http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Chemical_Properties.pdf http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/... See F

http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Chemical_Properties.pdf http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/... See Figure 2, Gas Permeability of Mylar vs. Temperature Helium permeates Mylar significantly faster than hydrogen over the full temperature range tested. It looks like the helium permeation rate is nearly double that of hydrogen near room temperature. Surprising! I would have guessed that lighter gases permeate faster. Note on the same figure that oxygen permeates faster than nitrogen even though it's heavier. There is more tabular data here: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690004779.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/196900... It confirms the same trend. Helium permeates Mylar faster than hydrogen.