Leo I have had expanding foam deteriorate badly with experiments on my pond from uv along and also with the white expanded open cell foam we call Styrofoam, but I have never noticed that with the blue or pink building closed cell foam. To test this further, two years ago I put a one foot square piece of blue board in the pond and left it for a year and there was no uv damage or water logging that I could detect. The floating island has been in the water now for a year and except for the turtle damage from them clawing their way on and off the island I see no damage. The exposed foam rocks are painted to resemble rocks and the paint insures that there won’t be uv damage. I use varnish with uv protection on my kayaks to insure the clear fiberglass on the wood won’t be uv damaged.
I did a water test on the Dow Blue Board about 10 years ago 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 it may be that this polystyrene’s absorption was nothing at all. The word STYROFOAM on the blue board foam is a trademark but the material is really polystyrene. This foam is inexpensive and comes in thickness up to 2 1/2 inches that I have seen in the lumberyards and perhaps more. Since it will not absorb any water worth mentioning I am planning on using it as a fitted flotation in the bow and stern of my new kayak.
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Dow STYROFOAM
Since its introduction in 1948, STYROFOAM Brand insulation, the Blue extruded polystyrene foam, has been a proven performer in millions of installation's worldwide. STYROFOAM brand insulation is made to offer reliable performance over the long term. It's unique closed-cell structure leaves no voids between the cells. The result: high compressive strength and unparalleled resistance to water penetration. Because it stands up to time, you can reuse STYROFOAM brand insulation in may applications, adding exceptional value to your project by saving both future replacement and disposal costs.
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 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.

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.