Cool Roof Selection and Applications

If you decide to consider a cool roof, these notes can help you select and install the right option for your building. The decision to make your existing roof a cool roof usually means deciding to coat the roof, replace the roof, or build another roof on top of the existing roof. If your roof is in good condition; has relatively few, easy-to-repair leaks; and has at least five years of expected service life, a cool coating may be a good option.

Note that the main reason for coating a roof is to extend its service life, and the energy savings alone will not normally provide sufficient financial reason to coat a roof to make it cool. If your roof is in poor condition, or is approaching the end of its service life, a roof re-covering (adding a new membrane) or replacement (removal of the existing roof, and installation of a new one) is the likely option. Roof re-covering or replacement gives you the opportunity to select any kind of roofing system and cool roof option you desire.

Above all, your roof must protect the building from the effects of weather, so be sure to select a roof system that will do this well. A durable roof is the result of the combined efforts of the building owner, specifier, manufacturer, and contractor. Insist that your installer follow all manufacturer installation procedures. Asking the right questions and being involved can help safeguard your investment. The following application notes will help guide you through this process.

Roof Coatings can be rolled, brushed, or sprayed onto most kinds of roofs. Any leaks or problems with the roof membrane must be repaired before coating the roof. If you plan to coat your roof, selecting the right coating for your particular roof is the most important thing you can do – cool roof or not. NOTE: Roof coatings are not the same as exterior paints. Ordinary paints are not designed to last on roof surfaces and will not provide protection. Three major kinds of coatings include acrylic, silicone, and urethanes, and cool roof formulations are available for each, Table 9. Aluminized asphaltic emulsions provide a glossy finish for some roof types, but fall short of meeting cool roof standards for low sloped roofs. Coating quality varies widely by manufacturer. Improper coating installation can produce coatings that flake off or wear out faster than they should. Insist that the installer follow the manufacturer’s recommended installation procedures, especially regarding weather condition restrictions during application and minimum coating thickness.

Roof Replacement or New Construction gives you an opportunity to select from many cool roof options. In some cases an existing roof must be removed and a new roof installed in its place. In other cases, it may be possible to build a second roof atop an existing roof, though this depends on your roof’s condition and local building codes. Regardless of how the roof is installed or replaced, here are some notes about cool roof options.

Single-Ply Membranes come in several types. TPO and PVC membranes are usually white and reflective, and do not require additional formulations or cool coatings. EPDM membranes are black, but can also be produced in white or cool colors. Durable cool EPDM options use black EPDM that is pre-coated with a reflective coating or laminated with a reflective material. The designer should consider membrane durability, since all membranes are not equally durable xvii.

Roof membranes that are attached with chemical adhesives must reach and maintain a minimum temperature to bond properly. To compensate for reduced surface temperatures, cool roofs that are chemically adhered might require warmer outdoor temperatures during installation to bond properly. Make sure your contractor follows the recommended installation procedures.

Built-Up Roofs can be surfaced in three ways, and each has a cool option. First, light-colored aggregate, such as marble chips, can replace the dark mineral aggregate commonly used with hot bituminous flood coat surfacing. It is usually impractical to coat a gravel-surfaced built-up roof with a cool coating. Doing so requires the removal of the non-embedded gravel, which may affect the roof’s fire rating. Second, reflective aluminum pigments can be added, at some cost, to an asphalt-coated smooth surface, however these have a low thermal emittance and are not cool. To make this surface cool requires an additional cool coating. Finally, cap sheets with white mineral granules can be substituted for those with dark mineral surface, also at a small premium.

Modified Bitumen roofs should have protective coatings to provide heat resistance, ultraviolet resistance, and fire resistance. SBS modified roofsxviii must be coated to prevent rapid ozone and UV degradation. APP modified roofsxix may be left unsurfaced, but this is not recommended because irregular surface cracks can develop that lead to premature aging. The protective surfacing layer can be made of aggregate, mineral, metal foil laminate, or smooth surfaced with a liquid coating. If a large amount of protective granules fall off, recoating becomes necessary.

Metal Roofs are often coated with Fluoropolymer- or Silicone-Polyester based paints, and many colors can achieve cool roof performance. Some manufacturers offer cool colors almost exclusively, since lower surface temperatures reduce color fading and cost premiums for the reflective pigments are modest. Unpainted metal roofs tend to have a high reflectance but a low thermal emittance, which prevents them from being considered cool in low slope applications.

Spray Polyurethane Foam roofs are typically coated and periodically re-coated with reflective coatings to protect the foam from UV and water damage. As with membrane roofs, there are many suitable cool roof coatings for spray foam roofs, with acrylic being the most common. Some slightly pitched foam roofs cannot be white according to building appearance ordinances, so gray or tan colors may be used instead.

Asphalt Shingles are not typically coated in the field and doing so may void the manufacturer’s warranty. If you have asphalt shingles and wish to make your roof cool, you may need to replace the shingles with reflective shingles. In such cases, it may be more cost effec tive to wait until the shingles reach the end of their service life before replacing them with cool shingles. Other shingles, like those made from wood, polymer, or metal, can be coated to achieve cool roof status.

Tile Roofs may retain their color or not with aging depending on the type of tile. The color of clay tiles depends on naturally occurring chemicals and minerals in the clay. The surface of clay tiles may be altered by glazing the tiles during the manufacturing process. Concrete tiles can receive a surface coating after being produced, or color can be dispersed throughout the tile as it is produced (color through). The color of roofing slate depends on its chemical and mineral makeup. Weathering slate exhibits color changes as it weathers, while permanent or unfading slate retains its color with weathering. Selecting cool roof tiles that retain their surface properties can yield better lifetime energy savings.

Cool Roof Maintenance As a cool roof becomes dirty from pollution, foot traffic, winddeposited debris, ponded water, and mold or algae growth, its reflectance will decrease, leading to higher temperatures. Especially dirty roofs may perform substantially worse than product labels indicate. Dirt from foot traffic may be minimized by specifying designated walkways or by limiting access to the roof. Steep sloped roofs have less of a problem with dirt accumulation because rainwater can more easily wash away dirt and debris. Some cool roof surfaces are “selfcleaning” which means they shed dirt more easily and may better retain their reflectance. Cleaning a cool roof can restore solar reflectance close to its installed condition. Always check with your roof manufacturer for the proper cleaning procedure, as some methods may damage your roof. While it is generally not cost effective to clean a roof just for the energy savings, roof cleaning can be integrated as one component of your roof’s routine maintenance program. It is therefore best to estimate energy savings based on weathered solar reflectance values rather than clean roof values.