I have a box set up for my grow and I have holes on the one side facing the wall to release some of the heat because I don't have a fan. I hear all the time a fan is vital but it doesn't get very hot in the box but it's not cold either. It remains pretty well room temperature. Is a fan that important?
You also want fresh air in there.... the plant "breathes" if its not getting a fresh source of air it will not be getting CO2 and such... also a fan will blow a breeze on the plant which strengthens the stalk and stems