How big is your grow room? If you run co2 you can have a closed room (no external intake or venting) as long as you can keep your temperatures in check. If you need to exhaust any heat, then you have to time your co2 so it doesn't come on while you are venting. There are controllers available that you can plug your exhaust fan, A/C, and co2 into that will shut off your A/C and co2 while venting but they are very expensive and usually not practical for small grows.