For the past 20 years, we have owned a number of sailboats over 30 feet that are in regulat use, but often stored for long periods. Non have had AC, and the humidity in SE Florida is second to none. We have never had a mold problem, although the boats are buttoned up tight for security and weather protection. Humidity is only a part of the problem creating mold IMHO. The spores need to be present to fester. We have always been carefull to give our boats an occasional wipe down over all the interior with a mild solution of bleach and water, and also a dose of bleach in the bildge water. Fabrics need an occasional vacuuming. The cabin stays sweet smelling and has that clean feeling. I have noticed on boats with ACs running a "sweat" on interior surfaces from condensation, which sems to me an invitation for problems? In this area some have severe problems with mold in homes, and some do not. So, maybe the heat and humidity is not the single causation? I still would rather a little mold than ice and snow? Good sailing Ron B