Low-E (low-emissivity) glass is manufactured by applying a Low-E coating, a microscopically thin, virtually invisible, metal or metallic oxide layer, directly on the surface of a glass pane. Low-E glass allows visible light to pass through it but blocks heat-generating ultraviolet light, requiring less artificial heating or cooling to keep a room at the desired temperature.
Window manufacturers apply Low-E coatings in either soft or hard coats. Soft Low-E coatings degrade when exposed to air and moisture, are easily damaged, and have a limited shelf life. Therefore, manufacturers carefully apply them in insulated multiple-pane windows. Hard Low-E coatings, on the other hand, are more durable and can be used in add-on (retrofit) applications. The energy performance of hard-coat, Low-E films is slightly poorer than that of soft-coat films.
The reduction in energy loss can be 30% to 50%. Low-E windows are a larger investment initially but will pay for themselves by reducing heating and cooling costs.
Thus assembling Low-E glass panes into an insulating glass unit can increase significantly thermal isolation of a building and consequently save energy and reduce costs.