles, the heightened solar reflectance comes from granules that contain solar-reflective pigments. Slate and tile products are available with solar-reflective surfaces that increase the number of "cool" colors from which the designer can choose. Additionally, the dense, earthen composition of slate and tile products provides increased thermal mass, which yields additional energy savings that are not captured through solar reflectance and thermal emittance measurements.

Single-ply: Single-ply roofing is a pre-fabricated sheet of rubber polymers. Single-ply roofing is laid down in a single layer over a 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 and single-ply thermoplastic. These roofing products can be specified with an ultra-violet-resistant and highly reflective surface.

Aging and Maintenance
Roofs are exposed to harsh climates, solar radiation and pollution, which can reduce the solar reflectance of cool roofing materials. Lawrence Berkeley National Laboratory studied the three-year aging and weathering of cool roofing membranes made of single-ply roofing at various locations across the United States. Results indicated that when washed with detergent, the majority of the roofs will still provide 90 percent of their un-weathered reflectance (in some cases an algaecide was required). Standard maintenance practices as suggested by the roofing manufacturer will keep your cool roof "cool" for a longer time.