Editing Radiant barrier (section)

==Reflectivity and emissivity==
All materials in existence 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 [[emissivity]] (also called "emittance"). Emissivity is expressed as a number between zero and one at a given wavelength. The higher the emissivity, the greater the emitted radiation at that wavelength. A related material property is [[reflectivity]] (also called "reflectance"). This is a measure of how much energy is reflected by a material at a given wavelength. 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 of thermal radiation|Kirchhoff's law]].{{citation needed|date=June 2018}}

Radiant barrier 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 [[Electromagnetic spectrum|spectrum]], in the range of 3-15 micrometres.{{citation needed|date=June 2018}}

Radiant barriers may or may not exhibit high visual reflectivity. 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. Therefore, it is possible to create visibly dark colored surfaces with low thermal emissivity.{{citation needed|date=June 2018}}

To perform properly, radiant barriers need to face open space (e.g., air or vacuum) through which there would otherwise be radiation.<ref>[http://www.rimainternational.org/pdf/ftc-letter.pdf FTC Letter], Regarding reflective insulation used under slab where no air space is present</ref>