The reflectivity of a radiant barrier is pretty easy to understand, the higher the reflectivity value, the more light energy(heat) the radiant barrier is able to reflect. If you look at different radiant barriers, from paints and sprayed on applications to foil rolls, you will quickly see that rolls of aluminum have much higher reflectivity values, making them more efficient than their counterparts.
But what about emissivity, why is this value so important?
Emissivity is basically the ratio of heat energy radiated by a material to the heat energy radiated by a true black body at the same temperature. Yea, I know thats a bunch of physics nonsense, so let me lay it down for you in layman’s terms. Basically, if we were to take a black object (theoretical) which reflects no light energy and absorbs all light energy that touches it, it would have an emissivity value of one (ε = 1). Any real object , weather it be black or not would have an emissivity value of less than 1.
So what this definition is really saying is that the darker an object is, the more light energy it will absorb. And subsequently, the lighter and more reflective a material is, the less light energy it will absorb. This why radiant barriers, even though they have little or no “insulative” properties, can effectively block out most of the radiant heat coming in or leaving a structure.