: Is this foam "closed cell"? And does it hold up with exposure to
: salt water? I don't have any experience with the stuff (hence, the
: question), but I do have some vague recollection of having read some
: previous discussion about not all foams being created equal... with
: "bad foam" being a kind that needs to be installed in a
: watertight compartment lest it become a soggy sponge... if not all foams
: are created equal, what kind is the "good kind"?

Good questions. Several are marine grade, closed cell products that are designed for this application, do not absorb significant amounts of moisture, are chemically inert and unafected by seawater. The stuff I used was Webac 151 with 15X accelerator left over from a chemical grout waterproofing job, but it was only available in 5 gal. kits for $450 or so. I have seen "marine" 2-component foam at TAP Plastics, Lowe's and Home Depot, but haven't looked close enough to see whether they actually are as advertised. If in doubt, call the manufacturer and see if their tech reps make the right noises.

These foams do not adhere well to the hull, they expand by a factor of 20 to 30 in just a few minutes, so the bond line is severely stressed. So there are interstitial spaces between the foam and hull that can trap moisture if it enters that space, e.g. after rolling practice. To avoid trapping that moisture, one strategy is to pop the hatches and thoroughly dry out the boat between outings. This is a good idea anyway, but by itself is probably insufficient to dry out the interstitial spaces.

The strategy I used was to do a small epoxy end pour a couple days after the foam was installed, to seal the area off and displace the air from these spaces. It has been almost 4 years now and there is no evidence of discoloration or rot, so I believe it works.

Then again, one could use air bags to accomplish the same purpose, but reserve the ability to remove them and completely ventilate the space. But they could be punctured or left behind by accident.

None of this is simple, is it?