Radiant barrier

A radiant barrier is a type of building product that reflects thermal radiation and reduces heat transfer. Since thermal energy is also transferred via conduction and convection, radiant barriers are often supplemented with thermal insulation products that slow down heat transfer via conduction or convection. A radiant barrier is a type of building product that reflects thermal radiation and reduces heat transfer. Since thermal energy is also transferred via conduction and convection, radiant barriers are often supplemented with thermal insulation products that slow down heat transfer via conduction or convection. Radiant barrier reflects heat radiation (radiant heat), preventing transfer from one side to another due to a reflective, low emittance surface. In building applications, this surface is typically a very thin, mirror-like aluminum foil. The foil may be coated for resistance to the elements or for abrasion resistance. The radiant barrier may be one or two sided. One sided radiant barrier may be attached to insulating materials, such as polyisocyanate, rigid foam, bubble insulation, or OSB. Additionally, reflective tape can be adhered to strips of radiant barrier to make it a contiguous vapor barrier, alternatively, radiant barrier can be perforated for vapor transmittance. 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. 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. To perform properly, radiant barriers need to face open space (e.g., air or vacuum) through which there would otherwise be radiation. In 1860, the French scientist Jean Claude Eugene Peclet experimented with the insulating effect of high and low emissive metals facing air spaces. Peclet experimented with a wide variety of metals ranging from tin to cast iron, and came to the conclusion that neither the color nor the visual reflectance were significant determining factors in the materials’ performance. Peclet calculated the reduction in BTUs for high and low emissive surfaces facing into various air spaces, discovering the benefits of a radiant barrier in reducing the transfer of heat. In 1925, two German businessmen Schmidt and Dykerhoff filed for patents on reflective surfaces for use as building insulation because recent improvements in technology allowed low emissivity aluminum foil to be commercially viable. This became the launching pad for radiant barrier and reflective insulation around the world, and within the next 15 years, millions of square feet of radiant barrier were installed in the US alone.Within 30 years, radiant barrier was making a name for itself, and was included in projects at MIT, Princeton, and Frank Sinatra’s residence in Palm Springs, California. For the Apollo program, NASA helped develop a thin aluminum foil that reflected 95% of the radiant heat. A metalized film was used to protect spacecraft, equipment, and astronauts from thermal radiation or to retain heat in the extreme temperature fluctuations of space. The aluminum was vacuum-coated to a thin film and applied to the base of the Apollo landing vehicles. It was also used in numerous other NASA projects like the James Webb Space Telescope and Skylab. In the vacuum of outer space, where temperatures can range from −400 to 250 °F (−240 to 120 °C) heat transfer is only by radiation, so a radiant barrier is much more effective than it is on earth, where 5% to 45% of the heat transfer can still occur via convection and conduction, even when an effective radiant barrier is deployed. Radiant barrier is a Space Foundation Certified Space Technology(TM). Radiant barrier was inducted into the Space Technology Hall of Fame in 1996.