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 = (Î”T ×— Area ×— Time) / Heat Loss

**Î”T:**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?