In an insulator, the forbidden energy gap between the valence band and conduction band is of the order of

Forbidden energy gap, also known as band gap refers to the energy difference   (eV) between the top of valence band and the bottom of the conduction band in materials. Current flowing through the materials is  due to the electron transfer from the valence band to the conduction band.

Insulators do not conduct electricity because a large amount of energy is needed for the electrons to cross the forbidden energy gap. Moreover the forbidden energy gap is the widest (>5eV) in case of insulators.