Materials vary enormously in their ability to conduct heat. Those that do not conduct heat well are called insulators. R-Value is the term used to indicate a material's resistance to heat flow or ability to insulate. The higher the R-Value, the better the insulator. Most insulation materials work by trapping pockets of air, which is an excellent insulator. Fiberglass does this by creating air pockets between spun glass fibers, and foam insulation contains air bubbles. Similarly, double pane windows work by trapping air between the panes.
Among insulating materials, R-Values can vary widely. This is the reason it is important to purchase insulation by the R-Value and not by the inch. R-Values of different materials can be compared while thickness cannot. For instance, two materials rated R-11 have precisely the same insulating ability while two inches of each may not. Take fiberglass and brick as an example. To achieve R-30 with fiberglass batts requires 8.5 inches, while it would take 60 inches of brick!
The "Material R/Inch" chart shows how many inches of a certain type of insulation it takes to achieve a specified R-Value.
In the "R-Values" chart, you'll see that R-30 requires 14.5 inches of vermiculite, nearly 8.5 inches of fiberglass batt or only 5 inches of urethane foam. Such comparisons are helpful in selecting insulation types because the type you can use may be limited by the space available.