You're probably more familiar with radiant barriers than you think. Those shiny shields designed to protect car interiors from overheating on a sunny day are very effective radiant barriers.
An attic radiant barrier works the same way as an automotive one. During summer months, a radiant barrier reflects the sun's heat energy back toward the roof, helping to keep the attic cooler. In wintertime, the radiant barrier reflects heat from the house back toward the living space.
Energy savings with radiant barriers will be greatest in hot climates where air conditioning systems need to operate frequently. In these areas, a radiant barrier can cut cooling costs by as much as 17%.