What Is Emissivity on a Standing Seam Roof?

Emissivity

• Emissivity is a property of all solids. It refers to the ability of a surface to radiate stored energy to the environment. Low emissivity allows objects like metal roofs to heat up significantly. This trait is often undesirable because the heat that roof is holding back will radiate into the home's attic and increase the attic air temperature. A variety of materials and coatings have been developed to increase the emissivity of metal roofs in areas where homes may become difficult to keep cool. In colder climates, low emissivity is highly desirable since it can contribute to the amount of heat held in a structure.

Reflectivity

• Reflectivity is the sister of emissivity, it is a term used to describe the willingness of a surface to return radiation in the direction it came. Reflectivity does not necessarily transfer the radiation to nearby objects. High reflectivity is desirable in a metal roof in many cases. If the heat from the sun is reflected before it can be absorbed, emissivity decreases from lack of opportunity and the roof remains cooler.

Cool Roofs

• Cool roofs are a greener option for metal roofing applications. These roofs are designed with high reflectivity and high emissivity, creating a surface that reflects much of the radiation that hits it. The radiation that is absorbed is radiated back to the environment much more quickly than in the case of a standard metal roof. Light-colored finishes enhance the ability of these roofs to stay cool.

Choosing Emissivity

• While it may seem that emissivity is a desirable trait that should be maximized, there are a variety of reasons why you might not want to install the most emissive or reflective roof. A highly emissive roof in a cold climate actually results in a cooler home because it is designed with the loss of solar heat in mind. When choosing roofing material, balance your heating and cooling needs before making a final decision on an emissive surface.