REFLECTANCE and EMISSIVITY – two important parameters of roof covering you might not be aware of

Roof coverings absorb energy of sunlight what causes the movement of covering particles, resulting in heating up of roof. The transformation of sunlight energy into thermal energy is known as photothermal conversion. During this process roofing materials heat up and give up absorbed energy in the form of infrared radiation or thermal energy. Therefore increased temperature of given object can be caused by the sunlight energy to which it is exposed and roof REFLECTANCE becomes a really important characteristic. If the material used for roof covering has high reflective properties most of the solar energy is reflected of the roof surface preventing from overheating.

Apart from high reflectivity, roofing materials should also have high EMISSIVITY, which is defined as  the ability to dissipate the heat absorbed from the energy of the sunlight.

In order to prevent the overheating of roof coverings, both reflectance and emissivity should be at a very high level. According to tests carried out on Cool-R Highly Reflective Waterproofing Coating*, the reflectivity of surface covered with this product is at the level of 85,7% (+/- 0,2), what means that over 85% of solar rays are reflected and do not contribute to overheating. At the same time emissivity of Cool-R is about 0,85 (+/- 0,03), which defines the ability to dissipate heat in the form of infrared radiation. Very high values of both parameters are resulting with the impressive Solar Reflectance Index (SRI) at the level of 107. This factor allows to maintain a low roof surface temperature.

For a better understanding of the importance of reflectance, emissivity and high value of SRI it is crucial to compare all those three factors with reference to typical roof covering materials.

Detailed values of compared parameters are presented in the table below:

*Test in accordance with the ASTM E1980-11 “Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces”

 

 

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