Windows are rated by the National Fenestration Rating Counsel (NFRC) and carry labels that indicate overall U- and R-value ratings. 

A U-value is the measure of the amount of heat (in BTUs) that moves through a square foot of window in an hour for every Fahrenheit degree difference in temperature across the window. In other words, the lower the U-value, the better the overall insulating value of the window.

Insulation is measured in R-values. R stands for the ‘resistance’ of the transfer of heat or cold through a solid object. So, the higher the R-value the better the insulation.