I am redoing a bathroom, converting a tub to a shower and I plan to install green-board above. The existing 1/2" drywall has some issues (flaking paint, nail pops). I'll be replacing the walls and the obvious choice would be to tear the ceiling out and replace it too.
But because there is loose cellulose insulation above the ceiling, I'm considering just applying a second layer of drywall (using long screws) on top of the current ceiling.
I'll be adding a vent fan, and I'm prepared to cut through two layers for that. I'm also changing overhead lighting so I'll probably use box extenders.
I've seen double drywall done for fire protection, so I know it's possible. Is it a bad idea in this situation? What problems might I encounter?