5 Tips for Specifying Cool Metal Roofs

Metal roofing has a reputation for being durable—both low- and steep-slope systems can remain operational for 30 years or more. But today's systems also meet increased demands for energy efficiency, says Scott Kriner, technical consultant with the Metal Initiative, Glenview, Ill.

Through the proper selection of surface finishes, metal roofs can have higher levels of solar reflectance and infrared emittance, resulting in lower surface temperatures, says Kriner.

To help AEC professionals sort through the numerous criteria to consider when specifying a cool metal roof, Kriner offers five important tips:

1. Make sure the roofing product is labeled as “cool.” The EPA's Energy Star program includes a roofing category that defines the requirements for energy-efficient metal roofs for both low- and steep-slope applications. A cool metal roof that carries this label can help Building Teams achieve tax incentives defined in the Federal Energy Policy Act of 2005. For commercial construction, tax deductions of $1.80/sf are provided for energy efficient improvements that result in a 50% reduction in annual energy costs compared to a standard building. Likewise, the Cool Roof Rating Council has a certification program that measures and lists reflectance and thermal emittance values of various types of roofing products.

2. Use durable and corrosion-resistant products. Metal roofing substrates include metallic-coated steel, aluminum, copper, zinc, and stainless steel. Metal roofs typically have longer useful lives due to the longevity of the substrates from which they are made. However, the corrosion resistance of a metal roof can vary depending on which substrate is specified. With steel roofing, galvanized and Galvalume sheet substrates are common. Galvalume is a steel product coated with an aluminum and zinc alloy that has two to four times the corrosion resistance of galvanized steel. Natural metals such as copper, aluminum, and zinc have much better corrosion performance than coated steel, but are not as strong as steel. This makes them less desirable for low-slope commercial roofing.

When a metal roof is to be used with a pre-painted finish, caution must be exercised in choosing the general type of paint system. A Kynar 500 or Hylar 5000 PVDF resin-based topcoat is the premium paint system for durability and fade resistance. PVDF systems are typically used on steep-slope roofing applications because of the durability of their colors. For low-slope roofing, silicone-modified polyesters are preferred due to their durability, resistance to fading and chalking, and low cost.

The Metal Construction Association has instituted a steep-slope metal roofing certification program that is performance based. Minimum performance guidelines are provided for metal substrates and paint finishes for these types of installations.

3. If using painted metal, don't be afraid to specify a dark color. Specifiers and designers should no longer assume that dark colors are hot. Infrared-reflective pigments introduced to the industrial finishes industry several years ago allow darker colors to behave much like lighter colors with respect to solar reflectance.

That means architects can specify darker colors, like earth tones, and still meet the solar reflectance and infrared emittance requirements set by Energy Star and CRRC. For instance, forest green and brown can meet Energy Star's solar reflectance requirement of 0.25 for steep-slope roofs.

4. Don't underestimate the effect of above-sheathing ventilation. Certain metal roofing products with raised profiles can take advantage of recently discovered benefits of the airflow between the roof deck and the bottom of the metal roof panel. This natural air ventilation helps reduce the heat flow through the deck into the space below. Combining the above-sheathing ventilation with a cool color or reflective granular-coated surface can result in a 45% reduction in heat flow through the roof deck.

5. Choose the correct metal roof for the climate and application. Metal roofs are available in a wide range of solar reflectance and thermal emittance properties. Specifying the ideal combination of these properties based on climate can increase the energy efficiency of the building. For example, in northern settings where heating is an important consideration, a metal roof with high solar reflectance and low thermal emittance may be desired to achieve maximum cooling and heating energy efficiency. An unpainted cool metal roof product would be most appropriate for this type of application. Conversely, for locations where cooling is of paramount importance, a cool metal roof with high solar reflectance and high thermal emittance would be preferred to help lower summer air-conditioning costs. Pre-painted metal with cool colors or granular coated metal panels should do the job.