As far as I'm concerned your Boxster should be by itself in a garage or barn "like I did" with mine and forgotten for the winter. All these cars you bring in are the culprit, the engines are hot the garage is cold and this forms humidity. the best is like 10/10ths said buy a roll of Tyvek and lay it on the floor. If I were you I would also insulate that garage.
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Frank
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