Some "ZipLoc" style bags are porous and meant to breathe. Be sure to verify before use.
I too use ZipLoc Freezer bags BUT also store inside large ammo cans with a good seal. Controlled temperature and humidity are the biggest factors for long term storage.
I have stored 25-06 match ammo this way since loading date of 12November1980 and it still shoots just fine, better than my old eyes.