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Definitions:
R-value is the numerical reciprocal of U-value. That is:
R = 1/U and U = 1/R
For Example:
If the U-value is 0.1, then the R-value is 10 (R=1/0.1)
If the R-value is 12, then the U-value is 0.83 (U=1/12)
One Btu is the amount of energy required to heat one pound of water from 58.5 to 59.5 degrees Farenheit. This is roughly the equivalent to the energy in one kitchen match.
(Of course the metric system u-value is different. To convert, multiply our standard u-value by 5.6783)
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R-value is a numerical expression of a material's resistance to heat transfer. It is the numerical reciprocal of U-value and to understand these terms, it is necessary to first explain the U-value.
The U-value of a material is the number of British Thermal Units (Btus) that transfer through a 1 square foot area in one hour when the temperature difference between the two sides of the structure is 1 degree Farenheit. The smaller the U-value, the better the insulation properties of the material.
We can use U-values to help calculate a building's energy loss. U-values can be used in multiplication and division. Complex computer programs assist the professional energy rater in this process but the fundamentals are simple. An example:
What is the energy loss in Btus per hour through 100 square feet of wall if its U-value is 0.05 (R-value = 20), the inside temperature is 70 degrees and the outside temperature is 30 degrees?
Answer: 100 x 0.05 x (70-30) = 200 Btus per hour.
The reason that we use two different values, U and R, to express the same thing is that they allow us to perform different types of calculations. As we just saw, U values can be used in multiplication and division problems. But U-values cannot be added or subtracted, R-values can!
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