Compare Low E2 Vs. Low E3 Windows

Residential Windows

by Emily Beach, Demand Media

Windows represent a major energy sink in the average home, with up to 30 percent of heated or cooled air lost through windows, according to the California Energy Commission. Low-emissivity coatings, which were first developed in the 1970s, promise to keep solar heat energy out of the home to improve the interior comfort and keep energy bills in check. Since these coatings were first introduced, glass manufacturers have added low-E coatings to multiple panes of glass within a single window to further improve energy efficiency.


Low-emissivity coatings consist of ultra-thin layers of tin dioxide, silver, or other metals applied to a pane of glass. Low-E windows are designed to reflect solar heat energy outdoors rather than allowing it to enter the home and introduce unwanted heat. These coatings, which are invisible to the naked eye, became fairly standard throughout the 1980s; by the 1990s, window makers were adding a second metallic coating to create low-E2 — or low-E squared — windows. In 2010, manufacturers introduced low-E3 — or low-E cubed — windows, which feature three coats of metal to further reduce solar heat transfer through the glass.

Visible Transmittance

Comparing low-E2 and low-E3 windows begin by comparing how much light is transmitted through the glass. Typically, the more layers of low-E coating applied to a window, the less natural light is able to enter the home. For example, a single pane of uncoated glass has a visible transmittance, or VT, rating of 90 percent, which means 90 percent of visible light enters through the glass, while 10 percent is reflected back out. A standard low-E2 window has a VT of 70 percent, compared to 66 percent for low-E3 windows, according to Milgard Manufacturing Inc.

The U-factor of a window measures how much heat is transmitted through the glass and is the inverse of the R-factor, which measures how well a surface resists thermal transfer. The lower the U-factor, the more efficiently the window blocks the passage of heat. Regardless of climate, a low-U-factor window outperforms one with a higher U-factor, according to the Green Building Advisor website. While a single pane of clear glass has a U-factor of 1.04, a low-E2 window measures 0.30, compared to 0.29 for low-E3 windows, according to Milgard Manufacturing, giving low-E3 windows a slight edge in this category.
Solar Heat Gain Coefficient

The solar heat gain coefficient, or SHGC, of a window, reveals the fraction of solar radiation admitted through a window. It ranges from 0 to 1, and the lower the coefficient, the less solar radiation is transmitted. A single pane of glass has an SHGC of 0.86, compared to 0.37 to 0.41 for low-E2 windows and 0.27 for low-E3 units, according to Milgard Manufacturing. In cold climate zones, homeowners should attempt to maximize SHGC to cut heating costs, particularly when choosing south-facing windows. In warmer climates, like California, a lower SHGC is more desirable to help cut cooling costs, particularly when choosing east- or west-facing windows. Green Building Advisor suggests aiming for an SHGC of 0.40 or lower in warm climate zones to maximize efficiency.