Hi, I am building a jumbo grow box, much like the one posted by badgr, but with some slight modifications. Today I just finished my ventilation system. I cut two eight inch by eight inch intake squares (one for flowering chamber and one for veg chamber) and placed light proof/ dark room vents (louvers). I have a TD 200 Mixvent S&P fan exhausting through an eight inch tube out the top of cab.. (476 cfm) I read the post VENTILATION 101, and my cab is slightly harder to open due to the air flow. Is this a problem? Do I need more intake? Is this positive or negative pressure? Any help would be appreciated.