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The term R-value is often used when talking about insulation and the qualities of a certain insulation material. Basically it is the measurement used for thermal resistance in terms of heath transfer issues. The measurement unit for the R-value in SI system is K*m2/W. When using this unit the R- value often is expressed if the value is for example 3. 345 RSI- 3.345 or in a simpler version R3.345. The non-SI measurement is R = ft²·°F·h/Btu.
The R-value is calculated by using the thermal conductivity which is marked with k and by using the thickness of the material which is market with a t. The formula for calculating the R-value is R= t/k.
Using R-value for comparison the insulation material per millimeter of thickness is mostly used for conductive and convective heat transfer and it cannot be used for radiant heat transfer.
The American government has implied a rule that the R-value of the insulation material has to be disclosed to the consumers in the advertisements and before the purchase to make it possible for home consumers to compare different insulation material in a proper way. Thus, all the packages of insulation have to carry a marking which tells the R- value of that particular material.
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