R-value, which stands for "resistance value," provides a means for quantifying the thermal resistance of an insulating material. If the R-value is high, the material is a good thermal insulator, and heat will not easily flow through it. If the R-value is low, the material is a poor insulator. RSI stands for "R-value Système International," meaning it measures the same quantity but uses the international metric system of units. Converting one to the other requires some simple math.
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Multiply the RSI by 6 to calculate a rough estimate of the R-value. For example, if RSI = 2, then R-Value = 2 --- 6 = 12.
Divide the original RSI value by 0.1761. This will calculate the R-value to an improved level of accuracy (if required). For example, if RSI = 2, then R-value = 2 ÷ 0.1761 = 11.36.
Take the R-value you just found and multiply it by 0.1761 to verify that you performed the calculation correctly. This should result in the original RSI value. For example, if you calculated an R-value of 11.36, then the original RSI = 11.36 --- 0.1761 = 2.
Tips and warnings
- The units of RSI, sometimes referred to as the Thermal Resistance Coefficient, are square-meter Kelvins per Watt, which can be written K m² / W. The units of R-value are Fahrenheit square-foot hours per British thermal unit, or °F ft² h / Btu.
- For improved precision, you can divide the RSI value by 0.1761101838 (instead of 0.1761) to calculate the R-value to a high degree of accuracy. R-values do not normally need to be this precise.
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