A whole new but familiar concept

For me, the discovery that radiant energy was a big part of the picture was something big and exciting, to say the least. I was starting to gain an intellectual understanding of something that my body had understood at a basic level for a very long time. I had, of course, felt radiated heat before, and have since learned to recognize it. All of us, from the simplest animals to human beings, know to stay out of the sun when it's hot. Why? Because we heat up in the sun. But few of us actually understand why that is. Why do we heat up in the sun? Because we are absorbing radiant energy. It is not because the air is any hotter in the sun; the air in the direct sunlight, and the air in the shade right next to it, are almost the exact same temperature. Instead of the air molecules bumping into us and heating us up (as most of us learn about in elementary science), a bombardment of photons, a.k.a. radiant energy from the sun, is impacting our bodies and causing them to heat up.
What this means for energy efficiency in the summertime in a sunny climate is this: if you're addressing only the hot outside air, which can affect your home only through the mechanisms of convection and conduction, you are acting like the lone cow out in the field standing in the sun on a 100 degree day, while the rest of the herd is standing in the shade: that is, you are ignoring the most important part of keeping cool.
So wouldn't it be great if there were a way to bounce that bombardment of photons right back into the sky where it came from? Wouldn't that significantly reduce the amount of energy being absorbed into a house, reducing the amount of electricity an air conditioner has to expend to remove that energy once it strikes something and becomes heat? Yes, and most definitively, yes! Enter the radiant barrier.
Officially, a radiant barrier is defined as any material that reflects 90% or more of the energy striking it, which implies that it absorbs less than 10%. There are different types of radiant barriers that you can place at various spots between yourself and the sun, including items as simple as lighter colored shingles, with the lighter color reflecting more of the sunlight than darker colors (although these don't tend to reach the 90% standard to be considered true radiant barriers) to silvered roof decking, to reflective paints, to large amounts of industrial-grade foil.