Confusion about the movement of electrons in insulators If electrons in an insulator cannot move, how do they leave it in discharging via humidity?
 A: If electrons in an insulator cannot move, how do they leave it in discharging via humidity?
They can leave it via moisture in air but first there must be a build up of net charge (and a high voltage generated) on the insulator for a discharge to occur, and that build up is more difficult the higher the humidity. First let's consider the process for building up charge and causing a discharge.
You are correct that the electrons in a good insulator generally don't move. That's what makes them good insulators. But it is possible to force electrons to move. One way is by friction (there are other ways too). When you rub a glass rod with silk cloth electrons are stripped away from the atoms in the glass and transferred to the silk cloth leaving the glass rod with more positive than negative charge.
This build up of static charge on objects is more difficult in a high humidity environment than in a low humidity environment such as indoors on a winter day. Water vapor in the air on a humid day is more conductive than dry air, allowing electricity to move more freely in the air minimizing the build up of static charge on objects when rubbing against each other. Since the build up of charge on objects is more difficult in a humid environment, the electrostatic field produced is weaker and it is less likely you will experience an electrostatic discharge on a humid day. Whatever charge has been built up will discharge, but you will be less likely to notice it because the discharge will be weaker.
So while electrostatic discharge on a humid day is possible it is much less likely than when the humidity is low, and would be much less noticeable. 
Hope this helps.
