What is the best way to add insulation to the outside of an icebox?
Depending upon where your icebox is located on board and what the restrictions are, you might be able to add sheet foam surrounding the box to increase the insulative qualities. We recommend rigid polyurethane foam, which can be purchased in various thicknesses. It's best to have at least six inches of insulation all together, so if your box already has some insulation, you won't need to add more than, say, a two-inch sheet. Remember, the thicker the insulation, and the tighter the seals on the lid or door of your box, the longer it will keep its chill.
Now, if getting a sheet of foam in there isn't an option, but your box does have some recesses around it, you should consider working with two-part pourable foam. (Be warned that you need to have good ventilation when working with two-part foam because the gasses produced can be noxious.)
The final option you have is adding insulation inside the existing box. Taking this route, of course, will cut down on the storage capacity, but the capacity in most boxes is underutilized anyway. It will be difficult to get sheet foam in there so try this approach: Purchase some reflective insulation, which basically looks like a cross between bubble wrap and aluminum foil. (It can be had inexpensively at most hardware and home improvement stores.) Cut it into sheets to fit the inside dimensions of your icebox and glue it in place with contact cement or a waterproof adhesive. Leave enough of the material to line the lid or door of your icebox and you've effectively increased the cool-keeping ability of your icebox.
Before taking any of the steps outlined above, I would recommend that you first have a look at the article that Tom Wood wrote on basic refrigeration. He talks about heat leaks and box efficiency, and it should give you a firm basis for making your modifications. Here's a link to his article: Refrigeration Part I. You might also want to have a look at Don Casey's book "This Old Boat," because he offers a thorough discussion of building and installing iceboxes and refrigeration systems. That book is available in the SailNet Store: "This Old Boat." Here's wishing you the best of luck with your project.