Heat travels from a warm area to a cool area by a combination of conduction, convection, and radiation. Heat flows by conduction from a hotter location within a material or assembly to a colder location, like the way a spoon placed in a hot cup of coffee conducts heat through its handle to your hand. Heat transfer by convection occurs when a liquid or gas (air, for example) is heat, becomes less dense, and rises. As the liquid or gas cools it becomes denser and falls.
Radiant heat travels in a straight line away from any surface and heats anything solid that absorbs its energy. When the sun heats a roof, it’s primarily the sun’s radiant energy that makes the roof hot. Much of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates its gained heat energy onto the cooler attic surfaces, including the air ducts and the attic floor. A radiant barrier reduces the rate heat transfer from the underside of the roof to the other surfaces in the attic. A radiant barrier works best when it is perpendicular to the radiant energy striking it. Also, the greater the temperatures difference between the sides.