The chart above provides generally accepted
thermal conductivities typical of the materials described. Do to the
variations in individual manufacturers formulations and production methods
significant variations can exist between apparently similar products. It
should also be remembered that thermal conductivity of the material is
only one of several factors effecting the heat transfer which takes place
in everyday objects. Depending on the materials involved, others factors
may include convection (in gases and liquids) and/or
radiation with varying emphasis on the related components
emissivity and absorptivity.
Convection
In some cases the contributions of
convection and radiation play only a minor part in comparison to that of
conduction. However, under some conditions, the effects of one or both can
be very significant. Convection is the term used to describe the motion
or, circulation current, which is set up in any gas or liquid as it is
heated or cooled. Convection is not, in itself, a singular heat transport
vehicle as is conduction and radiation. Instead, it greatly increases
conduction by constantly circulating colder material to the warm surfaces,
thus increasing the effective delta T.
A closer look at the role
of "trapped air" in traditional insulation materials provides a good
example of how convection effects heat transfer. As you can see from the
table above, air, by itself, is a very good insulator with a "k" value of
only .16. Further examination of the table shows that nearly all
traditional insulation materials have a higher thermal conductivity.
Therefore, one might reasonably ask, "Why use insulation at all?". The
answer is found when one also considers the effect of heat transfer
through convection.
The "k" value given for air (.16) describes the
amount of heat which will travel directly through perfectly still, and
dry, air. However, air used as an insulator never stays completely still.
Instead it sets up an active circulation as one side of the containment
chamber is heated. The heated air rises and the cold air falls. This
circulation constantly exposes the colder air to the warm wall, thus
increasing the delta T across that wall and greatly increases the rate of
heat transfer through the chamber. This is where traditional insulation
helps. In these materials the air is "trapped" on a great many small
chambers called "cells". While each cell still sets up its own convection
current, heat transfer is reduced in direct proportion to the size of the
cell. The smaller the cell, the greater the reduction in convection.
In standard insulation foam, the size of the
air-trapping cells is described in terms of the foam "density". A
high-density foam will have a greater number of smaller cells than will a
low-density foam. However, before jumping to the conclusion that the
highest density foam is, inevitably, the best insulator, there is one more
factor to consider. This is the thermal conductivity of the cell walls
themselves. These are typically PVC, polyurethane, polyethylene or
polystyrene and often have a greater thermal conductivity than does still
air. The greater the number of cell walls, the more material there is
present to transmit heat through conduction. This is why the best
insulation foams must reach a compromise between small cell size (ie.
high-density) and minimal cell wall material (low-density).
Radiation
Like conduction, radiation is a
unique and independent form of heat transfer. Ignoring the conflicts of
wave and quantum theory, it will suffice to say that radiation, in this
case, refers to the transmission of electromagnetic energy through space.
While the term radiation applies to the entire electromagnetic spectrum,
our concern is with that portion which falls between visible light and
radar, the infrared rays. Infrared rays are not themselves "hot", but are
simply a particular frequency of pure electromagnetic energy. Sensible
"heat" does not occur until these rays strike an object, thereby
increasing the motion of it's surface molecules. The heat then generated
is spread to the interior of the object through conduction. Therefore,
radiation is fundamentally different from conduction in that it describes
the transfer of heat between two substances which are not in contact with
each other. All matter above absolute zero (-456.7oF) radiate
heat to some degree. How much heat an object radiates is determined by
it's temperature, the temperature of the surrounding environment and the
object's emissivity factor.
Recently there has been a great
deal of emphasis placed on the importance of radiant heat "barriers" by
some insulation manufacturers. Such barriers inevitably consist of a piece
of aluminum foil glued to a traditional air-trap type insulation foam or
similar material. Although these manufacturers claim greatly increased
insulation performance as a result, the truth is that the improvement, if
any, is highly dependent on the application.
A radiant heat barrier
works by reflecting radiant heat back toward the source. It does not
reflect conducted heat, nor can it reflect heat within a solid object. In
a vacuum, such as outer space, radiant heat barriers are very effective.
In this air-free environment there can be no conduction (except through
solid objects) so all heat transmission is by means of radiation. A
radiant heat barrier on the outside of a space ship or satellite proves a
very efficient insulator by reflecting back up to 95% of the radiant heat
which strikes it. For this same reason, thermos bottles (ie. Dewars
flasks) are also coated internally with aluminum radiant heat
barriers.
However, once our
space ship returns to the atmosphere of earth, it's radiant heat barrier
becomes considerably less efficient. This is because the source of the
heat is no longer completely radiant in nature. The warm air is also
transferring heat to the ship's skin through conduction. How much good the
radiant heat barrier is now doing depend on several factors. If the ship
is parked on the runway in the bright sun, the percentage of radiant heat
to conductive heat would be very high and the barrier would still be quite
helpful. Once the ship is moved into a hot hanger, the conductive heat of
the surrounding air become much more dominate and the barrier contributes
little.
One example of the misuse of radiant heat barriers can be
seen in the common practice of laminating foil sheets between pieces of
standard foam insulation. In this case, all heat reaching the foil barrier
is conductive and passes straight through making the barrier useless.
Radiant barriers reflect infrared most effectively back into a vacuum. As
the density of the material in contact with the barrier increases the
effectiveness decreases. A barrier which would be highly efficient in
space would be totally ineffective if sandwiched between insulating
foam.
The chart below gives the infrared
radiation reflectivity (emissivity) of some common
materials:
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