Let’s revisit some 8th grade science for a minute for a little refresher on how heat goes from warm space to cold space, and how this relates to Radiant Barrier. Heat transfers via three different mechanisms; Conduction, Convection and Radiation.
The direct heat flow through a solid object like a wall or ceiling via surface to surface contact. A tea kettle sitting on a burner heats the water through this surface to surface contact.
The heat movement through air, occurring when air is warmed. The warm air expands, becoming less dense. The heater in your house heats the air by convection.
The movement of Infrared energy across air spaces from one warm object to another cooler object. Radiant heat has no temperature while traveling across the airspace but heats objects after striking them. Imagine the energy of the sun traversing the ice cold expanse of space then striking the earth and causing objects, like your roof, to heat up.
The heat we feel from a wood stove or a quartz space heater is radiant heat. ALL OBJECTS AND BODIES GIVE OFF RADIANT HEAT. Radiant Barrier REFLECTS radiant heat because 99% pure aluminum is 97% reflective and only 3% emissive. Fiberglass insulation only slows heat in the conductive and convective phases and eventually allows it through. The only way to stop RADIANT heat is by reflecting it. Enter Radiant Barrier.