Low-e glass improves the insulating value of a window, roughly as much as adding an additional pane of glass.
Low-e glass combined with argon gas boosts energy efficiency by nearly 100% over clear glass. Like all gases, argon gas will leak from the window over time. However, it is non-toxic and safe. Studies suggest a 10 percent loss over the course of 20 years, but that will reduce the U-value of the window slightly.
The added cost for low-e glass and argon gas fillings is only about 10 percent of the window’s overall cost – it’s an easy decision!