So as I have read. A fan need to be rated to completely replace the air volume in 5 minutes. Using my own tent set up:
5 x 9 x 6.5 = 292.50 cubic feet
292.50 / 5 min = 58.5 cubic feet per minute
The exhaust fan needs to be rated to at least 58.5 cfpm
So as mentioned any fan rated for and running above that number and all other things being equal will create a NEGATIVE AIR PRESSURE.
Why would i regurgitate all that?
My 292.50 cf are being exhausted not with a 58.5 cfm but with a 600 cfm and there is a VERY NOTICEABLE NEGATIVE AIR PRESSURE FROM THE LOOK OF THESE WALLS!!! There is at least 3" of bowing on all the walls. This fan is a 8" 600 cfm, I have it neck down to a 6" (port size on tent). There is one 3' x 6" screened flap open for intake.
....and there it is
Lol, or not. There seems to be some significant confusion in that post.
1. An air exchange every 5 minutes is a fairly random suggestion that can be found on the Internetz. You can also find the suggestion of 5 air exchanges
per minute - 25 times more. Neither includes much if any justification when you see it recommended.
2. one exchange every 5 minutes is WAY TOO LOW. I would think anyone who's actually put together and ran a grow and thought about these numbers would immediately know that this is a long ways from enough air exchange, even with separately ventilated light hoods. Experience rules here.
3. An air exchange every 5 minutes has nothing to do with negative pressure. I could setup a tent to turn the air over once per 5 minutes, but if I try to force a bunch of air in the intake, it will have positive pressure. I could hook a 1000 cfm exhaust fan to a 1'x1' tent, but if I have a 2000 cfm fan blowing in an intake, I'll have positive pressure.
Thinking a 60 cfm fan would suffice for the OP is a bad joke.