A cosy tiny house!
Here you can see the celotex infill of the timber and steal skeleton, this is 50mm thick with another 50mm covering the outside, getting rid of a lot of cold bridges.
It is important to seal all the joints with silicone or expanding foam, this reduces air flow in and through the walls, keeping it nice and warm inside.
I added an extra 100mm of glasswool into the service gap, mainly for acoustic insulation.
And finished off with a sheet of plastic to provide a barrier to warm wet air infitrating the wall from the inside, this is especially inportant with a steel framed building as I don't want water condencing near the steel structure.
As for the wall exterior there is a skin of waterproof membrain and a cladding of galvanised steal that should keep it dry inside.