U-Factor (U):
A window's U-factor rating is its singular most important rating. It is the rating that organizations such as Energy Star use to qualify windows as "Energy Efficient" windows. U-factor is a measurement of the number of BTUs (British Thermal Units) that pass through a square foot of a window every hour. The lower the U-factor the less BTUs pass through the window. The U factor measures the energy efficiency of the entire window. 


A Dishonest Sales Trick:
Most windows have a center of glass U-factor. Some salesmen point to their decent center of glass U-factor but don't mention their  whole window U-factor. Be sure to know which number your salesperson is giving you, because there is a difference and it is an important one. 


Insulated Sash & Frame (U) (CR):
Vinyl windows have empty cavities inside the frame and sash. Just like the walls of your home, these cavities can be left empty or filled with insulation. A window with an insulated sash and frame will do a better job insulating than a window without insulation and in doing so will significantly decrease a window's U-factor.


Air is Not a Great Insulator:
Some companies will try to convince homeowners otherwise.
But you would never leave your walls without insulation; so should it be with your windows. 
Need proof? Just check out the difference insulation makes in a window's U-factor!

Non-Metal Glass Spacers:
A glass spacer sits between 2 or more pieces of glass to allow space for an insulating pocket of air or argon gas. Non-metal or steel glass spacers conduct less heat than aluminum spacers and significantly reduce the risk of seal failure when expansion and contraction happens due to temperature changes. 


Inert Gas:
For years the gap between two or more panes of glass were filled with air. Today's energy efficient windows are filled with either Argon or Krypton gas to further reduce thermal transfer between the airspace. Argon is the more popular and most cost-effective option. Krypton is a better insulator but due to increased demand in the window and other industries, it has greatly increased in price. These gases are invisible to the naked eye, but greatly reduce a window's U-factor.