Radiant barrier

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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). The higher the emissivity, the greater the emitted radiation.

A closely related material property is the reflectivity (also called the "reflectance"). This is a measure of how much radiant heat is reflected by a material. The reflectivity is also a number between 0 and 1 (or a percentage between 0 and 100%). For a material that is opaque (that is, it does not allow radiation to pass directly through it), when the emissivity and reflectivity are added together, the sum is one (1). Hence, a material with a high reflectivity has a low emissivity, and vice versa. Radiant barrier materials must have high reflectivity (usually 0.9, or 90%, or more) and therefore low emissivity (usually 0.1 or less), and 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 on top of existing attic insulation, with the reflective side up, much of the heat radiated from the hot roof is reflected back toward the roof. 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.

One significant problem with placing the radiant barrier on top of the insulation, is that dust will eventually reduce the benefit. One solution to this problem is to use an approximate inch layer of Radiant Barrier Chips (RBC) on top of the reflective film. The dust falls on the chips, coating only the top sides, whereas the infrared light eventually hits the lower reflective surfaces of the chips and/or the reflective film, and is reflected back out. The chips are simply blown into place in a manner similar to blown insulation. Another method is to staple the foil to the rafters, which limits dust accumulation somewhat, or use radiant barrier paint. There must be air flow from the eve vents up through the rafter channels to ensure that condensation doesn't occur and to keep temperatures down so that the wood is not damaged. Insulating rigid-foam rafter channel stapled inserts are made that have a reflective side also. This gives three aspects to the installation: the airflow, the radiant barrier, and the substantial insulation of rigid foam. Incorporating a roof ridge vent with rafter channel air flow is an efficient method of controlling heat gain. Even if you cannot cover the entire underside of the roof, where ever these items are used, it will reduce the heat gain to the attic.