Guillermo writes from somewhere in Chile: "I know that the "U" value is reciprocal to "R" Value. I know the "R" value, but how do I calculate the "U" Value?"

Answer: The answer to your question can be easy or complicated, depending on whom you ask and the level of detail and accuracy you need or want. For example, here is a formula I found online: U = 1/ {(1/ho) + (l/ k) + (1/hi)}.

A simpler answer is to add up the R-values of materials, airfilms and air spaces from one side of the assembly to the other, then invert this number to obtain the U-value.

U-value indicates the thermal conductivity of a system, such as a wall assembly, while R-value indicates the thermal resistance of an individual material or component of the wall system.

R-value is the measure most commonly seen on insulation products. The higher the R-value the better the insulating performance will be. A well-insulated wall will typically incorporate R 13 to R 21 insulation. Well-insulated attics and cathedral ceilings may have R 30 to R 49.

You can find more information online at various Websites, including CLEAR (Comfortable Low Energy Architecture), a website hosted in the U.K. to help design energy efficient buildings, and Wikipedia, the free online encyclopedia. The Clear website explains the formula listed at the beginning of this answer.