How do you insulate between barn rafters metal roof with lathe to keep it from sweating so you can make a living space there?

You could have foam sprayed on the underneath side. The rafters are probably 3 or 4 feet apart so you can't use regular fiberglass insulation unless you add rafters. You can glue rigid foam panels to the strips. The roof sweats because the air inside comes in contact with it. Any insulation and a vapor barrier should stop this from happening.