Thermal emittance

Thermal emittance or thermal emissivity (<math>\varepsilon</math>) is the ratio of the radiant emittance of heat of a specific object or surface to that of a standard black body. Emissivity and emittivity are both dimensionless quantities given in the range of 0 to 1, representing the comparative/relative emittance with respect to a blackbody operating in similar conditions, but emissivity refers to a material property (of a homogeneous material), while emittivity refers to specific samples or objects.<ref name=":0">Eco-efficient materials for mitigating building cooling needs : design, properties and applications. Pacheco-Torgal, F., Alchapar, N. L. Amsterdam [Netherlands]. 27 February 2015. ISBN 978-1-78242-401-7. OCLC 905919280.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)</ref><ref>"The Difference Between Reflectance and Emittance". www.buildingenclosureonline.com. Retrieved 2020-10-24.</ref>

For building products, thermal emittance measurements are taken for wavelengths in the infrared. Determining the thermal emittance and solar reflectance of building materials, especially roofing materials, can be very useful for reducing heating and cooling energy costs in buildings. Combined index Solar Reflectance Index (SRI) is often used to determine the overall ability to reflect solar heat and release thermal heat. A roofing surface with high solar reflectance and high thermal emittance will reflect solar heat and release absorbed heat readily. High thermal emittance material radiates thermal heat back into the atmosphere more readily than one with a low thermal emittance. In common construction applications, the thermal emittance of a surface is usually higher than 0.8–0.85.<ref name=":0" />

High thermal emittance materials are essential to passive daytime radiative cooling, which uses surfaces high in thermal emittance and solar reflectance to lower surface temperatures by dissipating heat to outer space. It has been proposed as a solution to energy crises and global warming.<ref name=":5">Chen, Meijie; Pang, Dan; Chen, Xingyu; Yan, Hongjie; Yang, Yuan (2022). "Passive daytime radiative cooling: Fundamentals, material designs, and applications". EcoMat. 4. doi:10.1002/eom2.12153. S2CID 240331557. Passive daytime radiative cooling (PDRC) dissipates terrestrial heat to the extremely cold outer space without using any energy input or producing pollution. It has the potential to simultaneously alleviate the two major problems of energy crisis and global warming.</ref>