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A radiant barrier is a reflective insulation system, which blocks heat as opposed to absorbing it, which is what traditional insulation does. When the heat of the sun beats down on a roof, it heats the surface and that heat has to go somewhere. Some of the heat is conducted through the structure itself. Once inside the house the hotter air rises and the cooler air sinks. Eventually the heat in the attic will move down into the rest of the building through radiation. A radiation barrier stops the heat from going down into the house by reflecting it back to where it came from.