Reflectivity and Emissivity
All materials give off, or emit, energy by thermal radiation as a result of their temperature. The amount of energy radiated depends on the surface temperature and a property called the emissivity (also called the "emittance"). Emissivity is expressed as a number between zero (0) and one (1) at a given wavelength. The higher the emissivity, the greater the emitted radiation at that wavelength. A related material property is the reflectivity (also called the "reflectance"). This is a measure of how much energy is reflected by a material at a given wavelength. The reflectivity is also expressed as a number between 0 and 1 (or a percentage between 0 and 100%). At a given wavelength and angle of incidence the emissivity and reflectivity values sum to 1 by Kirchhoff's law.
Radiant barrier materials must have low emissivity (usually 0.1 or less) at the wavelengths at which they are expected to function. For typical building materials, the wavelengths are in the mid- and long- infrared spectrum, in the range of 3 - 15 micrometres.
Radiant barriers may or may not exhibit high visual reflectivity. This is because while reflectivity and emissivity must sum to 1 at a given wavelength, reflectivity at one set of wavelengths (visible) and emissivity at a different set of wavelengths (thermal) do not necessarily sum to 1. Though, it is possible to create visibly dark colored surfaces with low thermal emissivity.
To perform properly, radiant barriers need to face open space (e.g., air or vacuum) through which there would otherwise be radiation.
Read more about this topic: Radiant Barrier