I'm no physicist, but I don't think diffusion works that way. Even if the internal pressure is 1% of a standard atmosphere, if the density of helium inside is

I'm no physicist, but I don't think diffusion works that way. Even if the internal pressure is 1% of a standard atmosphere, if the density of helium inside is higher than the density of helium outside, the helium will still "want" to leave the box.