According to the Cool Metal Roofing Coalition, metal roofing offers tremendous advantages in energy performance. 

CMRC literature states that buildings consume one-third of all energy and two-thirds of all electricity generated. Cool roofs can help reduce energy consumption by lowering cooling loads, and the coatings and finishes available today qualify metal as a recognized cool roof product.

Energy Efficiency
A building’s cooling and heating costs can be effectively reduced by insulation under the roof surface. Adding increasing amounts of insulation is not always the best way to save energy. As part of total system design, a cool metal roof can be an economical method for better energy efficiency. 

How It Works
Cool metal roofing is available unpainted, with oven-baked paint finishes, or with granular-coated surfaces. This family of roofing can achieve solar reflectance of over 70 percent. Reflected solar energy allows the roof surface to remain cooler, which means less heat is transferred into the building. According to Oak Ridge National Laboratory, reflective roofs directly save up to 40^% in heating and cooling energy costs.

The infrared emittance of a roof is a measure of absorbed solar radiation that is reemitted from the roof surface to the sky. Emittance of metal roofing varies with the surface finish. Emittance of painted or granular-coated metal roofing can be as high as 90 percent.

Where annual cooling loads dominate, a highly reflective and highly emissive painted or granular-coated metal roof is optimal for reducing energy consumption. Where annual heating loads dominate, an unpainted metal roof is more desirable because of its low infrared emittance.

Source: Cool Metal Roofing Coalition