Question

Why the resistance of insulator remains unaffected with a change in temperature?

Answer 1

Although the resistance of a conductor changes with the size of the conductor (e.g. thicker wires have less resistance to current flow than thinner wires), the resistance of a conductor also changes with changing temperature. This may be expected to happen because, as temperature changes, the dimensions of the conductor will change as it expands or contracts.
However, materials that are classed as conductors tend to increase their resistance with an increase in temperature. Insulators, however, are liable to decrease their resistance with an increase in temperature. Materials used for practical insulators (glass, plastic etc.) only exhibit a marked drop in their resistance at very high temperatures. They remain good insulators overall temperatures they are likely to encounter in use.
In a conductor, which already has a large number of free electrons flowing through it, the vibration of the atoms causes many collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms jostle around in the material, the more collisions are caused and hence the greater the resistance to current flow.
In an insulator, however, there is a slightly different situation. There are so few free electrons that hardly any current can flow. Almost all the electrons are tightly bound to their particular atom. Heating an insulating material vibrates the atoms, and if heated sufficiently, the atoms vibrate violently enough to actually shake some of their captive electrons free, creating free electrons to become carriers of current. Therefore at high temperatures, the resistance of an insulator can fall, and in some insulating materials, quite dramatically.