Radiant barrier

Radiant barriers (also known as reflective insulation) are a type of thermal (heat) insulations that inhibits heat transfer by thermal radiation. Thermal energy may also be transferred via conduction or convection, however, and radiant barriers do not necessarily protect against heat transfer via conduction (without airspace facing the heat source) or convection (perforated).

There are many definitions of thermal/heat insulation and it is commonly misinterpreted as “Bulk/Mass/Batt Insulation”, which is actually used to resist conduction heat transfer with certain R-values.

Heat/thermal insulation is a barrier material which resists/blocks/reflects heat energy (either one or more of conduction, convection or radiation) to prevent its transfer through the boundary between two systems which are at different temperatures. Heat transfer always occurs from a region of higher temperature to one of lower temperature.

Radiant barrier (or reflective) insulation is heat/thermal insulation which reflects radiation heat (radiant heat), preventing transfer from one side to another due to a reflective (or low emittance) surface.

As such materials reflect radiant heat with negligible “R-values” they should also be classified as thermal/heat insulation.

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 emissivity (also called "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 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.

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.

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