In some situations, it is desirable to
keep heat from escaping a container or from entering an
area. Normally, heat is transferred by from one material
to another by conduction, convection and/or radiation.
Insulators are used to minimize that transfer of heat
energy. In home insulation, the R-value is an indication
of how well a material insulates.
Questions you may have are:
Where thermal insulation is used
If you have an object or area that is at
a certain temperature, you may want to prevent that
material from becoming the same temperature as
neighboring materials. This is usually done by employing
thermal insulation barrier.
If the air outside is cold, you may want
to protect your skin by wearing clothes that keep the
cold out and the body warmth in
If your house has cool air inside during
the summer, you may want to prevent the temperature from
becoming the same as the hot air outside by having the
house well insulated
If you have a hot drink, you may want to
prevent it from becoming room temperature by putting it
in a thermos bottle
Any place where there are materials of
two drastically different temperatures, you may want to
provide an insulating barrier to prevent one from
becoming the same temperature as the other. In such
situations, the effort is to minimize the transfer of
heat from one area to another.
How insulation works
Insulation is a barrier that minimizes
the transfer of heat energy from one material to another
by reducing the conduction, convection and/or radiation
Most insulation is preventing
conduction. In some cases radiation is a factor. A good
insulator is obviously a poor conductor.
Less dense materials are better
insulators. The denser the material, the closer its
atoms are together. That means the transfer of energy of
one atom to the next is more effective. Thus, gases
insulate better than liquids, which in turn insulate
better than solids.
An interesting fact is that poor
conductors of electricity are also poor heat conductors.
Wood is a much better insulator than copper. The reason
is that metals that conduct electricity allow free
electrons to roam through the material. This enhances
the transfer of energy from one area to another in the
metal. Without this ability, the material--like
wood--does not conduct heat well.
Conduction occurs when
materials--especially solids--are in direct contact with
each other. High kinetic energy atoms and molecules bump
into their neighbors, increasing the neighbor's energy.
This increase in energy can flow through materials and
from one material to another.
To slow down the transfer of heat by
conduction from one solid to another, materials that are
poor conductors are placed in between the solids.
Fiberglass is not a good conductor nor
is air. That is why bundles of loosely packed fiberglass
strands are often used as insulation between the outer
and inner walls of a house.
Heat cannot travel though a vacuum. That
is why a thermos bottle has an evacuated lining. Heat
cannot be transferred from one layer to the other
through the thermos bottle vacuum.
To slow down the heat transfer between
air and a solid, a poor conductor of heat is placed in
A good example of this is placing a
layer of clothing between you and the cold outside air
in the winter. If the cold air was in contact with your
skin, it would lower the skin's temperature. The
clothing slows down that heat loss. Also, the clothing
prevents body heat from leaving and being lost to the
Likewise, when you swim in water, cold
water can lower your body temperature through
conduction. That is why some swimmers wear rubber wet
suits to insulate them from the cold water.
convection is transfer of heat when a
fluid is in motion. Since air and water do not readily
conduct heat, they often transfer heat (or cold) through
their motion. A fan-driven furnace is an example of
Insulation from heat transfer by
convection is usually done by either preventing the
motion of the fluid or protecting from the convection.
Wearing protective clothing on a cold, windy day will
inhibit the loss of heat due to convection.
Hot and even warm objects radiate
infra-red electromagnetic waves, which can heat up
objects at a distance, as well as lose energy
themselves. Insulation against heat transfer by
radiation is usually done by using reflective materials.
A thermos bottle not only has an
evacuated lining to prevent heat transfer by conduction,
but it also is made of shiny material to prevent
radiation heat transfer. Radiation from warm food inside
the thermos bottle is reflected back to itself.
Radiation from warm outside material is reflected to
prevent heating cold liquids inside the bottle.
The R-value of a material is its
resistance to heat flow and is an indication of its
ability to insulate. It is used as a standard way of
telling how good a material will insulate.The higher the
R-value, the better the insulation.
The R-value is the reciprocal of the
amount of heat energy per area of material per degree
difference between the outside and inside. Its units of
measurement for R-value are:
(square feet x hour x degree F)/BTU
the English system and
(square meters x degrees C)/watts in the
Insulation for the home has R-values
usually in the range of R-10 up to R-30.
The following is a listing of different
materials with the English measurement of R-value:
|Hardwood siding (1 in.
|Wood shingles (lapped)
|Brick (4 in. thick)
|Concrete block (filled
|Fiberglass batting (3.5 in.
|Fiberglass batting (6 in.
|Fiberglass board (1 in.
|Cellulose fiber (1 in.
|Flat glass (0.125 in thick)
|Insulating glass (0.25 in
|Air space (3.5 in. thick)
|Free stagnant air layer
|Drywall (0.5 in. thick)
|Sheathing (0.5 in. thick)
The R-value is proportional to the
thickness of the material. For example, if you doubled
the thickness, the R-value doubles.
Thermal insulation is used minimize the
heat transfer in many everyday situations. It is done by
reducing conduction, convection and/or radiation
effects. The R-value is a standard of measurement of