John,
You are attributing to me something I didn't say.
The enemy is not weight but microbes that eat wood. My beef with foam in general is that it will pick up some moisture eventually and that moisture can support the microbial action we call rot.
The microbes need air and water to survive. If you cut either one of these they die. How much water do they need? I don't know but it isn't much. They are mighty small critters and thay can eat a hole in a boat if given the opportunity.
A boat hull that is light plywood backed up by building insulation is an ideal host to those microbes. The insulation will conceal the rot until it's all the way thru the luan.
Good luck

: 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.