Final answer:
The term you are looking for is 'dielectric constant' in general electrical context, or more specifically 'R-value' in the context of thermal insulation, which represents the thermal resistance of an insulating medium compared to air.
Step-by-step explanation:
The dielectric constant of an insulating medium is the ratio of the insulating value of the medium to the insulating value of air. In the context of heat transfer, the property you're asking about refers to the material's R-value, which is the ratio of the material's thickness (d) to its thermal conductivity (k).
The R-value measures the thermal resistance of an insulator and is inversely proportional to the rate of conductive heat transfer, meaning that a higher R-value represents better insulation. As an example, a typical fiberglass batt with a thickness of 3.5 inches might have an R-value of 11, while a 6.5-inch-thick batt would offer an R-value of 19, indicating better insulating properties.