WEll, you have two factors. One is CIRCULATION and one is VENTILATION.
Ventilation removes heat and air that has been depleted of CO2 by your breathing plants. Well, they don't 'breathe' but they transpire, whatever.
Circulation makes the temperature and humidity the same throughout the room, and gets stale air out of the corners so it can be removed and replaced with cool, fresh, CO2-rich air.

You will want a nice blower for your exhaust fan that sucks air out of the highest spot in your room, through a air-coolable hood if you are feeling sassy (it makes SO much difference, if you haven't bought a hood yet get an air cooled one!!!!), and then out. That also allows you to run a carbon filter to reduce smells, and you will find that flowering plants reek like the day after a skunk orgy.
stinkyattic Reviewed by stinkyattic on . new to growing, need some help hey guys, i want to start growing in my closet and i would like some help. my closet dimensions are about 3ft by 6ft. the ceiling is a little bit weird since there are stairs on the other side, so i plan to hang my lights as high as they can be, which is going to be about 5ft high. i just bought one 400w hps light system, is that going to work hanging about 5ft high in my closet? also, what are some tips on ventilation? i plan to get a thermostat set up with a fan to blow on the Rating: 5