I have slept on a boat
virtually every night for 20 years. Most of that time in cool, damp San Francisco
. I do understand this problem. Water
under a mattress on a boat is a very common problem, and there is a lot of misunderstanding about the source and best cures. But it CAN be completely eliminated with a bit of a scientific understanding of what’s happening.
The water does not come from the bilge
, or even from the damp air. It comes from YOU. At night you sweat, the resulting water evaporates off your hot body (that’s NOT a judgement amount your looks!) and moves as water vapor down through the mattress until it gets to a place cold enough to condense, usually the wood
, or FRP shelf the mattress sits on. Just about the WORST case is if you have a manufactured mattress with a vinyl layer on the bottom. Now the water accumulates INSIDE the mattress... Yuck! You can’t stop sweating, so this process is inevitable.
Over time lots of water can accumulate here. Enough to rot wood
, blister fiberglass
, and make life smelly and miserable for people onboard.
The solutions come in two flavors. You can try to increase air circulation below the mattress so the water that collects can evaporate during the day and disappear. There are many ways that have been sold
to do this, basically creating an air gap under the mattress. I have found them to be marginally effective, especially for a full time live aboard boat where the bed
is used every night, and in cold climates. Helpful, but not a fix.
Better, is to avoid the problem in the first place. For a berth that is a full time bed
, a waterproof mattress pad (NOT a “breathable” one!) will stop the migration of the water vapor down through the mattress. Lots of variations of these are available—usually sold
to people with continuance issues. The water, not finding anyplace cold enough to condense, will move up through the sheets
and blankets and leave into the cabin
air (unless you sleep with a rubber top sheet... but that’s s special kink).
For a berth than is used as a seat and not covered with bedding all day, you can actually disassemble the foam pad from the fabric
cover and wrap the foam in very thin plastic (disposable painter’s drop cloths work
great!). You’ll never know it is there, and it makes a great vapor barrier.
If you can get a vapor barrier between you and the bottom of the mattress, you will fix virtually all this problem.