This depends on what you are trying to protect. Positive pressure cleanrooms protect the process inside the room from any particulate outside the room. Negative pressure cleanrooms protect the user from the process.
Clean Environments have a range of rapid assembly positive pressure cleanrooms. In order to maintain positive pressure, you must make sure that you are controlling not only the amount of air that enters but crucially the amount of air that exitsthe room. If you have no control on how the air is exiting. You cannot balance the room and maintain a good level of positive pressure.