New windows have high tech low-E glass that reflects heat waves, which prevents heat loss in the winter and keeps heat out in the summer. Windows that are filled with Argon or Krypton gas:

• Argon and Krypton gas are both harmless, denser then air, gasses that insulate better.
• An added benefit is sound dampening- Argon for example can reduce unwanted sound from entering your home by 3-4 decibels.