Heating: save heating costs - attach emergency blanket
Just attach a silver / gold rescue blanket (which does not cost much) behind the heater. The silver side to the inside to reflect the heat and the golden - coated side to the outside. The advantage of this film is that the gold-coated side does not let the heat through in summer.
chris35: Yes, it should look like the insulation boards and help a bit, especially if the wall behind the radiator is thin and poorly insulated. However, the reflective coating only helps with heat radiation: the radiators are designed so that they primarily heat the passing air. So attach the foil well to the wall and not too close to the heater, so as not to obstruct the air flow. If the window sill above the radiator leads the heat outside (and there is still room), you may also glue the insulation under the window sill.
