Insulation resistance (IR) is a measure of the resistance between two isolated pins on the relay. Available insulator materials provide a wide range of insulation resistance (10^5-10^14W) in various environments.
The primary factor to limiting IR is bulk leakage and surface leakage. Bulk leakage is through an insulating material, usually after it has absorbed moisture from the environment.
Surface leakage is across the surface of the material, usually due to contamination such as salts or flux and/or other chemicals from fingers.
The properties of some insulators change with applied voltage, so that it is important to evaluate IR at or above the expected operating voltage.