I have an old utility cottage on my property I would like to use as an office/shop. I don't want to remove the roof or siding. It has metal roof over shingles and cedar siding over vertical wood planks. It is really old good hardwood. It does not leak.

I was thinking I could put a vapor barrier inside and use bat insulation on walls and roof and finish with car siding. Actually I don't care what it looks like inside - just want it functional. Or I could use the pink foam insulation and cut to fit between framing or just put it on the outside of the framing - maybe there would be less thermal bridging - again I don't care how it looks.

Floor is thick wood planks. I was thinking of putting down a layer of the pink foam and then osb or plywood on top.

Would any or all of this work???  Thanks