Reflecting cold

[Grimly Curmudgeon]

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cold? That's an interesting concept.

[Jules Richardson]

It's like photocopying cold to make it warmer...

[Chris J Dixon]

Perhaps it comes from the same school of thought that tells us electric current flows in the opposite direction to the electrons.

Chris

[dennis]

Oh come on, everyone knows its the holes that go that way.

[Brian Gaff]

Does not sound very scientific does it.

At that rate I could make a very interesting refrigerator. Brian

[Hugh - Was Invisible]

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> Reflecting cold? That's an interesting concept.

Reminds me. Under the lounge window we have a radiator about 8 feet long with a solid 9 inch block wall behind it. Could do with some reflective insulating stuff behind it.

[Davey]

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I need some as well, but I can't find any that is thin enough to slip back there. It's either too thick and won't fit, or comes in a roll from the kitchen dept. of Morrisons. Maybe I need to start glueing foil to cardboard.

[Paul - xxx]

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A friend of ours, no really, used some wide aluminiumium cooking foil, just draped and stuffed it down as best he could .. it worked so well he did it all over his house!

[Tim Lamb]

In message , harry writes

My wife has what must politely be called a *substantial* nose. Seated in front of our lounge TV, I often find her face shrouded in a scarf.

We have large window openings to either side and the loss of radiated heat causes her some discomfort.

Somebody once tried to explain *black body* radiation to me. AFAIR everything absorbs incident radiation at infra red wavelengths and then re-radiates it. The walls bounce back all they receive but the windows do not absorb much and consequently feel cold.

One of our physicists may explain it better:-)

regards

[Terry Fields]

I did see an expanation of this, but now can't recall all the detail.

As far as infra-red is concerned, the apparent surface temperature is a mix of the IR emitted by the body concerned, together with that transmitted through it and that reflected from it.

Imagine a surface at say 293K (20 degC), in surroundings also at 293K; the emitted, transmitted, and reflected radiation is all at 293K, so the sum total of the IR perceived as coming from the surface is what would be expected for 293K.

Now put the 293K surface in surroundings at say 200K. Although the surface is radiating at 293K, IR is being transmitted through it at

200K, and being reflected from it at 200K. The apparent surface temperature (which determines how much IR is radiated) is the sum of these, which will will be less than that expected for 293K.

For metallic things, there is no transmission of IR, so the apparent surface temperature is the sum of the emitted and reflected components. For foil placed behind a radiator, it will reflect the IR from the radiator back to it, but the other side will also reflect the lower IR back to the outside.

While it isn't 'reflecting cold' as such it is reflecting the lower energy received from outside, which would otherwise lower the heat radiated by the wall behind the radiator.

These are not trivial amounts: the average human body receives about

400W of radiant heat from indoor surroundings, and radiates about 330W in turn. To stay in heat balance, the difference of 70W is made up by burning food by the body.

Heat is radiated according to the fourth power. So turning down your heating at home from 20 degC to 15 degC means that your body will receive about 7 percent less radiated heat, and so to stay in heat balance this will have to be supplied by burning more food or wearing more clothes. This is 7 percent of the radiated heat, or about 23W, which is almost a one-third increase in the heat produced by food-burning!

I once had a suit that had a woven aluminium lining; it was brilliant, cool on hot days and warm on cold days, for the reasons given above. Terry Fields

[Martin Brown]

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>>>>> Reflecting cold? That's an interesting concept.

You want the version that comes in squares about 60cm on a side and 2mm thick. There is a roll version about 1mm thick that is not as good, It is surprisingly effective used on outside walls behind radiators.

It should be in season now next to rockwool and other insulation in B&Q/Wickes. Obvious keywords should get it.

[Davey]

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Thanks! I looked a few weeks ago, and did not see that on the manufacturers' websites. Maybe it's a seasonal thing. Thanks again.

[Roger Chapman]

IANAP

I am more than a little rusty on this so may have a few of the niceties wrong but AIUI:

Black bodies are so called because they absorb and radiate all frequencies. In the real world most objects reflect some frequencies rather than absorbing them (they must do one or the other) which is why we have coloured objects.

All objects above absolute zero radiate to some extent, even cold walls and substantial noses. Such radiation varies with the forth power of absolute temperature so you get quite a noticeable difference even with relatively minor differences in temperature.

Some surfaces radiate much better than others as anyone with a thermometer that uses infra red should have learned by now. I can get good reading from painted radiators but have to stick on a bit of black tape to get much response from copper pipes (a matter of emissivity I believe).

Glass reflects so it will radiate comparatively little so windows should appear colder. They should usually also feel colder to the touch as even double glazing tends to be a worse insulator than all but thinnest of solid walls.

All this off the top of my head so take it with a pinch of salt until a real scientist comes along and pulls it apart.

[Dave Liquorice]

Glass is complicated in that it's transmission/reflection abilties varies with IR frequency. I *think* glass will transmit the higher frequency IR radiation but reflect the lower. So sunlight with high levels of HF IR gets through this is absorbed by objects behind the glass which reraditates it at lower freqencies which the glass reflects. This is how greenhouses work...

Yep, and in the context of IR radiation anything that does go through isn't likely to come back.

[fred]

In article , Grimly Curmudgeon writes

That is MBS of course but if these products are installed in their very specific and controlled operating environment then they do operate in 2 ways that are equivalent to the what was quoted:

1. They reflect heat back into the room (on the inside)
2. They oppose radiation of heat to the outside world (the cold side)

Whether they provide these features effectively in everyday installations is another matter and it is an issue that I have actively questioned here in the past.

These product are required to be installed with a draught free cavity on both inside and outside, and specifically rely on the reflective surface remaining free from dirt for the life of the installation. I regard both of these conditions as highly unlikely to be achieved in typical installations and have been achieved in precisely none on the installations I have witnessed these products in use.

In short, waste of money, barely more effective than a cheap layer of ordinary bubble wrap in the same space. Better still, use some real, proven insulation such as PIR foam. Expensive snake oil.

[1970alr]

Even this stuff?

's only about 3 or 4 mm thick. If your radiator is that close to the wall it needs mounting further way to allow airflow up the back.

Andrew

[Davey]

Thanks, that looks good. They don't actually say how thick it is, just the l x w dimensions, but 3 or 4 mm would be fine. Having lived in the US until recently, I don't know the places to look for stuff over here yet, so guidance is appreciated.

--=20 Davey.

[PeterC]

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>>>>> Reflecting cold? That's an interesting concept.

some behind the rads. last year. Needs a good adhesive as it doesn't transmit much vapour.

[Ghostrecon]

If the body receives 400 and emits 330 the temperature of the body would rise until the themal losses = the gains at which time we would be in thermal equilibrium :-) Zeroth law IIRC

[Terry Fields]

"the difference of 70W is made up by burning food by the body"

Terry Fields