When we build balloon decor for events and put together bouquets, we often let the loose balloons float to the ceiling as we take turns bundling them together for deliveries and specialty bouquets. Here we have a few pretty colors placed together on the ceiling before we start the bundling process.
That said, many clients also ask us to fill their ceilings with helium balloons as well. This is certainly possible, but takes approximately 1 balloon per every square foot of ceiling space. That means that a 200 square foot room would take about 200 balloons to fill the ceiling. Another thing to consider is that helium balloons will slightly shrink as time goes on and the helium slowly leaks through the membrane of the latex balloon. That said, we often suggest getting even a few extra balloons to make sure that the ceiling is still covered when the event is done.
Another thing to consider with ceiling balloons is that helium will take them to the highest point in the room. So, if there is a vaulted ceiling or a pitch to the ceiling, then the balloons will all end up in a single space. If it is important that all of the ceiling be covered, then you may need to add in much larger balloons to fill space and ensure that more of the ceiling is covered.