U-Value is the inverse of R-Value, or the coefficient of heat transfer through a material. This means that we measure the temperature on the cool side of something like a wall assembly and compare it against the temperature on the warm side. We're finding out how much heat is passing through the assembly by doing this measurement.

U-Value = 1 / R-Value.

IMPORTANT: We want the LOWEST U-Value that we can afford in our building. Less U-Value is better! Again, insulators like fiberglass and foam have low U-Value compared to conductors like metal and stone.

It's also important to know that U-Value can be calculated for any material based on it's conductivity and its thickness.

The metric measurement of thermal conductivity is Watts per metre Kelvin or W/(mK) and the Imperial measurement is Btu per hour foot Farenheit or Btu/(hr ft F).

We need to know a bit of math to get to the U-Value of a material or assembly, and we'll see some symbols show up (gulp!) that come from physics (double gulp!). The Greek symbol for thermal conductivity is Lambda: