Ventilation is the second most important thing in an indoor grow. Keeping your room well ventilated reduces humidity levels and removes excess heat. Your plants need Co2 and unless you exchange the air frequently your plants will use up the Co2 in the room quickly. This can be mitigated by supplementing Co2 with a bottle. I wouldn't even think about running a LPG Co2 generator in a sealed room. Even if you do use Co2 you really need to keep your humidity levels down. If the humidity gets to high the stoma's on the bottom of your leaves will close reducing the amount of nutrients the plant can trans relocate. No nutes, no carbon, no grow. It's that simple. Also humidity levels have a lot to do with pest. Most pest like higher humidity. Spider mites in particular love the higher humidity levels. It actually affects their breeding cycle. When humidity levels are above 60 % in warm temps they can reproduce every three days making it next to imposable to get on top of an infestation. Long story short; you really need to vent your room.