Insulating Radiators

A reflector behind a radiator on an outside wall will reduce the amount of heat loss into the wall by about a quarter. It will also reflect some of the heat back into the room.
Rolls of heat-reflecting aluminium foil easily cut with scissors are sold with self-adhesive pads.
A cheap way of testing that the job is worth the expense is to make your own reflectors.
Cut one or more pieces of cardboard to fit behind the radiator and glue aluminium kitchen foil to one side. Slide the cardboard behind the radiator with the foil facing into the room. After about a year, the kitchen foil may oxidise and turn black so that it no longer reflects. You can then decide whether to replace it with the special heat-reflecting foil, which does not oxidise. It can also be positioned behind the radiator by sticking it to cardboard.