U-factor, U-value and R-value

What is the difference between U-factor, U-value and R-value?

U-factor and U-value are interchangeable terms referring to a measure of the heat gain or loss through glass due to the difference between indoor and outdoor air temperatures. U-factor or U-value is also referred to as the overall coefficient of heat transfer. A lower U-value indicates better insulating properties. The units are Btu/(hr)(ft²)(°F).

R-Value equals a measure of the resistance of the glazing to heat flow. It is determined by dividing the U-value into 1, (R-value = 1/U-value). A higher R-value indicates better insulating properties of the glazing. R-Value is not typically used as a measurement for glazing products. Both U-value and R-value are a measure of resistance to heat flow and are referenced here to help understand U-value.