Closed cell foam/water proof?

Having a 1648 Weldbilt hull getting mods for a jet. The shop cautioned against using under-floor foam sheets because "they eventually absorb water." Is there a better foam sheet made for this application that is completely water proof?

Decided to use 1/2 or 3/8 plywood with rib spacer blocks underside. That worked great on a previous hull, Lowe 1648. Floor was in two pieced fit to sides with finger holes on outer side for raising them if the hull needed repair, which it did twice.

The ribs are much wider on the Weldbuilt so there shouldn't be any sluff.

Conventional wisdom is that only closed cell foam should be used. However, there seems to be plenty of evidence that while it has the best water resistance it is not water proof. If constantly dunked it will eventually become water logged.

Also be wary of condensation ... thusly one of my favorite quotes, quoting myself that is:

"A dry bilge is a happy bilge!"

What about a rubber stall mat?

On my next build I am going to leave the floor screw heads exposed.

Then it will be no big deal to pull up the floor when you want to.

If you keep it covered, it shouldn't be getting much water in the bilge.

I used urethane roofing insulation. No issues so far.

Is there a retail source for buying urethane foamboard? I couldn't find it at home depot.

CedarRiverScooter said:

Is there a retail source for buying urethane foamboard? I couldn't find it at home depot.

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Roofmart is a Canadian company but I would imagine any wholesale roofing supplier would keep it.

jtf said:

What about a rubber stall mat?

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Not really familiar with these, but visualize something heavy. Would they be boyant -- I don't know.

Tractor Supply has them in black, probably not good in hot sun, could be used under floor if they don't absorb water. Similar to roll up camping mat, thicker/stiffer.

I'm going to say it would be totally dependent upon the material in question. I am in the HVAC industry and we use closed cell foam to insulate refrigeration linesets where condensation is a major concern. The type of material we use, trade-named "Armaflex", has an awesome ability to not soak up moisture. I know it can be obtained in sheet form as well, because I've had projects where it was spec'd to be the inner liner to insulate ductwork. In this particular form it is not extremely heavy. I know this particular type of material does not do well at all where exposed to UV, but kept out of the sunlight it will last for a very long time.

Personally, if adding flotation, closed cell foam is all I would ever consider using.

https://store.roofmart.ca/WebCatPageServer.exe?PROD_FULL&ProdInfo=00204008855 This is the stuff I used. IKO Enerfoil which is Polyisocyanurate foam.

As long as there is a way for it to drain then it should be okay but, it will retain moisture better than if it is just an air pocket. Plus you can't rinse it.