Understanding the Difference Between R-Values and U-Factors

Originally published by: Fine HomebuildingAugust 5, 2017

The following article was produced and published by the source linked to above, who is solely responsible for its content. SBC Magazine is publishing this story to raise awareness of information publicly available online and does not verify the accuracy of the author’s claims. As a consequence, SBC cannot vouch for the validity of any facts, claims or opinions made in the article.

Editor’s Note: The following article from the source listed above provides a brief overview of the difference between R-value and U-factor. For a more in-depth look at these two units and how they are calculated, read, ““Energy Code Math Lesson: Why an R-25 Wall is Not Equal to a R-20+5ci,” and “Wall Insulation Code Compliance: R U Confused Yet?

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.


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.


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.

Because doubling the R-value cuts heat loss in half, adding R-1 to R-1 will make a big difference; but adding R-1 to R-30 will reduce heat loss by only about 3%.

A good installation technique is the key to getting a good R-value. For instance, R-19 insulation installed incorrectly probably will perform at a level of R-13.