Re: lets talk about shipping or sea containers
Insulation, depending on how 'finished' you need the interior
Spray foam if you have the capability, but uneven and kinda ugly
if not covered.
Frame the inside walls and ceiling with 2x2's and use rigid insulation
panels between the studs.
Lay the container on it's side and spray foam the underneath [PITA]
or you can insulate from below if you choose pier and beam.
You can pack the underneath with fiberglass insulation before you
set it on a concrete slab, but suggest you plug the holes for the forks
and silicone around the bottom edge to keep critters from tearing
up the insulation over time.