: And using open
: cell building insulation is an invite to having your own interior water
: supply. That stuff will soak up water if given the chance. If it does it
: has to come out or you boat will rot.

Charlie the blue board discussion went on here several years ago and most believed it was an open cell foam that would soak up water. It is not. Blue board is used under concrete floors and next to foundations underground for insulation. If it absorbed water the insulation value would be negated.

Many years ago I did a water test on the Dow Blue Board by weighing a ¾”x1’x1’piece on a gram scales, then submerged it underwater for 7 days and weighed it again. It picked up it’s weight in water, which was like adding a feather to it. I let it set for seven days and it lost any water it had absorbed. This piece of blue board had some utility knife cuts in it that I could tell did have water trapped in them because of the coloration so I think that where the small weight gain was from temporary trapped water. I also use polyolefin (camping sleeping mats) for years in my boats for floatation because it is pliable and tuff. I purposely fill my boats full of water for safety test so both materials have been exposed to lots of water over the years. I'll check out the D-4 plans.

John

Invented by Dow more than 50 years ago and identified worldwide by the distinctive Blue** color, STYROFOAM* products are the most widely recognized brand in insulation today. In the early 1900s, The Dow Chemical Company invented a process for extruding polystyrene to achieve 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.