What is R-value and U-value of Concrete Slab
The R-value measures the thermal resistance or insulation effectiveness of a concrete slab or other material. It indicates how well it resists heat flow. On the other hand, the U-value measures the heat transmission through a concrete slab, either from the ground into a closed space or vice versa.
Lower density concrete offers better insulation than higher density concrete because thermal insulation varies inversely with density. To determine the ability of a reinforced concrete slab to resist heat transfer, it is necessary to calculate both the R-value and U-value of the slab under consideration.
R-value Calculation for Concrete Slab
To estimate the R-value of a normal concrete slab, the thermal resistance per inch of thickness (R-value) can be used. This value typically falls between 0.1 and 0.2, and it is multiplied by the thickness of the slab. The equation to compute R, as provided by ASTM C 168, can be used for this purpose.
The temperature difference between the outside and inside of a concrete slab is measured in degrees Fahrenheit. The area of the slab is measured in square feet, while the time is measured in hours. The heat loss is quantified in British thermal units (Btus).
In the ASTM C 168 document, two additional expressions are provided to calculate the R-value, which is the measure of thermal resistance. The R-value can be expressed in both imperial units and metric units.
U-value Calculation for Concrete Slab
The U-value of a concrete slab, as well as any other building component, is determined by its thermal resistance, also known as the R-value. The U-value is simply the inverse of the R-value. To calculate the U-value, the formula used is: [Provide the formula if needed].
The U-value unit is the inverse those of R-value:
Note
The R-value and U-value differ between the American and European standards. Converting an American R-value to a European U-value involves dividing 1 by the R-value and then multiplying the result by 5.682. On the other hand, converting a European U-value to an American R-value requires multiplying by 0.176 and then dividing 1 by the resulting value.