Hey ya'll! I have a grow room question I've been puzzling over- how do you prevent a humidifier system that pulls air from outside the grow room from creating positive pressure when the exhaust fan is off?

I run a ultrasonic mist maker in a water barrel for humidity, and currently have my intake fan ducted through it to blow the fog into the air (I run an intake fan as well as an exhaust but have the intake at a lower setting to keep negative pressure). I used to run my fans continuously, so the mist maker would just turn on and off via my humidistat and all was well, but I recently picked up a co2 sensor, and now since my fans aren't running continuously, I can only humidify when they're on. 

So now I think it makes sense to separate the humidifier from the ventilation system, but if I add another intake to pull air from outside into the humidifier barrel and into the grow room, the room would go positive in the case that the humidifier is running when the ventilation fans aren't. 

How do you deal with this? Any humidity system will push some amount of air into the room, and if the ventilation isn't always running wouldn't this reverse the pressure? I suppose I could add another lower powered exhaust fan to balance it out, but I'm just wondering if there's a simpler way. Or maybe the airflow from the humidifier isn't enough to cause an issue, either way I'd love some clarity!