R-value

A measure of a material's resistance to heat flow in units of Fahrenheit degrees x hours x square feet per Btu. The higher the R-value of a material, the greater its insulating capability. The R-value of some insulating materials is 3.7 per inch for fiberglass and cellulose,2.5 per inch for vermiculite, and more than 4 per inch for foam. All building materials have some R-value. For example, a 4-inch brick has an R-value of 0.8, and half-inch plywood has an R-value of 0.6. The below table converts the most common "R" values to inches. For other "R" values, divide the "R" value by 3 to get the number of inches.
"R"-Value Inches 3 1 11 3.5 19 6 52 18

This designates the resistance of a material to heat conduction. The greater the R-value the larger the number.