I'm in the St. Louis area (i.e. high humidity in summer months). My collection of some 5000 LPs – acquired over the last 50 years to now – is mostly (not box sets) wrapped in Sleeve City 5 mil sleeves, and there is no evidence of mold anywhere. In fact, recently I went through it all to catalog in Discogs. I pulled many discs out that haven't been played in ages (decades) to check runouts, etc for the proper Discogs entries. No sign of mold at all.
My records are all stored on Ikea Kallax shelves in HVAC controlled rooms. I'm not really worried about.