If the wetness is occurring from sleeping in the bunk it is due to the moisture
sweated from the body. The problem comes up again and again in forums
and the following is what I have posted before.
travels from warm regions of a structure to cooler ones (anyone who has been involved with the design of big freezers and cool stores in the meat and food
processing industries or with the design of vapour barriers in other buildings will be familiar with that). So the moisture
sweated from the body is driven from the warm part of the bed
towards the cold bunk top which forms a barrier to it progressing further and so it "puddles" (I am avoiding the misleading use of "condensation").
There are 4 solutions that I know of -
1. Ventilate the bunk top into the space below. This generally, in my observation, requires the likes of replacing the bunk top with slats, just drilling big and lots of holes is not enough. I, personally do not like this for modern boats as the space under bunks is normally not well ventilated so by doing this one is just transferring the dampness problem into the storage
space below. I have seen examples where this has been done though in custom built boats using slats for the bunk top and ventilating the space under the bunk up between the side lining and the hull
to ventilate out of slats into the cabin
space - the ventilation gets driven by convection and works well.
2. Raise the mattress on a grid or whatever so that there is good airflow under it so the moisture
driven down through the mattress is carried away. From what I have seen quite a lot of free air space is needed to provide adequate ventilation. Also, one needs to consider that if the existing fiddles on the side of the bunk are not high enough they will need to be raised else the mattress will not be restrained when the boat heels - not a problem on cats or for smooth water
mono sailors though. Also, if access to the storage
space under the bunk is through hatches in the bunk top then this method can be very inconvenient for access to the storage space.
3. Use a normal thick household type mattress which is made to go on a hard base. It should be thick enough and open enough in structure to self ventilate. It may not self ventilate sufficiently if one long side of the mattress is against the hull
side lining ie if it is not on an island berth.
4. Provide a barrier between the body and the top of the mattress so that moisture
cannot travel down through it - this is the method we use. This can be done by using the plastic/rubber materials used in hospitals/child beds/etc to prevent wetting from urine, blood, etc BUT all the materials of that type I have tried raise the humidity in the bed
to what can be for some an uncomfortable level even if light bedding is used over. They are also very "crinkly" when one moves in the bunk.
We needed to keep our bunk heights down (due to the way the heights turn out from sole height, the desired bunk width, and the shape in the side of the hull) and the fiddles on our bunks also being too low to allow us to lift
the mattress to ventilate underneath properly as in method 2 above. Also, access to the storage space under our bunks is through hatches in the bunk top.
After trying a number of materials we found that a very high quality, very soft and quite expensive "vinyl" (don't actually know what plastic it is) fabric
used for upholstering furniture worked perfectly - little extra humidity in the bunk and no crinkling sounds. Is just cut to shape, laid on top of the mattress and a thin under-blanket placed between it and the bottom bed sheet. I was led to using this material by a race
boat owner who had all the mattresses/settees upholstered using this type of material for ease of cleaning
and he reported no dampness underneath. Another forumite here uses rubber backed curtain material successfully, a material I had not thought of using and seems an equally good solution to try and likely cheaper.
It should be noted that heating
does not cure the problem. In fact it may make it worse if the space under the bunk is not also heated as otherwise the temperature gradient between the top of the mattress and the cooler bunk top will likely be increased providing more driving power of the moisture
from the sleeper towards it. Ventilating the cabin is also of little use.
Dampness under the mattress can also be caused by moisture condensing on the side linings of the hull and running down, but this will be obvious and the problem as outlined above will still predominate.