Why R-values are used
Now let’s discuss the use of thermal resistance (R-values) to compare insulating properties. It’s important to understand that no scales or units for thermal resistance have been named or established.* R-values are simply numbers obtained by dividing 1 by k or C. Well then, what good are R-values? Why use them at all?

The answer is that it is easier for most people to compare insulation properties using R-values than using k or C values. This is simply because R-values usually always include whole numbers, whereas k and C values are usually always fractional numbers less than one.

Most people have difficulty comparing three place decimals for values less than one. So, dividing 1 by k and C nearly always produces some whole numbers. (We can just round off any fractions to the nearest whole number.) Consequently, it’s easier to understand that one insulation 6 inches thick with an R-value of 22 is twice as effective as another insulation only 3 inches thick with an R-value of 11. The C values for those two insulations are 0.045 and 0.090 respectively-not easy to compare. But no matter whether you compare C values or R-values of the two, one is twice the value of the other, so their ratios are the same. It’s just easier to compare whole numbers.

Precautions for using R-values
Now there are some important precautions for comparing R-values. If you go to a building supply store and shop for insulation you will likely see R-values printed on the packaging (see Figure 7). These will enable you to readily compare one insulation with another. The differences are due mainly to different thicknesses. But please be aware that these R-values are all for the building trade, so comparisons are valid only for that industry. The same is true for engineering handbooks that list properties of various materials used in the building industry.

Figure 7. R-values on packages of insulation at a building supply store.

Consequently, that same insulation would have much lower R-values when used on an asphalt tank. That’s because temperatures encountered on asphalt storage tanks are radically different from temperatures encountered in the building trade. Thus, the R-values shown on the packaging at a building supply store are not valid for our industry.