waste of time and money ? ....

[Martin Brown]

Colour actually doesn't make all that much difference unless the surface finish of the radiator is polished mirror chrome like some designer ones intended for bathrooms. That really does hammer radiative output.

In the thermal IR band for emission almost every surface that isn't actually shiny metallic is a fairly good approximation to black. There are a few exceptions (black polythene bin bags are transparent to it).

I once built a PSU in a shiny new aluminium box and it ran into thermal overload PDQ - a coat of paint on the outside and it was fine.

[Martin Brown]

It would indeed conduct heat. In this scenario it gets warmed up by the radiant heat that it fails to reflect and the ambient air and cooled from behind by leakage through the foam on the back into the wall.

Either one would work on its own to slight advantage but the two together are better. The foil equalises the temperature across its surface by conduction and ends up warmer than the wall would.

They typically manage about 90% though which isn't bad.

At equilibrium the mirror surface of the foil ends up at a slightly higher temperature than the wall would do - at least it does if it is has a thin layer of insulating polystyrene foam on the back.

It is some help with exterior solid walls - no point in warming the outer surface of your house just for the hell of it.

[PeterC]

All my rads on outside walls have slightly insulated foil behind them. The walls are cavity with rockwool, but the heat would go through the inner leaf and then set up some convection - the air would percolate a bit through the rockwool. The exception is the rad in the hall as it's on an internal, single skin brick, wall. That does heat the other side - and the CU!