What material makes the best heat insulator?
Heat can be transfered in one (or in a combination) of three ways:
conduction, convection, and radiation. For a start in the answer to your
question, good heat insulators are substances that are poor conductors
Heat transfer by conduction requires that heat be moved between things
that are in contact with each other. For example, a glass rod is not
nearly as good at conducting heat than is a similar sized iron rod. If
you were to hold on to one end of a glass rod while you held the other
end in a flame for a time, you might feel little or no warmth at the end
you were holding. On the other hand, if you were to perform a similar
experiment with a common metal nail, you wouldn't be able to hold it
very long before your end got uncomfortably hot. This little thought
experiment is an illustration of heat transfer by conduction. Iron is a
better thermal (heat) conductor than glass.
If one were to melt glass, and then spin it into very fine fibers, and
then fluff and tangle the fibers together to form a blanket, one would
have essentially made a batt of fiberglass insulation such as might be
used to insulate the walls of a home.
The material works as an insulator because glass fibers are poor
conductors of heat AND, when fluffed into a blanket, they trap air
(also a rather poor conductor of heat) so that convection currents
cannot be easily established within the mass of material. The effect is
to greatly retard heat flow by either conduction of convection.
By the way, fibers of iron (like steel wool) would't make nearly as good
an insulator as glass because heat could more easily flow from
fiber-to-fiber where they are in contact with each other.
In addition to fiberglass, various foam insulation products exist. You
may be familiar with the common Polystyrene foam cup which is made of a plastic
that has poor thermal conductivity. When blown into a foam, the gas
pockets within the foam are tiny bubbles
which are in contact with each other but not connected so that
convection currents can be established. The result is a very good
thermal insulator that enables one to hold a very hot drink only a few
millimeters from one's fingers and just barely feel the warmth.
Sometimes clothes and winter boots are lined with fluffy or foamed
materials. Once again, the purpose is to separate you from the cold by a
barrier of insulation that interferes with heat transfer by conduction
Finally, some outdoor clothing contains a proprietary material referred
to as Thinsulate. It's is a very thin plastic film that causes heat to
be reflected back to its source. Thinsulate works by interfereing with
the radiation pathway by which heat can be transferred. Rather than
having the heat from your hands just radiate away, Thinsulate in gloves
reflects your own body heat back to you.
If you were to combine a good insulating material as described above
with Thinsulate, you would have the kind of clothes worn by those who
must work in any extremely cold environment.
I hope this answer is what you were seeking.
The "best" material is no material at all -- a vacuum -- because heat will
only be transported a vacuum by radiation not by the other mechanisms of
convection [gross movement of air] and diffusion[ the motion of air due to
the molecular collisions. I am assuming you mean heat insulator for air.
If a vacuum in not practical, of the commonly available insulators would be
polystyrene foam or fiber glass [ be careful with fiber glass it breaks off and is
very irritating to the skin].
In either case the outer surface should be wrapped in a piece of shiny
aluminum foil to reflect radiant heat to some degree.
You can pretty much tell this yourself. You know that your winter coat
is made out of cloth, or maybe plastic or rubber. These materials do
not let heat (or electricity) flow very well. They are insulators. On
the other hand, metals, like wire, or pots and pans, let heat (or
electricity) flow quite well. If you stick a metal spoon into a hot
water on the stove, pretty soon the spoon will heat up so that you will
have to let go, but a plastic or wooden spoon would be OK.
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Update: June 2012