STYROFOAM rigid foam insulation also helps protect your home from the damaging effects of moisture. In the early 1900s, The Dow Chemical Company invented a process for extruding polystyrene to achieve a closed cell foam that resists moisture. Recognizing its superior insulating properties, buoyancy and "unsinkability," it was originally adopted in 1942 by the Coast Guard for use in a six-man life raft. That was the start of many other wartime applications by the Coast Guard and Navy. Today, those same properties have found a home in your home – as insulating sheathing that resists moisture and reduces the potential for condensation in your wall where it can reduce the R-value of cavity insulation, contribute to the growth of mold and mildew, and can result in poor indoor air quality.

Blue board above. This is what I used in the bow and stern of my kayak. I shape fitted it by chalking and sanding. Lot of work. It is press fitted in and can't be glued in with some glues or it will be eaten. The Polyolefin butts up against the blue board to hold it in but the blue board can be taken out to inspect for moisture problems if need be.

Polyolefin was used on the sides. It is flexible and can be glued using contact cement. I bought mine at Wal-Mart and is sold as camping sleeping pads. I tested both the blue board and the Polyolefin underwater for a week. I weighed both on a gram scale before the submergence and after. Water retention was negligible. I did deliberate submergence test of my kayak with these materials in it and have experience no problems in three years.

I experimented with can foam for windows and the stuff just kept shrinking and changing form or a couple of weeks.
John