so balloons appear to have negative mass, it's actually just the result of having lower density than the air. the upward force balances out with the gravity where the lbs/in^3 figure of its entirety matches that of ambient air. it's exactly the same as how an empty tank underwater floats, and a water filled tank underwater sinks.

or I guess one could say it's the bottom side getting more compressive load from air than the topside, given the observable effect, whatever floats our zep...