High Thermal Emittance Roof

Aged solar reflectance tested in accordance with astm c 1549 astm e 903 or.

High thermal emittance roof. Thermal emittance is the specific term you should look for when shopping for a new roof. Mindful that reflectance and albedo can be different a combination of high albedo and high emittance resists solar heat gain most effectively. The thermal emittance of roofing surfaces is rated by the american society for testing and materials better known as astm. Sometimes the rating is expressed in a percentage.

Link between energy savings and emissivity. During the daylight hours a roof is constantly subjected to solar energy striking its surface. A roofing surface with high solar reflectance and high thermal emittance will reflect solar heat and release absorbed heat readily. Scientists use a number between 0 and 1 or 0 and 100 to express emittance.

They can also reduce the urban heat island effect. The use of area weighted averages to comply with these requirements shall be permitted. The thermal emittance of a roof or wall component is mainly affected by the characteristics of the layer that is exposed to the solar radiation. How they can be made cool.

Like solar reflectivity materials are rated on a scale between zero and one. 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. With the exception of a metallic surface most roofing materials can have emittance values above 0 85 85. Just like reflectance the emittance of the roof is measured on a scale of 0 to 1 with 1 being the highest.

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. One example is a metal wrench left in the sun which is hot to the touch because it has a low emissivity value. Painting a metal roof can increase its solar reflectance and thermal emittance allowing it to achieve cool roof status. However emittance also matters because the higher the emittance the more readily the roof surface will radiate long wave ir heat to the sky.

Ideally you ll want a roof that has equal reflectance and emittance because they complement one another. This physics related article is a stub. The term solar reflectance is a measure of the amount of that solar energy that is immediately reflected from the surface. High thermal emittance material radiates thermal heat back into the atmosphere more readily than one with a low thermal emittance.

Alternatively you can apply cool reflective coatings. Unpainted metals are typically good solar reflectors but poor thermal emitters so they rarely satisfy low slope cool roof requirements.