Metal Offers Durable, Energy-efficient Roofing Solutions
Warranties
Most architectural metal specifications call for the issuance of a finish warranty covering resistance to color fade, weathering and finish performance. PVDF finishes are typically warranted for 20 years when applied in coil form, and for five years when applied in spray-coat form, although some 10-year warranties are available. The most important performance characteristic is probably the finish resistance to color fade. Color fade is expressed in "Hunter" units, referring to the Hunter Colorimeter that is used to measure color variation. In terms of color fade, a lower number reading means higher performance. A rating of 5 or better is expected for PVDF over 20 years, while a rating of 8 or better is typical for polyester over 20 years. A rating of 5 or better is barely discernable to the naked eye. Another measure of finish performance is chalking, which is the result of a breakdown in the carbon bonds in the finish. This is evidenced by the white power that comes off the paint surface. Chalking is rated on a scale of 1 to 10. In this case, a higher rating connotes higher performance. Twenty year performance for PVDF is typically at least 8, with 20-year performance for polyester typically 5 or higher. Another parameter is coating thickness, which is critical to the overall finish performance. Standard specifications for PVDF call for a .2 mil primer coat and a .8 mil ± .1 mil. PVDF coat. Total dry film thickness should be 1.0 mil ± .1 mil.
Not all warranties offer the same coverage, and should be reviewed carefully to see whether, for example, coverage is prorated. Finish warranties should also reference paint adhesion, abrasion resistance and chemical resistance.
The Cool Metal Roof
Recently, coatings formulated for cool metal roofing panels combine "cool pigment" technology with high solar reflectance and high infrared emittance properties, thereby offering even greater payoffs in saving energy by reducing a building's cooling needs. Cool pigment technology strengthens paint and coatings with engineered formulations that reflect wavelengths in the infrared region. These infrared reflective pigments mean darker colors will function more like lighter colors in terms of solar reflectance, enabling architects to specify darker colors such as earth tones that still meet requirements of the federal energy programs.
Cool metal roofing refers to sustainable, energy-efficient roofing products comprising unpainted metal, prepainted metal, and granular-coated metal that are marketed in diverse finishes, colors, textures, and profiles. The term extends to substrates, underlayments, and insulation as well as the exterior roofing surfaces.
Basically, a cool roof is one that reflects and emits the sun's heat back to the sky instead of absorbing it into the building. "Cool" is measured by solar reflectance and thermal emittance in a range of 0 to 1. The higher the value, the "cooler" the roof.
The Cool Metal Roofing Coalition and Oak Ridge National Laboratory report reflective metal roofing can save up to 40 percent in summer cooling energy costs, and highly emissive metal roofs can reduce urban air temperatures by as much as 12 degrees. According to the coalition, these cool roofs with specially formulated coatings and construction methods engineered to enhance solar reflectance and infrared emission properties meet the EPA's Energy Star® Roof Products Program performance criteria for reflectivity and have emissivity as high as 90 percent.
Specifying the right combination of reflectance and emittance based on climate can increase the building's energy efficiency. In cold climates with high heating bills, a metal roof with high solar reflectance and low thermal emittance will achieve maximum cooling and heating energy efficiency. The most appropriate option: an unpainted cool metal roof product. But for warmer climates with steep air-conditioning costs, a cool metal roof with high solar reflectance and high thermal emittance is better.
In conclusion, a properly designed and constructed metal roof will last for decades without tearing, puncturing, burning, or shrinking. It will withstand ultraviolet degradation, chemical contaminants, and exposure to the elements. By their nature, metal roofs are fire- and wind-resistant, have durability and low life-cycle costs, and many are reflective, easily vented, and help reduce heat gain into a building. Roofing systems can also be designed to reduce a building's heating and cooling costs.
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