U-value and R-value

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

U-value refers to a measure of the heat gain or loss through glass due to the difference between indoor and outdoor air temperatures. U-value is also referred to as the overall coefficient of heat transfer. A lower U-value indicates better insulating properties. In Europe, U-value of glass & glazing is calculated in accordance with EN 673.

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, it is more commonly used to describe the thermal resistance of building materials such as insulation. Both U-value and R-value are a measure of resistance to heat flow and are referenced here to help understand U-value.