@billstevenson I have thousands of records, and have been using outer sleeves for ages. None have harbored mold, ever. The sleeves are open ended and my records are stored in an HVAC controlled environment. I suppose if people put their records in outer jackets and then keep them in cardboard boxes in a damp basement or garage they'll have issues. But I use the outer sleeves (Sleeve City 5mil) and will continue to do so. YMMV
The only mold I have ever encountered on records in 50+ years of collecting is on used ones I have bought from Discogs.

