The U-factor and R-value are both measures how much heat (or cold) can pass through a given piece of a building such as a window, wall or floor. The U-factor is the original measurement taken and is used to calculate the R-value. Let me explain…

The U-factor represents how much heat or cold can pass through an object so the higher the U-factor the *worse* the object (such as a window) is at insulating while the R-value represents how good the object is at insulating so the higher the R-value the *better* the object is at insulating.

The R-value is calculated using the U-factor’s reciprocal value. A reciprocal value is when you take a fraction and flip it. For example, a U-factor of 0.35 = 35/100 so it’s reciprocal R-value would be 100/35 = 2.9.

The R-value is a better presentation of the data than the U-factor because it increases with the quality of the window and the values are not small fractions. For this reason the R-value is used most often in labeling for windows, etc.

Reference: p. 33 “U-Factor and R-Value: What’s the Difference?” *Journal of Light Construction*, Vol. 31