Half Balloons
First, the balloon doesn't have two positions. Relatively instantaneously, it moves from the ceiling to the floor only if it explodes loudly or you didn't tie it at all (the truth is, in the second case, it's also wrapped in the room using a solid gas as a reaction motor). Otherwise, this process will take tens of seconds to tens of minutes depending on the rate of gas leakage from the shell.
Second, a gas ball in the room - aerostatically unbalanced system - its lift force exceeds the gravity of the gravity on which it operates. If you let it out in the open air, it will rise until it reaches a level of equilibrium, depending on the alteration of the atmosphere and the fall of the temperature of the gas; when it is released in the room, its lifting power is usually greater than necessary to achieve any semi-position.
To hang the ball in the middle of the room, you can balance it with the load - for example, extend the rope to tie it. There's no need for a lot of cargo. Air balloon It'll raise more than 5 g. The truth for obstination is he's not supposed to tread or do it minimally, the helium is still supertex.