A roofing surface with high solar reflectance and high thermal emittance will reflect solar heat and release absorbed heat readily.
High thermal emittance roof.
The thermal emittance of roofing surfaces is rated by the american society for testing and materials better known as astm.
The term solar reflectance is a measure of the amount of that solar energy that is immediately reflected from the surface.
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.
They can also reduce the urban heat island effect.
Sometimes the rating is expressed in a percentage.
Link between energy savings and emissivity.
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Just like reflectance the emittance of the roof is measured on a scale of 0 to 1 with 1 being the highest.
However emittance also matters because the higher the emittance the more readily the roof surface will radiate long wave ir heat to the sky.
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.
In general a cool roof is one that has relatively high solar reflectance and high thermal emittance.
Mindful that reflectance and albedo can be different a combination of high albedo and high emittance resists solar heat gain most effectively.
With the exception of a metallic surface most roofing materials can have emittance values above 0 85 85.
During the daylight hours a roof is constantly subjected to solar energy striking its surface.
Scientists use a number between 0 and 1 or 0 and 100 to express emittance.
The use of area weighted averages to comply with these requirements shall be permitted.
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.
One example is a metal wrench left in the sun which is hot to the touch because it has a low emissivity value.
Aged solar reflectance tested in accordance with astm c 1549 astm e 903 or.
High thermal emittance material radiates thermal heat back into the atmosphere more readily than one with a low thermal emittance.
Thermal emittance is the specific term you should look for when shopping for a new roof.
How they can be made cool.
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.
Painting a metal roof can increase its solar reflectance and thermal emittance allowing it to achieve cool roof status.
