A little on moisture transport (i like to talk about it but can never find anyone to listen
The principle to follow when insulating is to use a warm side vapour barrier, and avoid cold side vapour barriers.
If insulating against tropical heat, all is well. The aluminum hull
is the warm side, and for practical purposes, it is a perfect vapour barrier. You can use any insulation, include those that allow moisture wicking (diffusion) like rockwool, fiberglass
, or celluslose. As long as you use a moisture permeable wall panel, all is well. You could have moisture introduced in your insulation, and it will simply wick/diffuse through your insulation and wallboard into your cooler interior
If you use this system in a cold climate, you'll get a serious problem, of course, because moisture will diffuse from the warmer, higher-humidity interior to the cooler hull
. If the wall is cold enough to be below the dew point of the air in the interior, condensation
will occur. This moisture cannot go farther because the hull is a perfect vapour barrier, and you've got a 'condensation trap', where moisture will continue to build up until the temperature and humidity conditions change. The result is moisture problems (mold, wet insulation, corrosion
You can alleviate this condition by adding a warm side vapour barrier to slow down the rate of inbound moisture diffusion. But in the boat situation you'll still end up with moisture because none of it can get out through your perfect vapour barrier (metal hull).
If you wanted to wick it away, you'd need a path which would have be be laterally along the inside of the hull (because you don't want to make holes to let it out obviously). And you'd need low humidity (typically cold) area at the end of that path. I'm sure someone could figure out how to build something, but i can't see anything that would be easy.
Probably it would be much easier to use an insulation with very low moisture permeability, like a spray-on closed cell foam. But check its specs to make sure it is closed cell and low permeability - typical foam that you get at a hardware
store in a spray can is open cell and would be no better than fiber insulations.