The R-value stands for Resistance value and measures the effectiveness of a particular type of insulation in blocking heat transfer. This value is also closely linked to the metric unit known as RSI, which is °C m2 sec/J. The R-values are expressed in imperial units, specifically in °F ft² hr/BTU.

This value is also closely connected to the U-value, which is the reciprocal of the R-value. Additionally, the thermal conductivity of a material, which indicates its ability to conduct heat, is also linked to the R-value.

For example:

Windows also have a specific R-value that measures its effectiveness in retaining heat. R-values do not consider the heat generated by sunlight passing through a window, but they are important for evaluating the overall impact of having a window on a specific wall.

Materials with higher R-values are better at preventing heat transfer. Each insulating material possesses a distinct R-value per unit length. The resistance to heat transfer increases with the thickness of the material. Therefore, the R-values are expressed per inch, allowing for the calculation of total resistance by multiplying the R-value by the insulation thickness.

Calculating R-Value

This value is a mathematical calculation that evaluates the heat flow across a surface with specific temperatures on each side.

Formula: R-Value = (ΔT ח Area ח Time) / Heat Loss

  • ΔT: It is the temperature contrast between the interior and exterior of the house, measured in °F (or °C for RSI).
  • Area: Indicates the area of the wall (or ceiling) that requires insulation, measured in square feet (or square meters for RSI).
  • Time: The duration of the measurement in seconds is determined the same as RSI.
  • Heat Loss: This indicates the amount of heat lost through the wall and is expressed in British Thermal Unit (BTU).

Multiply the RSI value by 6 in order to convert it into an R-value. When you place one insulation material on top of another, the R-values of both materials simply combine.

Also See: What is Reactive Power?


Elliot is a passionate environmentalist and blogger who has dedicated his life to spreading awareness about conservation, green energy, and renewable energy. With a background in environmental science, he has a deep understanding of the issues facing our planet and is committed to educating others on how they can make a difference.

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