Here is my 2c from my little experience with dry ice thanks to Videoman's advice by placing dryice in a styrofoam box to control evaporation. After calculatios and some trial and error I have been able to maintain CO2 ppm levels above 1000ppm for over 8 hours with just 4 oz of dry ice placed in a small styrofoam box with a small hole in it. My grow room is 4x3x6. The main problem initally was that humidty shot up over 80% when the room was completely sealed. So i had to think of how to vent out the humidty without depleting all the CO2. I have made caps for my air in and outlets so that less air is exchanged and I am running my 12V fans on 3V in addition to that I had to quadruple the amount of dry ice from initially 1oz to 4oz. I was really surprised to see how steady the Co2 levels remain. By the way I store the the dry ice in another styrofoam box in the freezer and then take 4 oz chunks twice a day. I do wonder if it is really worth it thou...
Peace
Peace