The biggest difference between U-factor and R-value is that U-factor measures the rate of heat transfer (or loss) while R-value measures the resistance to heat loss.

R-value is a measure of conductivity. A product with high conductivity will transfer heat quickly, like a hot pan on the stove or a single pane of glass on a cold day. U-factor, on the other hand, takes into account more than conductivity. It also is affected by the airflow around the window u factor and the emissivity of the glass.

Emissivity is the ability of a product to absorb certain types of energy (specifically infrared) and radiate that energy through itself and out of a room. A product with high emissivity, such as one pane of clear glass, will transfer over 84 percent of the infrared energy from a warm room outside to the cold air. The lower the conductivity and emissivity of the glass, the lower the rate of heat loss and the lower the U-factor.

There have been significant technological developments over the last 10 years involving low emissivity (low-e) coatings on the glass. There are now many glass products available with these low-e coatings, which are typically used in dual pane windows and insulating glass units.