The ‘E’ in low-E refers to "Emissivity". Emissivity is a measure of a material’s ability to radiate energy. The lower the emissivity of a coating the better the glass performs in reducing heat transfer. Products with a low emissivity, or low-E coating, were specifically designed to reduce heat transfer in warmer and colder climates and reduce energy costs. Low-E glass works most efficiently in insulated glass units. This product is generally non-reflective and highly transparent.
The special coating on low-E glass reflects long wave heat radiation back into the room in winter conditions and for summer conditions reflects heat back outside. Low-E glass selectively allows specific portions of the sun’s energy or the ‘desirable’ wavelength components to be transmitted. When combined with spectrally selective tinted glass, low-E glass offers the ideal glazing solutions of low ultraviolet and high visible light transmittance, low shading co-efficient or solar gain and high insulation or ‘U’- values. This translates to a greater control of the internal environment, greater occupant comfort and lower heating and cooling costs.

Low-E glass is best utilized in double glazed units or IGU’s, though laminated and monolithic applications are possible with some interesting results (refer text). The surface position (#) on which the low-E glass will be placed is dependent on the climate.

Ideally for warm climates where summer heat reduction is a priority, the coating should be on surface (2) for maximum performance.

The coating on surface (2) minimizes heat gain because it reduces heat build up in the air gap and heat transfer to the interior. This coating should always be directly applied to a tinted glass as a soft coat application. The coating in this situation works in conjunction with the tinted glass to reduce heat gain. In these situations the substrate glass should be heat strengthened or toughened to avoid thermal breakage. Using surface (3) low-E or coating placement where cost is a major consideration, is effective for warm climate glazing with only marginal decreases in U-value and a higher shading co-efficient when compared to surface (2) coatings.

In cold climates where retention of heat is a priority, the low-E coating or panel glazed surface (3) is recommended.

In these situations we are relying on the effects of passive solar heat gain to naturally heat the building’s interior and the low-E coating to reflect any of the re-radiated heat back into the room