On my previous boat there were 4 vents on the roof with grilles on the inside, no problem on the bss as plenty of ventilation, when I had to take down the roof lining (it was toungue and grooved) to run some cables I found the vents didn't line up and the gap was filled with 1" polystyrene for insulation!
When the roof lining went back up the joining gaps were left open and the polystyrene fixed so it couldn't slide back and block them, after a particularly bumpy trip from wells to kings lynn with lots of punching through wave peaks there were lots of polystyrene beads in the top roof vents where the air had been sucked out of the cabin as the water roled over the cabin roof.
As said don't just assume when it comes to ventilation.