What does the U-value of insulation mean?
Tuesday, July 23, 2019
U-value is a measure of thermal conductivity, and is the inverse of the more-often-used R-value, which is a measure of thermal resistance. A higher U-value of an insulation material or assembly means more thermal conductivity—in other words, less insulation value. And a higher R-value means just the opposite: better insulating qualities.
Actually, it’s not quite that simple because U-value is a more complicated evaluation that includes consideration of convection and radiation heat losses, but they are approximately the inverse of each other. So, when evaluating insulation, a lower U-value and a higher R-value are what you want to see.
Although U-value is often used in energy efficiency calculations, most people are only familiar with the R-value of insulation. The only place we commonly see a U-value is on the HUD data plate of a mobile home, because HUD standards are based on U-values.
Conversion to R-value from the data plate from an of an older mobile home shown above is simple. So, for example, the R-value of the insulation of the walls of this home is 1 divided by .048, which is approximately R-20. The ceilings would be 1/.058 = approximately R-17, and the floors are 1/.034 = approximately R-30.
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