R-Value is a measure of thermal resistance used to compare insulating values. The higher the R-value number of a material, the better its insulating properties and the better the window will retain heat on a cold day or cooling on a hot day.

Many windows are rated in U-Factor (or U-value) or a measure of how well heat is transferred by a window, thus affecting heating and air conditioning costs. U-factor is the inverse of
R-value. The lower the U-factor, the better its insulating properties.