Foam for floataion

I built a pedal powered dinghy in 2021 and after some development it achieved all my objectives. I've passed it on to a new owner and have embarked on the build of a replacement. This time, rather than a dinghy, I'm building a catamaran. It will still be pedal powered but I've chosen the catamaran format to give me the ability to add sail assistance without too much risk of ending up swimming!
This time, rather than ensuring the hulls are water tight, I've decided to fill them with foam. I can't use expanded polystyrene because it absorbs some water between the beads which are packed together. An alternative type of insulation foam is PIR (polyisocyanurate) which is easily source in boards of the right sort of dimensions for my build. Despite much reading around the subject I'm unable to find a definitive answer as to whether it is truly waterproof or not. It is a closed cell foam so shouldn't absorb water on short term exposure but will longer immersion cause any problems?
Does anyone have experience or have heard of the use of this foam to fill a cavity where it will come into contact with water? Are there any other ideas or alternative foams which I can consider?
Below is a picture of one hull, constructed from aluminium in 2 section for ease of storage and transport.

 

If you're comfortable with fiberglass, you can do away with the metal skin and make the hulls from styrofoam with plywood support webs. I have paper copies of an article on how to make Gary Dierking's "quick-ama" but can't find it online anymore; that would work. I don't know if it's a good idea to make such a thing in take-apart sections; I think it would require doubling the number of cross-beams (akas).

Maybe someone else on here can locate those ama instructions. It's a lot like some surfboard construction.

 

Check out extruded polystyrene (XPS)- you might have to shape it, but if you get some thinner sheets you might be able to bend them to fit. Since Clark left the surfboard blank business, XPS has become a go-to for surfboard makers. You live in a cold weather area, so you should be able to pick up some pink or blue insulation foam at big box stores, or insulation supply stores. It comes in all sorts of thicknesses. I used it in this sailing canoe- not for big load bearing stuff, but for deck and hull bottom support w/ 4mm okume it worked nicely, . It’s not perfect, but it does work. (I think it’s still ok’d for floatation use- you can check that out.) You can use either epoxy or gorilla glue. If you’re going to glue it, remember to score it so the glue holds. Cool stuff- Google XPS surfboards or catamarans- IIRR, it was used for Class A catamarans decades ago.

 

Any of the foams will work unless you plan to leave it outside in the weather exposed to water constantly. If it only has the opportunity to see water when in use, then none of them will absorb water.

 

Yes, it will only see water when in use. PIR comes with a foil on the faces but I also think I should coat the edges in a waterproof PVA like Titebond III. I'm doing a flat roof in fibreglass soon so I may have the option of epoxy. Don't tell Mrs Kirk but I'm prepared to skimp on the quality of the roof if it improves the boat.

 

Use 2 pound pour foam. No foam is absolutely watertight and the best way to build is to make watertight boxes with submersible vents.

But pourable foam is okay unless under constant immersion.

Not sure exactly what or how you are building, but for aluminum another way to go is to lay thick plastic or battens on the bottom and pour foam over it which provides a barrier and you can also then put a garboard drain on the bottom and provide a pathway for any water to exit faster..

If water gets in and leaves slowly, it will soak into the foam some..

Two Part Polyurethane Expanding Foam - Boat Builder Central https://www.boatbuildercentral.com/product/two-part-polyurethane-expanding-foam/