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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.


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.