Radiant barrier
From Wikipedia, the free encyclopedia
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.
The main problem with placing the radiant barrier on top of the insulation, is that dust will eventually reduce the benefit. It is better to staple the foil to the rafters or use radiant barrier paint. Even if you can not cover the entire underside of the roof, where ever the paint or foil is used, it will reduce the heat gain to the attic.
[edit] External links
- This entry incorporates public domain text originally from the Oak Ridge National Laboratory and the U.S. Department_of_Energy. [1]
