Energy-Efficient Insulation

We are planning a complete renovation of a caboose into a guest home/office in New Jersey.  We plan to install electric heat, possibly with some in the floor.  Since the shell is metal and lined with wood, what would you suggest as the most energy-efficient insulation?  We expect to be removing the interior and replacing it with wood. It does not appear that there is enough air space to use blown insulation without removing the wood. We would appreciate any suggestions to take on this project.   Thank you, Eloise Hagaman

The Money Pit Answer

First, congratulations!  This sounds like a really fun project.  That said, keep this in mind: Electric heat is no doubt the most expensive way to heat any building.  If you're planning to use this caboose year-round, you might want to rethink whether or not you should use electric radiant heat. 
Another option that comes to mind is a ductless heat pump.  This is also electric, but it costs only half as much to operate as radiant heat.  Since your caboose is presumably a relatively small area, this might be the most efficient way to accomplish the talk.  In terms of insulation, that spray foam insulation might be more feasible than you think.  When those walls are stripped, you should be able to easily apply spray insulation to the interior.  It comes with the added benefit of soundproofing, and is a very energy efficient insulation.  I recently had Icynene spray insulation installed at my house, and the resulting impact on energy bills has been nothing short of dramatic. Good luck - and please pass along pictures when the project is complete!