Comment by londons_explore
7 years ago
The seal works against air but not hydrogen or helium because the molecules are so small they'll diffuse through pretty much anything.
It's really hard to contain helium. It'll leak through pretty much any solid material.
is mylar special? a good mylar balloon can float for weeks
It's not special, just less porous compared to latex. Mylar balloons still lose gas after time.
It's the aluminium on the mylar.