Why is electric field in an insulator non zero? I had read from several sources that electric field inside a conductor is zero. This is attributed to the fact that the electrons are loosely bound to the nuclei and they are free to rearrange themselves until the net field becomes zero.
But in an insulator the electrons are tightly bound to the nuclei. So they can resist movement even at more intense fields. So net field is not zero.
But my problem is, if there is an electric field in an insulator, then the nuclei create a field in the opposite direction to hold the electrons.  So net field becomes zero.
Shouldn't this be a universal condition? 
Please explain in as much of a non textbook way possible.
 A: This is a great question! The simplest way in which materials respond to the external field is via dipoles. There may already pre-existing dipoles in the bulk of the insulator that point in random directions. And in the presence of the external field, the dipoles align to oppose it and some new ones may get formed. The amount of new dipoles formed and the ones that are already present depends on the material properties and can be calculated quantum mechanically. 
So as a response to the external electric field, the field generated by the dipoles aren’t enough to balance it all out, rather just reduce it. This is because there are bound states of electrons in the system that have no net dipole moment. So these don’t contribute in the reduction of the field. 
A: This is a deep question. The atoms or ions that make up an insulator are not free to move. They are restricted by the electric field and Pauli exclusion to be in a bound state. If they are moved from equilibrium there is a restoring force proportional to the displacement. In a conductor the highest energy electron orbitals have a sufficiently high kinetic energy to not in be a bound to a particular atom. They are delocalized. Also metals usually consist of atoms that only weakly attract their valence electrons. The conduction electrons are only bound to the crystal as a whole by the work function. They are thus capable to completely compensate for low frequency, below UV, external fields 
A: So, one important thing- the fact that the electric field inside a conductor is zero is true only for a perfect conductor, i.e., one which is assumed to have an infinite charge to counter any external electric field.
Now regarding your argument for the insulators- then the nuclei create a field in the opposite direction to hold the electrons. So-net field becomes zero. 
It's important to remember that electrons are not fixed in their orbit and (assuming a classical picture), are rotating around the nucleus. Hence, when the field due to the nucleus and the electron is time-averaged, that will just be a zero! Hence, on sufficiently large time scales, the atom is doing nothing to counter the external electric field (We are assuming an ideal insulator which doesn't get polarized at all).
