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(At Wonder Windows we think this is very important and so should you!) 
This is a measurement the rate of heat transfer. How heat escapes from your home to the outside. Very important to Homeowners in the Northeast. Window U-Factor values can range from 0.25 to 1.25. Just remember The lower the U-factor, the better the window keeps heat in your home. The more heat kept in your home the lower your heating bill.

For Energy Star qualification in the Northeast a window must have:
a minimum U-Factor of ≤ 0.27
Obviously the lower the U-Factor the greater the cost of a window. LOWER IS BETTER