... the way we use it, the moisture doesn't build up. There is always some ventilation. Also my hull and decks are mostly foam core so they're good insulators. These are used just to take the chill off for a few hours at a time, in Spring or Fall, or even on a cool summer evening. I wouldn't even think of using them full-time in winter.

If we were living aboard or full-time cruisers I'd invest in a "closed loop" heating system that re-heats interior air and has separate venting to the outside for combustion intake and exhaust by-products.