I knew a guy thirty years ago who built a Gillmer design steel
schooner and spray foamed before fitting out the interior
. He lived on board in the north east states and wintered in the caribbean
. It was brilliant stuff and nowadays the modern foams are that much better. Closed cell foams in the two pound variety are even given structural ratings in the building trades and do not require vapour barriers. They adhere tenaciously to any substrate and can give a 5R rating per inch. I would be hard pressed to do a retrofit job on a boat that had the hull ceiled as without the complete envelope the expense wouldn't be warranted. When curing they only give off water vapour and are inert at final cure. You can buy DIY porta packs of foam from Dow, Tiger Foam or Zero Draft
and do a decent job. If I had a newly completed hull, wood, steal or GRP without the interior
, I wouldn't hesitate for a minute in hiring a spray foam contractor and paying the buck and a half per square foot (one inch thick) to have it applied before fitting out the furniture down below.