Just a simple note about energy efficient glass. First, Low-E. Low-E stands for low emittance. Emittance is lowered by an invisible coating of metallic oxide layers on the glass panes. These layers suppress radiant heat flow out of the home by reflecting energy back into the house during winter season. During the summer, the energy is reflected away keeping the house cool. Of course, wanting to use the sun’s heat during the winter to warm up the house doesn’t work well with Low-E glass. “You can’t have it both ways,” is what my wife told me.
Next, ATG. In Low-E glass a gas called Argon is added. Argon does not have odor, color and is not flammable and is made by removing nitrogen and oxygen from air. Argon makes it difficult for warm or cold air to pass through and results in about a 40% gain in efficiency and is Energy Star certified.
Finally, ATG4. Like ATG, but instead of Argon, uses Krypton. Krypton gas is denser than Argon. Denser gas makes it more difficult for warm or cold air to pass. This also meets Energy Star standards.
Simplified and paraphrased from Glass On Web.