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What is R-value and U-value of Concrete Slab per inch, Calculation

what is r value and u value of concrete slab

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What is R-value and U-value of Concrete Slab

R-value is the measurement of concrete slab (or other material) capacity to resist heat flow i.e. it measures the effectiveness of insulation or thermal resistance. However, U-value is the measurement of heat transmission through concrete slab from ground into the closed space or vice versa.

Due to the fact that thermal insulation varies inversely with density, lower density concrete provides better insulation than higher density concrete. Finally, it orders to decide the on the ability of the reinforced concrete slab to resist heat transfer, it would be required to calculate R-value and U-value of the reinforced concrete slab under consideration.

R-value Calculation for Concrete Slab

A reasonable R-value for a normal concrete slab can be estimated using an R-value, the thermal resistance per inch of thickness, between 0.1 and 0.2 and multiplying it times the slab thickness. however, the following expression can be used to compute R, this equation is provided by ASTM C 168:

where the temperature difference (between outside and inside of concrete slab) is in degrees Fahrenheit, the area is in square feet, the time in hours, and the heat loss in Btus.

ASTM C 168 is also provided two additional expressions to compute R-value and can be found in ASTM C 168 document.

The R-value imperial unit and metric unit is as follow: 

U-value Calculation for Concrete Slab

The U-value for a concrete slab (and any other building component) is the inverse of its R-value and can be calculated using the following formula:

The U-value unit is the inverse those of R-value:

Fig. 1: Understanding U value


It should be known that, the R-value and U-value according to American Standard is different from that of European Standard.

So, to convert an American R-value into a European U-value, divide 1 by the R-value,then multiply the result by 5.682. To convert a European U-value to an American R-value, multiply by 0.176, then divide 1 by the result.

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