Both low-E windows and argon gas windows are designed to keep light and heat from going through the glass. However, the way that they go about it is slightly different. With low-E windows, a metallic additive is placed inside the glass when it is produced. With argon gas windows, argon gas is pumped in between the window panes. This is only possible with windows that have more than one pane. With low-E windows, something is actually inside the glass, while with argon gas windows, the argon gas is simply in between the different layers of glass.