An Oldie but a goodie !!
Approximately 5% of heat loss from radiators goes through the wall, even walls that are insulated !! One ‘quick trick’ is to install tin foil behind the radiators thus convecting the heat back into the house. Its really simple, get a roll of tin foil, cut a piece to size for the back of the radiator, then drop it down behind the radiator and adhere it to the wall, job done !! Now just sit back and watch your heating bills go down !!