# Understanding the Difference Between R-Values and U-Factors

**Originally published by**: Fine Homebuilding — August 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.

**U-factor**

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**

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.