My entire collection resides in the Japanese Resealable Outer Sleeves that are sold by Elusive Disc. I've never had a problem with mold.
These Sleeves are perfectly sized in that they will accept singles, and even all doubles and some triples fit. They are durable, crystal clear, the adhesive strip does not fail. I've always been 100% satisfied with them.
Beware of others, the ones MusicDirect sell are not the same, are inferior IMO, and they're even too small-tight for just singles, let alone doubles.