Cool Metal Roofing

A cool roof reflects more of the sun's energy and releases absorbed heat efficiently. Metal roofing can be made "cool" through bright natural metal surfaces or, more often, through factory paint finishes formulated with infrared-reflective pigments, so that even medium and dark colours reflect more heat than conventional paints.

Key measures

• Solar reflectance: the fraction of sunlight reflected, from 0 to 1.
• Thermal emittance: how readily the surface releases absorbed heat, from 0 to 1. Bare, unpainted metal has low emittance; painted finishes usually have high emittance.
• Solar Reflectance Index (SRI): combines the two into a single value, typically 0 to 100, used in green building programmes. Lawrence Berkeley National Laboratory provides an SRI calculator.

Benefits

• Lower roof surface and attic temperatures.
• Reduced cooling energy in warm climates. Oak Ridge National Laboratory research reported in 2008 found substantial summer energy savings for cool-coloured metal roofs installed with an air gap.
• Reflective metal finishes keep much of their reflectivity over time compared with some other roofing types.
• Contribution to reducing urban heat islands.

Considerations

• Oak Ridge reported smaller savings in winter than in summer.
• Results depend on insulation, ventilation and roof construction, not the surface alone.

Many coil-coating resin manufacturers and paint makers market cool-pigment versions of their standard colour ranges.

Metal compared with other roofs

Reflective metal roofs are reported to keep around 95% of their reflectivity over their life, compared with roughly 60 to 80% for some other roof types, and reflective paints are reported to cut cooling bills considerably in hot climates.