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Message: <blockquote data-quote="onthewater102" data-source="post: 457775" data-attributes="member: 13702">The 2 part expanding urethane is my choice - it doesn't react to gasoline should any spill (insulation boards will dissolve into goo and their type of foam isn't allowed in the lower portions of the hull per federal guidelines.) It does complicate things a bit though - too much foam poured into too small a volume will exert a significant amount of pressure as it forms, and if not properly contained/confined it will fill a cavity completely creating drainage issues through the area.No foam is 100% closed cell, so any of them will take on water over time if left submerged. The closed cell expanding foam tends to do a better job of trapping water around itself and therefore gets a bad rep for becoming water logged when the material itself isn't to blame - it's poor preparation of the area and poor design.</blockquote>