You'll sometimes see a thin, reflective insulation barrier between a radiator and the outside wall of a building. The Dead Men installed these to reflect the radiators' radiant heat into the room. Typically, a freestanding cast-iron radiator gives off about 40% of its heat by radiation and 60% of its heat by convection. These reflective barriers aren't thick enough to take the place of true inner-wall insulation, but they will direct the radiant energy that's flowing from the hot metal toward where the people are, rather than toward the cold wall. And the older those walls are, the less chance there is that they will contain any insulation.
In New York City, the Empire State Building got a green retrofit, and one of the things they did to save energy was add reflective insulation barriers behind the radiators (there are about 6,500 of them) on the building's outside walls. These barriers make sense. They're not expensive, they're usually very easy to install, and they're as green as can be.
Want to learn more about making your steam-heating system more efficient? Check out Greening Steam: How to Bring 19th-Century Heating Systems into the 21st Century (and save lots of green!).