Insulating behind the radiator - How much distance to leave?

Hello everyone, I am about to spend my second winter in my rented old building apartment. After the "normal" renovation (wallpapers, floor, etc.) is completed, I now want to take care of radiator insulation. Unfortunately, as was common back then, the wall behind the radiator is extremely thin. I have therefore looked around a bit on the internet and found these styrofoam boards covered with aluminum foil. What do you think of these things? Is it worth it (better than nothing) or is it a waste of money? How much space must/should still be between the radiator and the wall? I don’t have much space behind the radiator. I would have to measure more precisely... But roughly estimated on average maybe 1.5-2 cm. I cannot remove the radiators (rented apartment -> no access to refill water and extremely heavy radiators). The space around it is also very limited, so only flexible material comes into consideration. Who can give me some tips here?? Thanks in advance!!