How much insulation do you need? Answers to that question usually include a reference to R-value. Energy codes typically require builders to install insulation that, at a minimum, is rated at a certain R-value.  Most builders and homeowners have a straightforward understanding of R-value: “The higher the R-value, the better the insulation.” But what is the technical definition of R-value?

Simply put, R-value is a measure of a material’s resistance to heat transfer. R-value was first defined in 1945; before then, engineers described a material’s resistance to heat flow by referring to the material’s U-factor.

What is U-factor?

U-factor is literally a factor—it’s part of a mathematical formula involving multiplication. (We all remember factors from the 7th grade, right?) It can be found in the well-known heat loss formula used by designers to determine transmission losses through floors, roofs, and walls:  Q = A • ΔT • U

In other words, the rate of heat flow (Q) through a building assembly (in Btu/h) is equal to the area of the assembly (in ft²) times the ΔT (in F°) times the U-factor (in Btu/ft² • hr • F°). Delta-T (ΔT) is the difference between the outdoor temperature and the indoor temperature. For example, if the outdoor temperature is 20°F and the indoor temperature is 70°F, then the delta-T is 50 F°.

The U-factor was invented to make this formula work for the peculiar British (or Imperial) units we use in the U.S. It’s the number that you have to use to multiply a certain product (area times delta-T) in order to end up with a value we are interested in: Q, or the rate of heat flow in Btu/h.

U-factor is still useful; readers interested in learning more about the topic should read “All About U-Factor.”