ᐅ Insulating Behind Radiators – How Much Clearance Should You Leave?

Hello everyone, I’m about to face my second winter in my rented old apartment. After finishing the usual renovations (wallpaper, flooring, etc.), I now want to improve the insulation behind the radiators. Unfortunately, as was common back then, the wall behind the radiator is extremely thin. I did some research online and found these Styrofoam panels covered with aluminum foil.

What do you think of these? Are they worth it (better than nothing) or just a waste of money? How much distance should there ideally be between the radiator and the wall? I don’t have much space behind the radiator. I would need to measure more precisely, but roughly it’s about 1.5–2 cm (0.6–0.8 inches) on average.

I cannot remove the radiators (rented apartment – no access to refill the water and the radiators are very heavy). The space around is also very limited, so only flexible materials are an option.

Can anyone give me some advice on this?

Thanks in advance!