Low E Window Facts
In an insulated glass unit (IGU), the glass surface acts to absorb heat energy or shift that energy away in the form of moving air. Low E (low emissivity) coating applied to the glass surface helps to reduce the penetration of infrared and ultraviolet light. The protective layers, actually thinner than a human hair, act to prevent excess heat from entering the room in warm weather and also act to reverse heat loss from inside the home in the colder months.
Generally, light colored materials have a lower emissivity and dark colors a higher emissivity. Low E was developed to lower the amount of ultraviolet and infrared light that can pass through the glass. In turn, the amount of visible light is not compromised. Ultraviolet light is the wavelength that fades fabrics in the home. Heat transfer occurs with infrared wavelengths. Short-wave infrared is referred to as solar infrared. Long-wave infrared is produced by heat radiating from warm objects.
Two different coatings work in unison on the glass layers. Passive Low E coatings maximize solar heat gain to produce a desired heat gain in the home which reduces heating costs. Solar Control Low E coatings are made with air conditioning in mind. Solar Control greatly limits the amount of heat transfer. The two layers on the glass function simultaneously with Solar Control being the layer closest to the sun (#2 surface) and Passive being the layer furthest from the sun (#4 surface).