More about heat transfer in attics
Radiant barriers and conventional insulation both reduce cooling bills by reducing the amount of heat that is transferred from the attic into the house. They differ in the way they reduce the heat flow. Conventional insulation traps air within the insulation, reducing heat transfer by air movement (convection). The insulation fibers or particles also partially block radiation heat transfer through the space occupied by the insulation. A radiant barrier reduces the amount of heat radiated across an open air space next to the radiant barrier. The performance of a radiant barrier depends on two material properties, the emittance and the reflectivity.
The emittance (also called the emissivity) is the property that determines how well a radiant barrier will perform. This property is a number between 0 and 1, with lower numbers indicating better potential for performance because the lower the emittance, the lesser the emitted radiation. It is not always possible to judge the emittance by visual appearance, as the heat transfer is longwave radiation and not visible radiation. There are materials than can appear to be shiny and aluminum colored, but may be coated such that they actually have high emissivity. Measured emittance values should be part of the information provided by the manufacturer.
A closely related material property is the "reflectance" (also called the "reflectivity"). This is a measure of how much radiant heat is reflected by a material. The reflectivity is also a number between 0 and 1 (sometimes, it is given as a percentage, and then it is between 0 and 100%). For a material that is opaque (that is, it does not allow radiation to pass directly through it), the emittance and reflectivity add up to one. Hence, a material with a high reflectivity has a low emittance, and vice versa. Again, the reflectivity of concern is in the longwave spectrum and thus how reflective it appears based upon the part of the light spectrum that humans can see is not applicable.
|Radiant barrier materials must have high reflectivity (usually 0.9, or 90%, or more) and low emittance (usually 0.1 or less), and must face an open air space to perform properly.
Interior radiation control coatings must have a low emittance (usually 0.25 or less), and must face an open air space to perform properly.
|The effective emittance and reflectivity of a radiant barrier in an attic environment may change if the surface gets dirty or is torn. If the surface is dirty, the performance and energy savings may be cut in half.