Radiant barrier

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. 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 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. ==History==

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. ==Applications==

Space exploration 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. 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 Window treatments Window glass can be coated to achieve low emissivity or "low-e". Some windows use laminate polyester film where 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==

Roofs and attics When radiant solar energy strikes a roof, heating the roofing material (shingles, tiles or roofing sheets) and roof sheathing by conduction, it causes the underside of the roof surface and the roof framing to radiate heat downward through the roof space (attic / ceiling cavity) toward the attic floor / upper ceiling surface. 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 that 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. This is different from the "cool roof" strategy which reflects solar energy before it heats the roof, but both are a means of reducing radiant heat. According to a study by the Florida Solar Energy Center, a white tile or white metal cool roof can outperform a traditional black shingle roof with a radiant barrier in the attic, but the black shingle roof with a radiant barrier outperformed the red tile cool roof. For installing a radiant barrier under a metal or tile roof, the radiant barrier (shiny side down) should not be applied directly over the roof sheathing, because high contact area reduces the efficacy of the metallic surface as low emitter. Vertical battens (aka firring strips) may be applied atop said sheathing; then OSB with a radiant barrier may be put atop the battens. The battens allow more air space than construction without battens. If an air space is not present or is too small, heat will conduct from the radiant barrier, into the substructure, resulting in unwanted IR shower on lower regions. Wood is a poor insulator and so it conducts heat from the radiant barrier to lower surfaces of said wood, where it, in turn, sheds heat by emitting IR radiation. 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 as a facing for attics. For a traditional shingle/tile/iron roof, radiant barriers may be applied beneath the rafters or trusses and under the roof decking. This application method has the radiant barrier sheets draped beneath the trusses of rafters, creating a small air space above with the radiant barrier facing into the entire interior attic space below. Reflective foil laminate is a product commonly used as the radiant barrier sheet. Another method of applying a radiant barrier to a roof in new construction is to use a radiant barrier that is pre-laminated to OSB panels 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 a radiant barrier in an existing attic, it may be stapled to the underside of the roof rafters. This method offers the same benefits as the draped method in that dual air spaces are provided. However, it is essential that the vents 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 SHINY SIDE DOWN to the underside of the roof with an air space facing down; thus dust won't defeat it, as would be the case of a SHINY SIDE UP barrier. 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 homes with air-conditioning duct work in the attic in the hottest climate zones, such as in the US Deep South, could benefit the most from radiant barrier interventions, with annual utility bill savings up to $150, whereas homes in milder climates, e.g., Baltimore, could see savings about half those of their southern neighbors. On the other hand, if there are no ducts or air handlers in the attic, the annual savings could be even much less, from about $12 in Miami to $5 in Baltimore. Nevertheless, a radiant barrier may still help to improve comfort and to reduce the peak air-conditioning load. Shingle temperature One common misconception regarding radiant barrier is that the heat reflecting off the radiant barrier back to the roof has the potential to increase the roof temperature and possibly damage the shingles. Performance testing by Florida Solar Energy Center Attic dust accumulation When laying a radiant barrier over the insulation on the attic floor, it is possible for dust to accumulate on the top side. Many factors like dust particle size, dust composition and the amount of ventilation in the attic affect how dust accumulates and thus the ultimate performance of a radiant barrier in an attic. A study by the Tennessee Valley Authority mechanically applied a small amount of dust over a radiant barrier and found no significant effect when testing for performance. However, TVA referenced a previous study which stated that it was possible for a radiant barrier to collect so much dust that its reflectivity could be decreased by nearly half. It is not true that a double-sided radiant barrier on the attic floor is immune to the dust concern. The TVA study Both the American Department of Energy (DOE, Energy Efficiency & Renewable Energy Department) and the Ministry of Natural Resources (NRCAN) state that these systems are not recommended for cold or very cold climates. Canada Canada is considered to be a cold climate, so these products do not perform as promoted. Though they are often marketed as offering very high insulating values, there is no specific standard for radiant insulation products, so be wary of posted testimonials and manufacturers’ thermal performance claims. Research has shown that the insulation value of reflective bubble foil insulations and radiant barriers can vary from RSI 0 (R-0) to RSI 0.62 (R-3.5) per thickness of material. A study conducted by CMHC (Canada Mortgage & Housing Corporation) on four homes in Paris, ON found that the performance of the bubble foil was similar to an uninsulated floor. It also performed a cost-benefit analysis, and the cost-benefit ratio was $12 to $13 per cubic metre RSI. The effective insulating value depends on the number of adjacent dead air spaces, layers of foil and where they are installed. If the foil is laminated to rigid foam insulation, the total insulating value is obtained by adding the RSI of the foam insulation to the RSI of the dead air space and the foil. If there is no air space or clear bubble layer, the RSI value of the film is zero. ==See also==