waste of time and money ? ....

You're learning :-)

do people waste their time sticking a shiny reflective material between radiators and the external wall ..... I think they do

If you have 9 inch solid walls with no insulation, isn't this still worthwhile ?.

I think you're wrong

HTH

tim

being in thatb situation, I did put foil behind the radiators. When I redecorated, the foil wentb under the wallpaper - it's neater that way.

Unfortunately that configuration doesn't really work. The mirror surface needs to be presented to the thermal radiation. The foil in that position will merely equalise the temperature of that patch of wall.

Heat is just infrared thermal radiation at a lower frequency than you can see - it is still reflected by a mirror metallic surface.

There is usually a thin (too thin really) layer of insulation as well.

It isn't a huge improvement but it is helpful to add a reflector with a thin layer of insulation behind it. Snag is that you cannot really add thick enough insulation. But as Tesco says every little helps.

It is pointless on interior walls but it does prevent some heat from escaping from exterior ones. I have also used the stuff to prevent my workshop becoming insanely hot inside on a sunny day.

If you have an IR camera you can see radiators that are mounted on solid outside walls by their effect on a nice still day. A decent non-contact thermometer ought to show it too since they are big areas.

The heat is still escaping on a windy day too but you just can't see it.

If you have good cavity wall insulation then I doubt it makes much difference. Plenty of homes don't though. Mine are mostly solid walls.

Page 5. Many of the materials do well at long IR wavelengths.

How do you know?

Bill

Disordered infrared radiation can certainly be heat, but a great deal of heat is instead based on the kinetic or vibrational energy of matter.

I was going to say that most heat *wasn't* in the form of electro- magnetic energy, but then I decided that integrating the 2.7K cosmic background over the entire universe, then working out the comparable mechanical energy, along with that from disordered electronic and nuclear excitations wasn't worth the bother.

#Paul

I don't think foil does much good anyway. Most of the heat from a radiator comes from the air in contact with it rising into the room. If you stand in front of the radiator you don't get much radiant heat like you do with an electric bar fire. Foil only reflects radiant heat.

And it reflects it back into the radiator which is pretty senseless unless you are on a single brick outside wall with no insulation

On say a decent plasterboard and insulation wall heating the wall just makes for better convection from behind the rad.

First, you need to establish how you are going to model the situation. Let us start by making the arguably plausible assumptions that by an ability to reflect radiant heat, the shiny material can alter the heating process, and so it will be particularly effective if near a radiant heat source (where radiant heat flow is high).

Note that if you make different assumptions about the situation, you will, as a result, come to a different conclusion. In practise you will be somewhere between the two cases, but maybe more towards one than the other. You could, if you wanted, and were used to doing such things, easily throw together a rate equation type mathematical model that covered both the above cases; and look at its steady state behaviour.

#Paul

It will stop all the radiant heat flow, Shirley, by relecting it. What its own temperature is at that moment is not relevant. The shiny will heat up from conduction of the air touching it. If the shiny (in the extreme case) were all that separated the room from a sub-zero outside, then it would handily conduct heat from the air inside to the air outside.

You are not quite right there Tim. An imperfect reflector will have partial black body characteristics, and of course if you paint over the foil it is for that reason even more useless.

So it wont reflect *all* of the radiant heat. But so long as te wall is insulated it is useless anyway, because a wall that heat up will improve rear radiator convection

Well, there used to be some kind of laminated black stuff you could stick on that stopped the heat getting to the wall, but the black material was supposed to store the heat and lose it gradually upwards. I guess the heat went somewhere, but not convinced it was in any way storing it. Brian

I was making that assumption yes

It is the normal situation where people put foil behind rads

HTH

tim

And if they painted the radiator (which we should actually call 'emitter') matt black then it would be more efficient at heating the air surrounding it.

Most females would argue to the contrary though ...