High Thermal Emittance Roof

In general a cool roof is one that has relatively high solar reflectance and high thermal emittance.

High thermal emittance roof. In common construction applications the thermal emittance of a surface is usually higher than 0 8 0 85 except for those layers that are based on metallic components for example aluminum shingles. Sometimes the rating is expressed in a percentage. Mindful that reflectance and albedo can be different a combination of high albedo and high emittance resists solar heat gain most effectively. A roofing surface with high solar reflectance and high thermal emittance will reflect solar heat and release absorbed heat readily.

How they can be made cool. The term solar reflectance is a measure of the amount of that solar energy that is immediately reflected from the surface. Thermal emittance is the specific term you should look for when shopping for a new roof. The thermal emittance of roofing surfaces is rated by the american society for testing and materials better known as astm.

You can help wikipedia by expanding it. Materials lacking 3 year aged tested values for either solar reflectance or thermal emittance shall be assigned both a 3 year aged solar reflectance in accordance with section c402 3 1 and a 3 year aged thermal emittance of 0 90. During the daylight hours a roof is constantly subjected to solar energy striking its surface. However emittance also matters because the higher the emittance the more readily the roof surface will radiate long wave ir heat to the sky.

A surface with a high thermal emittance will both absorb heat and radiate back into the environment. With the exception of a metallic surface most roofing materials can have emittance values above 0 85 85. Scientists use a number between 0 and 1 or 0 and 100 to express emittance. Like solar reflectivity materials are rated on a scale between zero and one.

Painting a metal roof can increase its solar reflectance and thermal emittance allowing it to achieve cool roof status. They can also reduce the urban heat island effect. The use of area weighted averages to comply with these requirements shall be permitted. One example is a metal wrench left in the sun which is hot to the touch because it has a low emissivity value.

Ideally you ll want a roof that has equal reflectance and emittance because they complement one another. Link between energy savings and emissivity. Unpainted metals are typically good solar reflectors but poor thermal emitters so they rarely satisfy low slope cool roof requirements. Aged solar reflectance tested in accordance with astm c 1549 astm e 903 or.

Alternatively you can apply cool reflective coatings. High thermal emittance material radiates thermal heat back into the atmosphere more readily than one with a low thermal emittance.