A radiant barrier is a single reflective surface that faces an open space. Its purpose is to reduce summer heat gain and winter heat loss, and it's usually installed in attics. Radiant barriers are always installed with the reflective surface facing on open air space.
Here's how this type of insulation works: Radiant heat moves in a straight line away from surfaces, heating anything solid that absorbs the energy. So when the sun heats your roof, this happens mostly because of radiant energy. A lot of this heat travels through the roof to the inside attic side, where it's radiated onto anything cooler, like the attic floor. A radiant barrier minimizes radiant heat transfer from the underside of the roof to other attic surfaces. It can effectively block 90 to 97 percent of radiant heat striking its surface, which translates to lower energy costs.
However, keep in mind that radiant barriers don't provide much thermal insulation.