Foil behind radiators?

As the wp article explains, "radiators" put out very little heat by radiation - so foil doesn't provide much insulation.

A cheapo bit of polystyrene will do the job fine though.

Cardboard insulates. Foil doesn't to any significant extent, but does reduce the board's flammability some. There are better insulators, but cardboard works.

NT

It's good thick cardboard with two layers of airgap, IYSWIM.

They do produce some thermal radiation though so the foil is actually helpful and also easier to clean than bare expanded polystyrene. Anything much above 55C produces enough to be noticeable.

The foil backed polystyrene is particularly effective for preventing ingress of heat from metal garage doors in mid summer. The door ends up much hotter as a result.

Expanded polystyrene is a lot more effective as an insulator than cardboard.

Agreed they are basically actually 'convectors' not 'radiators'; however if you can get your hand behind a radiator flat on the wall, it will be normally be pretty warm to the touch... I would guess most of that's from radiation, and tryingf to stop it leaching to the outside can't be a bad thing?

Double glazing? You posh bastard !