I’ve been reading up on using foam in marine applications for flotation. The basic idea is that if your boat springs a leak and it is made of materials that are heavier than water (like the steel in my pontoons and boat/house frame!) then it will sink. But if you fill the pontoons with a dense foam that can sit in water for a long time and not absorb the water, then even if the pontoons have a leak, the boat can still float.

In my internet research recently, this was the best link I found explaining about using foam for flotation:
https://www.glen-l.com/weblettr/webletters-7/wl55-flotation.html

The most interesting part was the recommendation to use a non-combustible variety of urethane foam. I’m assuming this refers to standard polyurethane foam, which is easily sourced. Based on this page, urethane is highly toxic and not normally used in foam applications.

The other interesting note was that at less than 4lbs per cubic foot density, the foam can absorb large amounts of water over time and thus have substantially reduced flotation properties. 4lbs per cubic foot comes out at about 310kg per cubic meter (if I did my math right).

A quick search in Thailand gets me to this vendor:
http://www.bkk-panelandpipe.com/product_detail.php?p_id=6&pt_code=30
(I’m sure there are many more.)

The link above is for polyurethane block foam. They say that they have densities in the range of 80 to 300 kg/cubic meter. So the very high end of this is within a few percent of the 4lbs per cubic foot.

I don’t know why it didn’t occur to me to look into this before. But I guess that since we are experimenting with prototypes, now isn’t too bad a time to explore it.