Ok. Co2 is heavier then oxygen ( see link) Answers.com - What is heavier oxygen or carbon dioxide

so if by the time your co2 gets to the ceiling your girls are bathed deep in co2 and the drop ceiling will help act like a buffer. There will be dissipation through walls etc..... I would say its a waist of time unless you want air tight. just my opinion based on scientific principle.:stoned: