Insulation: Reflectix or Sheet Styrofoam
I've lived aboard a Caliber 28' sailboat in Boston harbor the last three years and I'm amazed how much juice I use for the two oil-filled radiators. The boat isn't insulated; it's a plywood-cored deck. So I've taken down the fabric from the ceiling and removed the veneer ply from the interior cabinhouse sides, in prep for insulating.
I'd use either reflectix or 1/2" unfaced styrofoam boards (kerfed on the back to meet contours). Foil-faced sheet insulation may have an advantage as contact adhesives or hot glue may melt the unfaced styrofoam. I'd prefer the sheet insulation because then I can bond white laminate to it, thus finishing the interior sides and for removable ceiling panels. It will also form a more palpable thickness as I'm replacing several portlights.
My concern, and it's remote, is that if it catches fire, the fumes are highly toxic/caustic, though the reflectix and for that matter, the fibreglas may be equally toxic. Also, I haven't compared the R-values of the two -- I thought there might be other issues I haven't considered and about which you might know.
I'd appreciate any and all comments/guidance you might have on this process.