So what is this Low-E glass thing all about? Let’s break it down now. Low means “little” nor “no”. E stands for “emissivity”. Emissivity is the ability of something, in this case glass, to emit energy. I know, it sounds counter intuitive, and it kind of is. You see, it depends on which side of the glass you’re on. The emissivity is the amount of energy that is absorbed and emitted (later). So, this glass is going to absorb low amounts and emit low amounts later.
Here’s an even more confusing definition:
“Low-emittance (Low-E) coating are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow.” – http://www.efficientwindows.org/lowe.cfm
Again, think of the sun shining from outside of the window. The heat from the sun gets reflected by the metallic coating on the interlayer of the glass. In fact, Low-E glass targets infrared light, as that is what seemingly creates most of the heat in a building. It not only blocks radiant energy (light) from coming in, but it blocks energy from leaving the building.