The magnitude of the energy gap for an insulator is

Energy gap between the conduction band and the valence band is known as forbidden energy gap. 

Conductors:

In conductors, the conduction band and the valence band overlap, which indicates that the valence electrons can easily move from the conduction band and are free to conduct.

Insulators:

In insulator, there exists a large band gap between conduction band and valence band Eg (Eg > 3 eV), that result in no free electrons in the conduction band, and therefore no electrical conduction is possible.

Semiconductors:

• In semiconductor, there exists a finite but small band gap between conduction band and valence band (Eg < 3 eV).
• Because of the small band gap, at room temperature, some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band.

Therefore, the forbidden energy band gap in conductors, semiconductors and insulators are in the relation Eg1 < Eg2 < Eg3.