Insulation is best determined by:

The determination of insulation effectiveness is primarily assessed through its R-value, which measures the material's thermal resistance. The R-value indicates how well the insulation can resist heat flow; the higher the R-value, the better the insulation's performance. This is a critical factor in building design and energy efficiency, as effective insulation can significantly reduce heating and cooling costs by minimizing heat transfer between the conditioned space and the outside environment.

Other aspects, such as the number of batts, the width of batts, and the I Value, do not provide a comprehensive measure of insulation efficiency. While the quantity and dimensions of insulation batts can contribute to the overall insulation quality when installed correctly, they do not account for the thermal resistance properties of the material itself, which is what the R-value encapsulates. The I Value, which may pertain to other specific metrics in insulation material, is not a standard measure used to determine the effectiveness of insulation in the same way the R-value is understood in building practices.