ᐅ Drywall installation above a recessed mirror cabinet – does it need to be airtight?

Hello everyone here,

I want to extend a drywall panel above the mirror cabinet in my bathroom, now that the wooden framework is finally finished, keeping it flush with the same depth. The idea is to make it look as if the mirror cabinet is fully recessed into the wall.

Does the cavity that will be created between the wall and the drywall panel need to be completely airtight? There will be no insulation or other material inside that could absorb moisture.

This question arises because there is quite a bit of electronics inside the mirror cabinet, and ventilation holes lead upwards into this cavity... There won’t be any spray water exposure—only concerns about humidity.

Since I would have to cut into the existing insulated drywall wall to make the mirror cabinet deeper, I would need to seal this opening in the insulated drywall wall completely airtight.

I have attached a small sketch showing what it looks like (only the upper part of the construction that this concerns).

Thanks!
Regards, Benji179