Not when it's solid CO2 vapor and sealed up inside a greenhouse/tent, it drifts downwards
here's just one vid of how the vapor drifts if sealed inside something:
http://www.youtube.com/watch?v=HyO8L8TRHgU&feature=related
Everyone does need to be aware that breathing in the CO2 vapor can be very dangerous as it will suffocate you as well as the pests so don't allow yourself to breath it in unless you want to lose braincells and then die...
The problem with that vid though is that they have fans around the plants moving the vapor around so it doesn't cloud like I do. This vid's creators were probably doing it to get more CO2 to their plants as they are indoors instead of trying to suffocate pests...
What I do is I hang metal containers (cans usually) above my plants filled with hot water. I drop dry ice in those (using oven mitts and a face mask) and get the f--- out of there. I seal off my greenhouse meaning no windows are open and I cover the entire thing with a giant plastic tarp that I tuck under the greenhouse itself which seems to seal the thing off and keep a nice full cloud of the vapor in there until I'm ready to take the tarp off after a little more than 5 minutes
I'll try to get a vid next time I do it