The R-Value is the thermal resistance of a glazing system measured over 1 hour through a 1 sq. ft. area of the glazing unit for each 1° F temperature difference between the indoor and outdoor air. Essentially, it is a measure of resistance to heat gain or loss (insulative ability). The higher the R-value, the less heat is transferred through the glass.

R-Values rather than thicknesses can be compared for different materials, since 6″ (15.24 cm) of fiberglass (R-19) might compare with 12″ (30.48 cm) of wood or 18″ (45.72 cm) of stone.