What does the U-Factor of a vinyl window mean?

What does the U-Factor of a vinyl window mean?

The U-Factor measures the rate at which heat transfers and is an indicator of how well a window insulates.

The lower the U-Factor the better the window insulates. According to Energy Star, U-Factor values generally range from 0.25 to 1.25.

U Factor
ecoshield
Ecoshield Windows