Low E Glass: Window Sunscreen


The Simple Explanation of Low E Glass

Low e glass is the mainstay of energy efficient windows. Energy performance testing, certification, and labeling on windows is made up of these Energy Performance Ratings.

U-Factor, Solar Heat Gain coefficient, Visible Transmittance

Ringer Window Energy Performance Certificate

U-Factor, Fading Transmission, Solar Heat Gain coefficient, Visible Transmittance, Energy Performance Ratings for Windows

Energy Performance Rating Definitions

Here is a little energy lesson for you to better understand Low E Glass. I promise to describe and use very little math so don’t turn away yet. This is very simplistic to better help homeowners better understand energy efficient windows in laymen’s terms. So, forgive me students of science fair projects, physics students and professionals, my intentions are to simplify definitions to better help non-scientific consumers better understand windows.

Low E Glass, Low-E Glass, Low E Windows? Which is correct?

The window glass, not the window, has a low e coating to help reduce heat transfer to the other side. Low e coating is not a tint or film. It is part of a state-of-the-art process to coat the glass surface. Many people do searches on low e windows, which is fine, but low e glass is the window component that is Low E. Low-e glass with a hyphen is proper, but most people searching for windows search for low e glass (no hyphen). For search purposes, I’ll leave it as Low E glass, no hypen, so others can be educated on low e glass in windows. You scientific guys can stick to Low-E.

What is Low E Glass?

Low E stands for Low Emissivity. Emissivity refers to radiant energy sent out or through. Radiant Energy we refer to is light/heat from the sun, heat from a furnace. For windows, the emissivity refers to the radiant energy transfer through glass. Put this altogether – Low E Glass (in windows) allows optimized energy from the sun transferring through the glass into your home. Of course, in winter time we don’t want that precious heat emitted from the furnace to transfer out through the glass, so, in our case, Low E Glass works outside to inside and inside to outside.

You’re doing fine if you are still with me. Patience. We’ll get through this like childbirth. Painful, but the end result is beautiful. OK. I’ve gone too far.

Sunlight: UV, Visible, and IR Light

Yes, but, sunlight is broken down into 3 different light spectrums (ranges). Not all light spectrums are created equal.

Blue Chair with faded spots and faded blue stool

Chair & Stool exposed to sunlight with damaging UV thru windows without Low E glass.

UV Light

Ultraviolet – low spectrum light -not visible to the naked eye. I’m sure you’ve used sunscreen to keep from getting a sunburn when outside. When you put sunscreen on your body, you are blocking the UV rays that cause havoc to skin cells… These UV rays also fade fabrics and other materials by breaking down molecules (which I assume is what they do to skin cells). This is a bummer because we tend to have very expensive furniture, draperies, paintings, and other materials next to windows and you notice over time, the surface colors of these items fade.

Visible Light

This is the light we see. Red, orange, yellow, green, blue, indigo, violet and all visible between colors. We want as much of this light passing through our windows as possible. Except when bathing (At Ringer Windows we have additional glass options to help with obscuring that image for privacy).

Infrared Light or IR

This is heat – you can’t see it, but you can feel it. This is the heat energy. We want it to stay inside during the cold months in the winter. During the hot months, we do everything possible to keep it out. In Texas, this is most of our year.

OK, Now you’re thinking like a true scientist. Hang in there!

We Want Light to Come Through Our Windows

Here is the crux of issue with windows. Although windows are for letting in the light, we want only the visible light to come through our windows. In the summer time, we really want the heat (IR light) to stay out, and in the wintertime we want the heat to stay in. AND, we really don’t want or need the UV sunlight to come in because we don’t want our fabrics and paint to fade.

The Beauty of Low E Glass: It’s Like Sunscreen For Your Windows!

Low E glass contains two to three layers of silver nitrate (depending on which type is used) that are molecularly bonded to the glass. Low E glass is slightly darker than clear glass, but not by much. The Low E glass coating is micro-thin, never peels, and is nearly invisible. Its powerful component drives efficiency in all our replacement windows and new construction windows.

In summer, it’s what keeps the heat out & in winter, it helps keep the heat inside.

Low E Glass

Low E Glass in Summer

Low E Glass

Low E Glass in Winter

The superior low E glass in Ringer Windows blocks most harmful UV and infrared spectrums while allowing most of the good visible (desirable) light to pass.

You are doing fabulous! You’re almost to the end.

Ringer Windows Options for Low E Glass


Low E Glass spectral response for 366 glass in relation to other glass

Spectrally Smart 366 Low E Glass

Ringer Windows offers several Low E glass options, the basic Low-E 270(LoE2-270 Glass), the spectrally smart Super Low-E 366 (LoE3 366 Glass), and the Anti-glare 340 Low E Glass(LoE3 340 Glass).

The 270 low E glass blocks 86% of UV and IR light. This slightly lower cost glass is adequate for use in fully shaded or light protected or sheltered areas (you know if you have those areas).

The Spectrally Smart Low-E 366 (triple coated glass) stops a 95% of the harmful infrared and UV light, making it the most efficient Low-E glass we’ve ever seen!


240 Low-E Glass is recommended for those seeking a much darker look or serious glare control glass. In addition to being a great thermal low e glass, it also reduces the amount of visible light that passes through. LoE3 340 reduces the amount of visible light entering from outside producing a darker room. 340 Low E Glass is only recommended in rooms where you prefer a PERMANENT reduction in visible light such as media rooms or computer rooms. In reference to a call I received last week, at Ringer Windows we have NEVER had a full house window replacement job use ALL 340 low e glass.

Examine more Cardinal LoE Performance Stats.

low e windows, low e glass, energy efficient windows

Low E 270 Glass

low e windows, low e glass, energy efficient windows

Low E 366 Glass

low e windows, low e glass, energy efficient windows

Low E 340 Glass
*Great Anti-Glare*

Low E glass is the main ingredient to great energy efficient windows for your home. Select a super low e glass for maximum efficiency. Honestly, the cost difference is minimal and the energy efficiency is best with the best coating you can get.

Whew! You made it! I encourage questions and comments because your replacement windows investment is very important, and it is important you do your homework before buying replacement windows.

Call 512-989-7000 to visit our Showroom & come see our different low e glass for yourself!

3 windows left to right, low e glass 270, low e glass 366, low glass 240

Low E Windows From Left to Right: LoE 270, LoE 366, and LoE 340