The best thing to do would be to use polyurethane spray foam directly on the hull. While insulating the boat, it also provides buoyancy (like Roger Taylor did with MingMing).
@rek haha oh wow that's wild. i guess the foam layer would also act as a potential bumper in case of *knocks on wood* sea-faring logs etc?
@rek My concern with that (from thinking and talking about green building for years, e.g., container houses) would be that the water vapour permeability of the foam (small but not zero) would eventually allow too much water vapour to get through and condense against the cold hull causing the foam to become water logged.
@rek I don't know of a tested solution but wonder if a thin layer of vapour-open material immediately against the hull could somehow be ventilated sufficiently to prevent water build up. I do know of a metal roof on a house near Cambridge, England, which uses something like that though I'm not completely sure of the details.
@edavies I was under the impression that if you didn't cut into it, or sanded it or wtr that it wouldn't be weak to water. But I've zero experience with the stuff, this is just from what I've read.
Tbh i haven't decided what we'll do. My first idea was to layer sheets of reflectix and polyethylene foam, with each layer glued down. I have rolls of the stuff with me rn
@rek All plausible insulation materials will let water vapour through eventually. How long it takes will depend on the material, of course, and the long-term history of the humidity of the air in the boat and the temperature of the hull so it's a matter of risk rather than being necessarily a problem.
The PNW in winter seems near a worst case, though. Maybe you'd be OK if you went somewhere with warm waters and dry air every so often to dry it out.
@rek That's more of a problem than a Quebec winter because of the higher absolute humidity outside and therefore higher relative humidities in the cabin once the air's been heated a bit.
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