Need to dissipate heat and provide CO2.

Is there any alternatives to putting holes in my walls for ventilation tubes and in-line fans? Also, should I buy a CO2 tank and have it slowly release gas throughout the day or are there some cheaper methods of producing CO2. I've heard of yeast producing CO2 and Mr Green says in one of his videos that spraying bubbly or carbonated water on your plants will release CO2. Anyone know if Mr green is right?

My 400w Metal Halide light is heating up my 4 by 4 foot closet so I'm wondering if there are any ways I can decrease the amount heat in the room.