We're liveaboards in the Chesapeake bay. The nights can get pretty cold, and like most boats, we don't have much insulation. We do have shore power though, and with shore power comes an (a drum roll please)...
ELECTRIC BLANKET!!!
Oh glorious warm berth... After shivering in the salon (I exaggerate), we go into the aft cabin and slide into the pre-warmed bed and skoosh down into warm blissful sleep.
Only one problem. The blanket keeps the warm air trapped against our skin. That's its job. So where does the humidity and air actually go? Down into the mattress. And, if the mattress is resting on the cold fiberglass, it condenses into water and BANG. The cabin starts smelling like a dank mildewy basement.
We use
Den-Dry. It lifts our mattress about three quarters of an inch above the fiberglass, and lets the air move freely, carrying the humidity away before it can condense, and what little does condense on the fiberglass evaporates easily, because it's not soaking into the mattress. It's sort of like hypervent, but cheaper and a bit stouter, with cross venting channels.