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Radiant Barriers Or Reflective Barriers

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Radiant Barriers Or Reflective Barriers Radiant Barriers or Reflective Barriers ―Radiant barriers or reflective barriers inhibit 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 or convection..‖ About Radiant barriers or Reflective Barriers 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 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. History In 1860, a French scientist named Jean Claude Eugene Peclet experimented on the insulating effect of high and low emissive metals facing air spaces. Peclet experimented on 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 a radiant barrier in reducing the transfer of heat. In 1925, German businessmen Schmidt and Dykerhoff filed for patents on reflective surfaces being used 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, CA. Plot no.221, Road No.2, Kathwada GIDC,Ahmedabad-382330.Website: www.patidarcorporation.com Applications Space Exploration For the Apollo program, NASA helped develop a thin aluminum metalized film, which 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, in addition to numerous other NASA projects like the James Webb Space Telescope and Skylab. In the vacuum of space where temperatures can range from 250 °F above to 400 °F below zero, 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. Textiles As far back as the 1970s, sheets of metalized polyester called space blankets have been commercially available as a means to prevent hypothermia and other cold weather injuries. Because of their durability and light weight, these blankets are popular for survival and first aid applications. Swarms of people can be seen draped in reflective metalized film after a marathon, especially where the temperatures are particularly cold, like during the annual ING New York City marathon which takes place in the fall. Window Treatments Windows glass can be coated to achieve low emissivity or ―low-e‖. Some windows use laminate polyester film wherein at least one layer has been metalized using a process called sputtering. Sputtering occurs when a metal, most often aluminum, is vaporized and the polyester film is passed through it. This process can be adjusted to control the amount of metal that ultimately coats the surface of the film. These metalized films are applied to one or more surfaces of the glass to resist the transfer of radiant heat, yet the films are so thin that they allow visible light to pass through. Since the thin coatings are fragile, and can be damaged when exposed to air and moisture, manufacturers typically use multiple pane windows. While films are typically applied to the glass during manufacturing, some films may be available for homeowners to apply themselves. Homeowner applied window films are typically expected to last 10–15 years. Construction Industries Roofs and Attics Radiant solar energy strikes the roof, heating the shingles, felt paper and roof sheathing by conduction, and causing the underside of the sheathing and the roof framing to radiate heat downward through the attic toward the attic floor. When a radiant barrier is placed between the roofing material and the insulation on the attic floor, much of the heat radiated from the hot roof is reflected back toward the roof and the low emissivity of the underside of the radiant barrier means very little radiant heat is emitted downwards. This makes the top surface of the insulation cooler than it would have been without a radiant barrier and thus reduces the amount of heat that moves through the insulation into the rooms below the ceiling. This is different from the cool roof strategy which reflects solar energy before it heats the roof, but both are means of reducing radiant heat. According to a study by Florida Solar Energy Center, a white tile or white metal cool roof can outperform traditional black shingle roof with a radiant barrier in the attic, but the black shingle roof with radiant barrier outperformed the red tile cool roof. Some roofing companies even offer the option of installing both a radiant barrier and a cool roof, for optimum performance. There are a wide variety of factors to consider in deciding whether or not to use a cool roof and/or a radiant barrier. For installing a radiant barrier under a metal or tile roof, the radiant barrier may be applied directly over the roof sheathing. Then furring strips (1x4s) are applied over the radiant barrier before the metal or tile roof is applied. The furring strips ensure that the radiant barrier faces into a sufficient air space. If an air space is not present or is too small, heat may be able to conduct through the radiant barrier. Since the metal in the radiant barrier is highly conductive, the heat transfer would all be through conduction and the heat would not be blocked. According to the US Department of Energy, ―Reflective insulation and radiant barrier products must have an air space adjacent to the reflective material to be effective.‖ The most common application for a radiant barrier is facing into attics. For a traditional shingle roof, radiant barrier may be applied over the rafters or trusses and under the roof decking. This application method has the radiant barrier sheets draped over the trusses of rafters, creating a small air space above with the radiant barrier facing into the entire interior attic space below. Plot no.221, Road No.2, Kathwada GIDC,Ahmedabad-382330.Website: www.patidarcorporation.com Another method of applying radiant barrier to the roof in new construction would be to use radiant barrier that is pre-laminated to OSB or roof sheathing. Manufacturers of this installation method often tout the savings in labor costs in using a product that serves as roof decking and radiant barrier in one. To apply radiant barrier in an existing attic, a radiant barrier may be stapled to the underside of the roof rafters. This method offers all the same benefits as the draped method, in that dual air spaces are provided. However, it is essential that the vents still be allowed to remain open to prevent moisture from being trapped in the attic. In general, it is preferred to have the radiant barrier applied to the underside of the roof with an air space facing down to prevent the accumulation of dust, preventing the radiant barrier from conducting. The final method of installing a radiant barrier in an attic is to lay it over the top of the insulation on the attic floor. While this method can be more effective in the winter there are a few potential concerns with this application, which the US Department of Energy and the Reflective Insulation Manufacturers Association International feel the need to address. First, a breathable radiant barrier should always be used here. This is usually achieved by small perforations in the radiant barrier foil. The vapor transmission rate of the radiant barrier should be at least 5 perms, as measured with ASTM E96, and the moisture in the insulation should be checked before installation.
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