Radiant barriers or reflective barriers work by reducing heat transfer by thermal radiation. All materials give off, or emit, energy by thermal radiation as a result of their temperature. The amount of energy emitted depends on the surface temperature and a property called the emissivity (also called the "emittance"). The emissivity is 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 a number between 0 and 1 (or a percentage between 0 and 100%). At a given wavelength the emissivity and reflectivity values sum to 1.
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 microns.
Radiant barriers may or may not exhibit high visual reflectivity. This is because while reflectivity and emissivity must sum to unity 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 unity. Thus, it is possible to create visibly dark colored surfaces with low thermal emissivity.
Radiant barriers must face an open air space to perform properly.
On a sunny summer day, solar energy is absorbed by a roof, heating the roof sheathing and causing the underside of the sheathing and the roof framing to radiate heat downward toward the attic floor. When a radiant barrier is placed directly underneath the roofing material incorporating an air gap, 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.
On residential homes radiant barrier is typically installed one of two ways: Radiant Barrier Decking or Radiant Barrier Attic Foil.
New Construction: A product called Radiant Barrier Decking is used. This product is made by laminating a highly reflective piece of aluminum foil to one side of OSB board or plywood. The foil side will face the attic which creates the required air space. It is important to not that if the structure is located in a humid area, the radiant barrier should be perforated to ensure proper passage of moisture. Often times pre-laminated radiant barrier sheathing will have vapor barrier properties because the lamination adhesive will fill in the perforations. It is often better to use a perforated raflect radiant barrier and staple it to the roof decking before installing it on the rafters.
Existing Homes: Radiant Barrier Attic Foil is installed inside the attic. This product is a tarp-type material with a layer of aluminum foil laminated on both sides to create a double-sided radiant barrier. The radiant barrier foil material can be either stapled to the bottom of the roof rafters or laid out over the existing insulation. Both methods provide the required air space. If put over existing insulation, care must be taken to insure moisture is not trapped in the insulation by using a perforated product that is NOT a vapor barrier. Radiant barrier foil can either be installed by a professional installer or as a do-it-yourself project.