Traditional fiberglass insulation is the most effective type for helping outdoor wood furnaces retain high temperatures. While polyurethane foams that spray on are popular, difficulties arise as temperatures reach the highest degrees.
One anecdote about using spray foam insulation with an outdoor wood furnace ended with the spray foam crystallizing to the boiler's edge, while the rest of the insulation had basically disappeared between the outside of the boiler and the exterior shell. Spray on foam works well for wooden dry kilns, but those do not reach the same temperatures that a furnace does. Insulation is too expensive to have to replace it as a result of material failure.