Low-e, or low-emissivity, windows have a coating of invisible metallic oxides that allows natural light through, but protects people and building interiors from damaging ultraviolet and infrared light. They control radiant heat, lowering utility costs in both cold and hot seasons.
Without a metallic oxide coat, windows are naturally emissive, with heat transferring easily through the glass.
The metallic oxide coat sits on the inside or outside of a single-pane window, or between the panes of a double-pane window. This microscopic layer acts as insulation by reflecting heat to the inside of a building or home.
In addition to improving heating and cooling efficiency, low-emissivity windows help homeowners avoid sunburn and faded furniture caused by incoming sunlight. They also reduce drafts and condensation build-up. Low-emissivity windows have some drawbacks, including reduced radio and cellular signal passage.
Two types of low-emissivity coatings are passive coatings and solar control coatings. Passive coatings are useful in cold climates, while solar control coatings are ideal for warm or hot climates. Several methods are used to apply these coatings, the most common of which is called pyrolytic CVD.
Low-emissivity windows were first created in the 1980s to keep homes warm in the winter. Emissivity building codes vary depending on region and climate.