sealed room problem

We are in the process of setting up a sealed room and have problem. The room we are using is an add on attached to the laundry room. We are using a portable dual hose ac with the intake pulling from the wash room and exhaust blowing into the attic. When the doors are closed and the ac is on air is getting pushed into the room from under the floor, we put a pond liner down and it blows up like a balloon. My guess is the positive pressure in the attic is pushing air where ever it can go and causing it to come up from under the floor. My partners guess is that the ac is causing a vacuum and pulling the floor up. Despite me telling him that this is physically impossible, his mind is made up. Any help or explanation would be greatly appreciated.
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two things to check. 1st you need to vent both hoses outside. so many reasons why i wont bore you. 2nd if the air in basment is warmer than room the warm air will push in because warm air rises and exspands. i see this at work when i open the doors to my paintbooth. the car is hot , the air in the car is hot and so is the air under car and the car has plastic cover on it. this car was just baked. when i open doors colder air rushes in and the plasic on the car blows up like a ballon. but i think yiur problem is your vent.

Thanks for the heads up. Guess we need to go back and figure out the ventilation for the ac and lights. I wonder if we could vent both into the garage, would be much easier and stealth. Maybe we could equalize with a fan and vent it outside from there. I know my description is vague but I really appreciate the help.