What is a Cool Metal Roof
What is a Cool Metal Roof?
For the benefit of our new readers or those who missed our previous articles, here is a short article on what cool metal roofs are about.
Cool roofing is defined differently by various performance criteria in codes, standards, and incentive programs. In general, a cool roof is one that has relatively high solar reflectance and high thermal emittance.
During the daylight hours, a roof is constantly subjected to solar energy striking its surface. The term “Solar Reflectance” is a measure of the amount of that solar energy that is immediately reflected from the surface. It is not reflected heat, but reflected electromagnetic energy from the sun. Solar reflectance is reported as a decimal (0 – 1.00) or as a percentage (0-100%). The solar energy that is not reflected away from the surface is absorbed into the outer surface of the roof product and is converted into heat. The heat can be removed by convection as air flows over the surface, or by conduction through the roof material into the sheathing below. The energy that is left can also be re-emitted to the night sky in the form of infrared energy. That re-emitted energy is referred to as thermal emittance, which is also expressed as a decimal (0 – 1.00) or as a percentage (0-100%).
A cool metal roof with high solar reflectance and high thermal emittance would have a lower surface temperature as compared to a roof with low reflectance and low emittance. In the case of a cool metal roof, a lower surface temperature translates into less heat gain into the attic space or living space below the roof. The result is a cooler living space and lower cooling/heating energy consumption.