In a grow room like mine you have to have windows facing the South so the sun will be able to hit the roses, and you need to clear out all of the air from the room all the time.
I have as exhaust fan that will clear out the air every two minutes. What I have done was to go to this website: http://www.industrialfansdirect.com/CFM_Calculator.html. It has a calculator that gives you the CMF or Cubic Feet per minute. They sell fans by their CFMs.
I decided to buy this fan because it was the cheapest and it had a big enough CFM to clear my room quickly:
Because I have 420 Cubic Feet of air in my room, and this fan can clear out 210 Cubic Feet every minute, that is how I Know that it will clear out the air in my grow room. In two minutes.