Properly controlling moisture in your marine refrigeration compartment(s) will improve the effectiveness of your air sealing and insulation efforts, and these efforts in turn will help control moisture.
A good strategy for controlling moisture in your marine refrigeration is to ‘damp proof’ it with a vapor barrier (or vapour barrier, if you are Canadian, eh).
Vapor barrier is typically a plastic sheet that resists diffusion of moisture through walls and doors of your compartment to prevent “interstitial condensation” – moisture in the walls and doors themselves.
It protects against corrosive damage caused by humidity, moisture, oxygen, salt spray, aromas, grease and other airborne contaminants and heat transfer.
What moisture can do to your refrigeration compartment
The laws of physics govern how moist air reacts in various temperature conditions. The ability of air to hold water vapor increases as it warms and decreases as it cools. Once air has reached its ‘dew point’ the air can no longer hold all of the moisture in it and the moisture condenses on the first cold surface it encounters. If this surface is within your refrigeration compartment wall cavity, the result can be wet insulation and framing.
In most climates, properly installed vapor barriers can be used to reduce the amount of moisture transfer. They work with your insulation to reduce the opportunity for condensation in your refrigeration or freezer.
Protecting your marine refrigeration compartment with vapor barrier
You don't want your beautiful compartment insulation work to go to waste, getting soaked with moisture over the years and robbing its insulation R value! So, use a moisture barrier as the first layer on the outside between your insulation and compartment outer wall, and as the last layer on the inside between the insulation and compartment inner wall. Voila! Your insulation is encapsulated to stop airflow between the layers, dramatically reducing the chances of moisture invasion.
The best and easiest vapor barrier to use is a 6 mil poly. You can find it at any home renovation store; it is the same material home builders put over insulation before they install drywall! Tape it into your compartment and lid using vapor barrier tape and you are all set - protected again moisture in your marine refrigeration.