My boat is laid up on the hard for winter here in NY. Yesterday I removed a panel of the headliner on my boat and I found a lot of condensation on the underside of the deck in the cavity above the headliner. This is a new boat to me, before I got it it had been covered and closed up for about 18 months, it had quite a funky mildew smell. My wife and I have been cleaning, drying and ventilating ever since and I think we have finally won the war on the stink. So, now I am always on watch for moisture and mold. This seems like a great spot for mildew and mold to grow over the winter. I am thinking of installing some louvered vent covers throughout the boat to allow the cavity up there breathe and dry out. I'm afraid insulation will just give the mold more surface area to grow. At the time I found the condensation the outside temperature was warmer than the inside temperature due to the winter cover. Even with warming and cooling cycles, there's no place for that moisture to go.