1. Radiation. This is the movement of heat as infrared energy passes through glass.
  2. Air Leakage. The passage of heated air through and around cracks and weather stripping.
  3. Conduction. The direct transfer of heat through the window to the outside.
  4. 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.