Your going to leak more air through your ductwork. When it comes to sealing a room I've learned a few things. Air tight reflectors are not air tight. Run a fan blowing INTO your complete light cooling setup and seal off the exhaust end. Feel around the edges of the glass and any duct connection you made. You will be surprised at how many leaks their are. I use tape the tape the reflectors shut once the glass is clean and bulbs are installed/tested...then use silicone caulk on the duct connections where they leak. Also, the poofy silver or black insulated duct is misleading. The inner duct almost always has pinholes and rips in it and leaks into the fiberglass insulation which slows it down to a very dispersed leak that's hard to notice. I cut the insulation off and tape up all the holes/rips then reinstall the insulation jacket.
Window AC's will always leak air through the vent even when shut, and also through the Styrofoam surrounding the blower fan. Pull the AC out of its cabinet and tape all around the seams and tape the vent shut.
Use the red "Zipwall" heavy duty zippers, they have the most aggressive adhesive and tightest zippers. Obviously it will leak CO2 and just air in general, but it's the best thing. I'm guessing you are building a "room within a room" with the panda film....make sure to fully seal/tape every seam of the film, make sure the zipper door goes all the way down to the floor and put a metal rod across the bottom to hold the door down the floor (CO2 will always sink and find the lowest way out). Also get a door "sweep" that installed on the bottom of the door to the room and seals the bottom crack, and seal around the door with foam if it's lose. Even if the CO2 gets out of your panda room, it will stay in the main room.
Using the above techniques I've more than doubled the life of a 20lb CO2 tank in grows.
