If you have an AC unit and a dehumidifier, I don't see the need in ventilation. In fact, with CO2, you really DON'T want it to be vented out. Lots of people, myself included, have truly 100% sealed rooms with no ventilation. IMO that is the best way when using CO2, AND you have a way to climate control with an AC. If I were to vent, it would only be a cool tube for your light, but that is not venting the room, just the light.