A radiant barrier reduces the heat transfer from the underside of the roof to the other surfaces in the building like the deck substrate, duct work and ceiling materials. When the sun heats a roof, it's primarily the sun's radiant energy that makes the roof hot. Starting from the super heated roof surface (200 degrees), building envelope components then radiate gained heat energy to relatively cooler surfaces and spaces. The building interior is now penetrated and occupant comfort is sacrificed.