Low-E
A microscopically thin metallic coating on glass that reflects heat while letting light through.
Low-E (low-emissivity) is an invisible metallic-oxide coating bonded to window glass that reflects radiant heat back where it came from while still letting visible light pass. In winter it bounces your furnace's heat back into the room; in summer it reflects the sun's heat away — so the same glass cuts both heating and cooling loss. Low-E is the single biggest reason a modern double-pane window outperforms old clear glass, and it's also available on [low-e storm windows](/glossary/storm-window) that deliver much of the benefit at a fraction of replacement cost. It lowers a window's [U-factor](/glossary/u-factor) and lets you tune solar heat gain for your climate. See whether new windows are [worth the cost](/guides/are-replacement-windows-worth-it).