Air Leakage (AL):

Did you know even a new window will leak air? Anywhere a window has moving parts it will have a small gap to allow the window to operate. But some windows have larger gaps than others, and the leaking air can be costly as your temperature-controlled air from the inside of your house leaks outside.

Air leakage ratings are represented by the number of cubic feet of air that can exit a window every minute.

The industry standard calls for windows with an air leakage number of no greater than .30, which translates into 2.25 gallons or 24 soda cans of air going through each window every minute. Some windows offer much lower air leakage ratings such as .02. 

Air escaping through windows and doors can account for up to 35% of heating/cooling loss.
According to DoItYourself.com