Thermal emittance orthermal emissivity () is the ratio of theradiant emittance ofheat of a specific object or surface to that of a standardblack body. Emissivity and emittivity are bothdimensionless quantities given in the range of 0 to 1, representing the comparative/relative emittance with respect to a blackbody operating in similar conditions, butemissivity refers to amaterial property (of a homogeneous material), while emittivity refers to specific samples or objects.[1][2]
For building products, thermal emittance measurements are taken for wavelengths in theinfrared. Determining the thermal emittance andsolar reflectance of building materials, especiallyroofing materials, can be very useful for reducing heating and coolingenergy 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.[1]
High thermal emittance materials are essential topassive daytime radiative cooling, which uses surfaces high in thermal emittance and solar reflectance to lower surface temperatures by dissipating heat toouter space. It has been proposed as a solution toenergy crises andglobal warming.[3]
{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)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.
