If you sit next to a window and it is cold outside, your indoor air could be causing a breeze. Depending on the type of glass you have in your window, the effect is dramatic.
If the surface of your window is much colder than the room temperature, a circular convection current forms around the window. Room temperature air flows by the inner glass and gets colder. Colder air is heavier and actually drops to the floor. The warmer air on the floor below creates an upward movement of air, forming a circular movement around cold windows. The colder the inner glass, the stronger the drafts. This is even assuming the window had no drafts to begin with, which is usually not the case when the windows have not been replaced.
As a comparison, here are different types of glass and their inner temperatures assuming 0 degrees outside and a 15 mph breeze.
Since the lower temperature causes more drafts and convection of air, the Soft Coat Low E glass is well worth the money.
Ask your sales representative about other features on Colorado Window & Siding, Inc. windows that will further improve the insulation of your home.