What Is Low-E Glass?
Larry Wachtel, JELD-WEN architectural consultant, clears up Low-E Confusion:
Not all Low-E coatings are created equal, nor is the Low-E coating on glass units manufactured today the same as it was as recently as two or three years ago. Advances in glass coatings technology and stronger regional energy code requirements have helped to create a new generation and more 
sophisticated array of Low-E glass options. Choosing the best option for your next project could now be compared to pairing the perfect wine with your entrée at a new restaurant. So how do we know which Low-E is best suited for that next project? Let’s begin by understanding just what Low-E is and how it works.
Low-E, meaning ‘low emissivity’ is an extremely thin layer of metallic particles, or more commonly several layers, applied to the glass which, in simple terms, allows the glass to act like a sieve. Long wavelengths, or heat, are filtered out, while short wavelengths (the visible light spectrum) are allowed to pass through. However, Low-E is is much more than that today. By changing the types of materials used in the ‘stack’ or layers of Low-E, or by increasing or decreasing the number of layers, we can now get more specific in choosing glass which will meet our exact project needs. Need high visible light but low U values? There’s a Low-E for that. Need greater protection from fading? There’s a Low-E for that. And it gets even more specific than that. Adding argon gas to the captive air space, as we all know, will improve insulating value. Adding various tinting agents to the glass itself will allow for even further refinement of the glass’s performance.
Low-E is not the same as tinted glass. Tinting is the adding of alloying materials to the glass itself. The depth of color of tinted glass will change with glass thickness so that a sheet of 3mm glass will have a lighter tint than that of a sheet of 6mm glass. Beware of that. Small windows next to large fixed units or doors can have different tints, since the standard glass thickness of smaller panes is typically thinner than that of larger ones. Low-E, on the other hand, is applied to the glass, and therefore will have a similar appearance regardless of glass thickness. Also, tinted glass tends to absorb sunlight and will get very hot when installed as a single pane, hence tinting does not improve insulating value.
Solar Heat Gain can also be controlled by the use of Low-E coatings combined with the use of tints, and can even be influenced by which glass surface the Low-E coating is placed upon. Additionally, since less than ½ of the total solar energy spectrum is visible to the human eye, solar performance of glass can be visually deceptive. Darker tints don’t necessarily mean significantly better SHGC values. For instance, green tinted glass will allow 77% visible light transmission, while grey glass only allows 45%, yet the grey glass only improves SHGC by 2 percentage points! A better way to improve SHGC, without compromising visible light transmittance, is through SHGC specific Low-E coatings. The chart to the right compares clear, tinted and some of Cardinal Industries Low-E options.
Perhaps one of the least often discussed elements regarding glass performance is the comfort level of the occupants. If the inside glass temperature of an insulated unit is significantly lower than the room temperature, it can give the occupants the feeling that the room is colder than it actually is. At 0 degrees F outside, the inside surface of double pane glass can be as much as 30 degrees warmer than single pane glass but still 25 degrees lower than Low-E coated glass. The converse can be true during hot summer months. Low-E has the ability to keep the temperature of the surface of the glass facing the interior very near that of the room itself, regardless of outside temperatures. Low-E squared glass can keep the inside glass temperature within 1 degree F of a room with no windows.
This is but a small sampling of how specific the new generation Low-E coatings can get. For more information visit Cardinal Industries.