In order to understand how and why radiant barriers are so effective at reducing heat transfer, you must first understand how heat transfer actually works.
How Heat Transfer Works:
There are three basic types of heat transfer from least efficient to most efficient; conduction, convection and radiant. I am going to use a kitchen analogy to help explain the different types of heat and the different methods of controlling them.
Conduction is the least efficient method of transferring heat and in our kitchen analogy, that would be an equivalent of boiling water in a pot over a stove. The fire heats the pot and the pot transfers its heat to the water and the water eventually comes to a boil. The key point here is that conduction is the transferred through direct physical contact.
Convection is more efficient than conduction, and in our kitchen analogy would be the use of an oven to heat some food. Convection heat transfer is the process by which heat is circulated in an area through the air and/or fluid. The key point here is that heat is transferred through air/fluid circulation.
Radiant heat otherwise known as thermal radiation is the most efficient type of heat transfer and the best example in our kitchen analogy would be the use of a microwave. Radiant heat is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation. In a home or building, radiant heat refers to heat radiated from materials, incandescent lights and the sun. It is also important to note that radiant heat moves through the air without heating the air, instead it will heat the first surface that it touches, and this surface will then re-radiate that heat in the space making it feel warmer when you are in that space. This principle causes interior spaces to feel hotter in the summer and reverse during winter months where interior heat is radiated towards the outside of the structure.
The following images help to explain the difference between conductive, convective and radiant heat transfer.