For what it's worth:

I am, among other things, a "Certified Infrared Thermographer"
(Infraspection Institute). I have several years experience with using
infrared cameras to inspect electrical systems, building envelopes, and
industrial processes. In addition, I have taught these skills to others in
formal classes.

In simple terms, there are three methods of transferring heat: 1) Conduction
2) Convection 3) Radiation

1) Conduction - If two materials are touching each other heat will be
transferred from the warmer to the cooler.
2) Convection - If two materials are separated by a liquid or gas the heat
can be transferred from one material to the other through the motion of the
liquid or gas.
3) Radiation - All materials give off infrared radiation from their
surfaces. This radiation is just below the lower end of visible light and
extends down to microwave frequencies. The qualities of the surface
determine the "Emissivity" of the surface. A perfect radiator has an
emissivity of one. A perfect cold dark body has an emissivity of zero. Real
world objects are between these two extremes. Highly reflective surfaces
usually have a very low emissivity and will not radiate heat efficiently nor
absorb very much heat energy. This is why gold foil is used to cover certain
areas of satellites. It protects them from the suns infrared radiation.

It's also why the space shuttle must roll over with it's top facing earth
and it's bay doors open soon after reaching orbit. It's cooling radiators
(high emissivity) are inside the cargo bay and they must be kept exposed to
the outside and facing away from the sun at all times. Since they have no
means of removing heat by conduction or convection in outer space, they must
do it by infrared radiation.

After that very simplified beginning, on to reflective barriers.

A reflective barrier can greatly reduce the heat gain to a cool object from
infrared radiation. The question is, where to put it? If you have an
insulated ice box you could put it on the outside surface. It would then
reduce heat gain from infrared radiation. However, it would do nothing to
reduce heat gain from convection. The warm air surrounding the outside of
the box would transfer heat to, and through, the reflective surface just as
if it was painted flat black. In addition, since the reflective foil was in
contact with the outside of the box, conduction would just move the heat
right into your ice box.

You could put it on the inside of the box, with the shiny side facing out.
In this case the foil would be in contact with the material forming the box
and again conduction would pass the heat right through the foil. The same
problem occurs no matter where you place the foil. Conduction or convection
always wins.

The only solution I know would be to build an inner box, cover it with shiny
foil, and surround that with a layer of vacuum to eliminate convection and
conduction. Even then, heat gain through infrared radiation would be the
least of your concerns. The closest practical solution is to use insulated
vacuum panels like those built by Glacier Bay and others. They have a real
life insulating rating of R50 per inch. But, you will pay for that luxury.

I can think of one place a reflective foil might help. If you had a freezer
or refrigerator, with adequate vacuum or foam insulation, that had one side
facing the inside of you engine room then foil on that surface facing the
engine would reduce infrared heat gain to the box when running the engine.
But that can also be covered with Mylar faced noise control foam with even
better results.

The bottom line: Most heat gain to a refrigerated box is through convection
and conduction, not infrared radiation. There is no free ride and reflective
foils will not noticeably improve the insulating qualities of the typical
boat ice box.

Rusty O


Reflective barriers
"Skip Gundlach" skipgundlach sez use my name at earthlink dot fishcatcher
(net) - with apologies for the spamtrap wrote in message
...
One of my respondents to my question about vacuum panels is very high on
"Heat Shield" (www.heatshieldmarine.com) - a mostly radiant barrier, to my
expectations, but their claim is that in conjunction with foam, it has the
effect of tripling the foam R value.