im currently runnin a setup that will help you right here i think.

you can check out my log but this is my 2nd closet grow. led's and t5's i have 0 fresh air in there right now but as you can see there is still nice growth.

I put a ExHale CO2 Bags co2 bag from my hydro store in there as a subsitute so the plants can absorb some kind of air.

have a fan just to give airflow around the top of the cannopy.

i wish i had fresh air inside. it is key and will keep the plants happier. your plants will still grow with out it and you can make it happen.

i have my room completely sealed with a 4" comp fan just pulling air out. doesnt do much but my room stays a constant 80f.
hope this helps.:thumbsup:

like everyone else says tho. it is key and will make for a happier plant but that doesnt meean they wont grow with out it.