R-Value: It's More Than a Number
R-values were created in the 1970s as a measure of thermal value for insulation. More specifically, they measure the ability of a given material to restrict heat flow. The higher the R-value the less heat the insulation conducts. A good analogy by HGTV is to consider what would happen if you put a blowtorch on one side of tinfoil; you would feel the heat, but if you put that same fire on the other side of a wall board you wouldn't feel the heat. The wall board would have a higher R-value. The recommended R-value for any given building depends on where you are and what part of the building is being insulated. For example: The floor of a building in Southern Florida should only have R13 insulation while an uninsulated attic in Northern Michigan is should have R49-R60.
Although R-values are considered to be the standard measure for insulation, they are not the whole story when it comes to choosing insulation. The biggest problem with R-values is that they treat all materials the same and are not necessarily an accurate measure of how a given insulation material will work over time.
The Monolithic Dome Institute explains that the R-value test accounts only for heat transfer, not air flow or moisture resistance. What about when an insulation material is exposed to wind and moisture? If air can easily come through the insulation, it isn't as effective. A material that can absorb water or doesn't provide a seal against water vapor won't provide the same sort of protection when it's even slightly wet from normally occurring water condensation. How well an insulation works also depends on other factors such as how the building is constructed, what materials may be combined for insulation, and how carefully the insulation is installed.
The goals of insulation are to reduce heat loss and control surface temperature. The best way to do this is by creating a heat sink that keeps an average temperature by holding a limited amount of heat and then radiating it out as the temperature changes. Examples of building materials that do this are concrete, brick, and adobe. While these materials may not have high R-values, their ability to diffuse heat makes them better insulators than other materials such as fiberglass, cellulose fiber, or rock wool. Selecting a solid insulation makes sense not only from an ecological standpoint, because it utilizes recycled materials, but also because it provides superior protection compared with other insulation.
You can find more information at the Department of Energy's Energy Saver Page. Learn more on adding insulation to an existing house or insulating a new house. And remember that along with R-value, air sealing and moisture control are important to your home's energy efficiency, health, and comfort.