Most people are familiar with the term R-Value when discussing insulation properties. An R-Value is a measure of an insulation product's resistance to thermal heat transfer. It's a measure of how long it takes a heat "absorbing" product to absorb 100% of its potential and begin to transfer the heat out the other side of the product (referred to as thermal resistance). R-Values are calculated via a controlled test under the following conditions:
Isn't it odd that products designed to protect us from the heat are rated using a test that is performed under 75 degrees Fahrenheit with 50% humidity? An interesting fact is that as the temperature rises, the R-Value of an insulation material decreases.
Basically, a product with a higher R-Value is better than a product with a lower R-Value. A higher R-Value product takes longer to absorb 100% of it's potential and therefore, is more effective at "delaying" the transfer of heat. R-Values themselves don't mean much alone unless they are used to compare the effectiveness of different R-Value rated products.
Radiant barriers do not "absorb" heat. Instead, they retard heat flow by two means - by reflecting radiant heat away from its surface or by reducing the emission of radiation heat from its opposite side and therefore, have no R-value rating like bulk insulation products.
Installing a radiant barrier between the heat source (sun) and an existing R-Value rated insulation will improve the effectiveness of the R-Value rated insulation. Why? Because only 3% of the heat is now hitting the R-Value rated insulation and therefore, it will take longer for it to absorb 100% of it's potential thereby holding the heat longer before passing out the back side into living spaces.
In summary, you increase the R-Value of your existing insulation by installing a radiant barrier foil insulation product. Studies have found the following about radiant barriers to be true: