What R Value Is & How It Relates to K Value

R Value is a term typically used in the building industry to classify insulation performance. Its most common use is to compare insulating materials at low temperatures, such as those to which home insulation would be subjected. 

The R Value formula is:

In other words, the R-Value is the reciprocal of the K Value. Because the K Value term, BTU·in/hr ft²°F, is per inch of insulating material thickness, R = 1/K is also true.

However, to determine the K Value when the R Value is known requires that the insulation thickness also be known. Therefore:

Here are several examples of applying the formula:

Example 1

Example 2

Example 3