Thermal-Resistance Ratings: U-factor vs. R-value

U-factor and R-value measure heat transfer and heat resistance,respectively; understanding the difference will help you select energy-efficient windows and insulation.

Both U-factor and R-value are measures of a material’s insulating performance. The former is a measurement of heat transfer; the latter measures heat resistance.

U-factor is generally used to compare the performance of windows, U-factor first was invented by engineers and scientists as a measure of the rate at which heat flows through 1 sq. ft. of material. The direct opposite of an R-value, a lower U-factor rating means that less heat will be lost. U-factor ratings generally fall between 0.20 and 1.20. A window that has a U-factor of 0.40 or less is considered energy efficient.

R-Value was created as a way to sell insulation, R-value is a measure of resistance to heat flow.  The higher the number the greater the insulating value.  If you double the R-value heat loss is cut in half.  Regardless of the R-value a good installation technique is key.  For example if not installed correctly an R-19 insulation might only perform at a level of R-13.