Hi,
Here's more on the "U" Factor for windows: The U-Factor measures the rate of heat transfer. Therefore, the lower the U-Factor the lower the amount of heat loss and the better a window is at insulating.
A window that has high conductivity transfers hear quickly the same as a frying pan transfers heat quickly to scrambled eggs or as quickly as a single pane of glass on a cold day.
The U-Factor takes into account more than conductivity, it also is affected by the air flow around the window and the emissivity of glass.
Emissivity is the ability of glass to absorb certain types of energy, specifically infrared, and radiate that energy through itself and out into a room. A window with high emissivity, such as a single pane window, will transfer ±84% of the energy of a warm room outside to the cold air.
A window with low emissivity and low conductivity has a lower U-rating because that window has a lower the rate of heat loss.