We lived aboard for many years, at times also in very cold climates. We were frozen in a few times with ice thick enough to stand on all around the boat
Your body gives off a considerable amount of moisture when you sleep. What happens is when this moisture comes in contact with any cold surface, such as what the mattress is sitting on, this cold surface will cause the moisture to condense onto the surface. The mattress sitting on top, insulates the base it sits on enough to ensure the base stays cold enough to cause the moisture to condense.
You can see why many of the suggestions others have made might work
. It depends on how cold the climate is that you are in, as to how far you have to go to solve the problem. Anything you do to create air circulation under the mattress will help to raise the temperature under the buck, which will help you avoid the condensing. It will also help to remove the most air exchanging it with the air in the cabin
which will have lower humidity. In cold climates the air in the cabin
should already be pretty low in humidity, unless you're heating
with something like a Force 10 propane
, which is marginally vented. Propane
gives off lots of moisture when you burn it.
So, ventilating, or adding some kind of air space, or putting insulation
onto the surface of the bunk, like armaflex, can also help, however, if it's cold enough outside, you may find this is not enough to completely solve the problem.
What we did was to add a heating
element. We used restive wire, meant to be used on 12V, that came from the agricultural industry. This is used to heat seed beds. We made loops about 12" apart running back and forth under the mattress. It drew about 2- 3 amps for our queen size bed
, if I remember. We were on a dock
with AC, so this was not a consideration. You may be able to do the same thing by buying
a 12V heating pad. I think they sell these for the RV market. Anything that raises the temperature enough so that the base is above the condensation
temperature will completely solve the problem. Doing this means you don't have to worry about the bedding, such as your comforter, blocking airflow. It won't matter, since the base is above the temperature which would cause condensation
On another note, Amaflex can be used on hard surfaces like your hull
sides to prevent condensation. Most boats have no insulation
. When you live aboard in cold climates, many things like hull
sides, metal hatches and more, will be cold enough even to condense moisture. It's a common problem. All of these issued can be solved
, but since boats are not generally used in cold climates, builders don't see the need.