A low-emissivity, or low-e, coating is a thin layer on glass designed to reduce transfer of long-wave thermal radiation. It can remain largely transparent to visible light, so a window still lets daylight in.
Visible light and thermal radiation differ
In a multi-pane window, the coating faces an interior cavity and reflects or emits less room-temperature infrared energy than untreated glass. Depending on the product, a coating can also control how much solar heat enters. Those are related but distinct design goals: keeping indoor heat in during a cold night and limiting solar gain on a hot afternoon.
The whole window counts
A low-e layer does not stop every form of heat transfer. Air leaks, frames, pane spacing and gas fill also shape the window’s performance. The appropriate solar and insulation characteristics depend on climate and placement. A coating is one carefully chosen part of a complete glazing system.
Related: how anti-reflective coatings reduce glare.