Drift Away has three levels. The upper level is the flybridge and sundeck, and right now that's covered in snow and doesn't enter into this. The main level is the helm station, galley, and main saloon. The helm is heated by an electric heater and the main saloon by a diesel space heater. Both do an admirable job of keeping things livable. Last night it was 15 degrees outside, and the main level was 57 degrees. Yeah, I know it sounds chilly, but you get used to it. Really.
The lower level has the engine room, and three staterooms with ensuite heads. All are heated with electric heaters. This is the problem spot. The outside walls are, of course, the hull. Unlike the main saloon walls which are a sandwich of fiberglass with an air space in the middle, this is solid fiberglass. Fiberglass is NOT a good insulator. The warm air inside the boat hits the cold fiberglass and the result is condensation. Constant condensation results in mold and mildew, and also wet things inside the lockers and cabinets.
Briefly, on another topic, the weather here has been like upstate NY, where we both come from. We've gotten about three feet of snow so far this season, and it's been pretty cold so it hasn't been melting much. The dogs seem to really enjoy the snow and love to play in it.
We, on the other hand, don't find it so much. The heads are about 150 feet up the docks.