To understand how Radiant Barrier works, it is important to remember that heat always gravitates to cold. If you are trying to keep your home cool in the summer, heat is trying to get in. If you are trying to keep warm in the winter, heat is trying to get out. Radiant Barrier reflects 95 percent of the heat back to its source regardless of the season.
Traditional insulation merely delays this transfer of heat.
The sun's radiant heat goes through solid objects like roofs and walls. The radiant heat enters an attic, warms up the air inside the attic and then hits traditional insulation like blown-in or batt fiberglass. This traditional insulation slows down the transfer of this heat from the attic to the inside structure, but does not stop it. How effectively it delays the heat is measured in a unit of resistance called R value.
Traditional insulation acts like a sponge, and as a result of it storing this radiant heat, the temperature in the attic can get much hotter than the outside ambient temperature. Worse, because this heat is stored in the traditional insulation, the AC unit can run day and night trying to combat the heat that is still entering the living area even thought the sun has set.
The Dept. of Energy estimates that homeowners can see up to a 50 percent reduction in energy bills by using Radiant Barrier with or without existing foam or fiberglass insulation.