Much. Are you drawing air into your space passively? Does your exhaust fan run when the lights are off? If you are dealing with outside air that is being pushed into the tent from the wind then you could try a couple of different things IMO. 1. You could extend 4" flexible tubing in an s pattern using say 8-10ft. This would make it tougher for the wind to push through, but will make it harder to draw passive air through. or 2. You can build a damper with a piece if 4" duct pipe, piece of sheet metal cut round to fit inside, and a piece of small tubing for the center shaft. I mention building your own because I and alot of the peps on here seem to custom build our own stuff because what we need is not always available. Or you can buy a damper and close down the vent so that you get a happy medium of just enough air when lights on/off. Also, if not already, put your fan on the timer with your light so it shuts off with the light. I'm thinking the only way a flapper would help is if you were having a problem with air coming back through your exhaust opening. Extra legths of flex tubing also is a VERY good muffler on your fans. I have extra lengths that I make sure they turn or U shape before they exit the house and you can hardly hear my fans running outside, and are quieter inside the house. Just whats works for me. Peace and good growing...