Low-E stands for Low Emissivity and is a fairly new advancement in glass technology. Basically, it’s a microscopic, metallic coating—applied to a surface of glass that reflects and re-radiates heat energy either into or out of a home depending on climate conditions.Using Low-E is an excellent way to increase the energy efficiency of a window. In fact, it’s so effective that every A1 window with Low-E is ENERGY STAR®-compliant in every region of the U.S.