Where to place first fan within grow room.

Ok growroom is a closet with very solid doors. I keep them closed except twice a day to let fresh air in. I only have a small fan inside blowing gently on the plants 18 hours a day along with lighting. I have a small nutone 50cfm fan and am looking to install it. Not sure if I should use it to intake air or extract at this point. The closet doenst seem to get overly hot, however I must admit I have no thermometer or humidity devices monitoring it. I have an attic overhead of the closet, or could take air in with a little manuevering. Your thoughts????

I'd use it for intake though you really do need both, heat or no heat. While I don't know the size of your closet, 50cfm seems to me like its almost not even worth hassling with unless its a very small closet.