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 fiber glass and cellulose,2.5 per inch for vermiculite, and more than 4per 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 hasan R-value of 0.6. The table below converts the most common "R" values to inches.