[ahecht]

Don't forget that helium molecules are MUCH smaller than O2 of N2 molecules that make up the air, and so something that is air tight might not be helium tight. I know that many plastics can easily be inflated with air but hold helium like a mesh bag. You can also see that helium balloons will lose their helium before they lose the N2 and O2 that is also in them, as a large balloon that has deflated will never rise as fast as a newly inflated balloon of the same size. For more info, I'd try to get a copy of the Junkyard Wars episode where they tried to make a dirigible.