It is the resistance a material has to heat transference. The higher the R-Value, the better the insulating capability. 

It is the rate of heat transfer that results from a temperature difference across a window or the rate that your window loses heat. The smaller the U-Factor, the less heat transfer between the inside and outside due to a temperature difference. This is especially important during the heating season because of the vast temperature differences inside and outside - especially in Minnesota.

Radiation. This is the movement of heat as infrared energy passes through glass. Air Leakage. The passage of heated air through and around cracks and weather stripping. Conduction. The direct transfer of heat through the window to the outside. Convection. When air gives up its heat to cooler glass, the cooled air moves to the floor. The movement sucks new warmer air toward the glass that is in turn cooled, and moves to the floor, creating a draft.

In as little as a day, we can replace old, drafty windows. To reduce discomfort, during our window installation, only one window is removed at a time.  The new window has been prepped prior to insertion, immediately placed and sealed, minimizing heat loss.