Why does it take longer for a foil balloon to lose its helium than a rubber one?

Question

Why does it take longer for a mylar balloon (a foil balloon) to lose its helium than for a common rubber balloon? 

Answer 1

In addition to what Serge said, modern Mylar balloons have special coatings to enhance the already low permeability of Mylar film. DuPont states, "Marked reductions in gas permeability (up to a factor of 100) can be obtained through polymeric coatings, metal foil laminations, or
vacuum metallization." Also the pressure inside Mylar balloons is much lower than latex balloons since the balloon is only slightly inflated above atmospheric pressure to give it shape whereas the latex balloons must have greater pressure to "stretch" the balloons. The greater pressures help force the helium through the latex in a matter of hours!

Answer 2

The rubber and foil balloons are essentially membranes, and helium molecules escape through pores in the membranes. Rubber pores are bigger than foil pores. this is also why auto tires are often filled with nitrogen rather than air which is a mixture of nitrogen, oxygen and other gasses. The nitrogen molecule is bigger and escapes through the tire more slowly than smaller oxygen molecules.