I am also in the middle of trying to figure out how to do an air exchange in the grow room I'm building in a 7x7 outdoor shed. My original thought was to use an inline ventilation fan down low with a chimney on top for exhaust and have that going for much of the day. But when I started adding climate control to the build (humidifier, dehumidifier, AC, heater, circulation fan), I realized I wanted this thing sealed and insulated.
This opens up the problem of getting fresh air into the shed. The single-hose portable A/C I am using will create negative air pressure inside the sealed shed, so I am considering leaving the built-in vent, rather than blocking it up, if we can find a way to damper. Otherwise we will make a dampered 4" vent.
In any event, then plan as of now is to open up the 6x6 doors twice a day for 20 min and let the circulation fan blow things around. Seems like
@Dr. Who does something similar, except with a ventilation fan twice a day, and now I am considering that. I am forgoing CO2 for now.
In any event, the shed arrived yesterday, and the build will commence on Monday or Tuesday, depending on when my help can get here. I will start a new thread on this forum to document the build and hopefully help someone else out down the road.