This can't be a good insulation barrier. It will have a lot of thermal bridging, so might as well not insulate with rockwool inside. Rockwool will mostly help w

This can't be a good insulation barrier. It will have a lot of thermal bridging, so might as well not insulate with rockwool inside. Rockwool will mostly help with sound there. The metal will also have another issue with condensation as the two different sides will have different temperature and moisture and it will condensate.