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insulation.
The term thermal insulation can refer to materials
used to reduce the rate of heat transfer, or the methods
and processes used to reduce heat transfer. Thermal
insulation is the method of preventing heat from escaping
a container or from entering the container. In other
words, thermal insulation can keep an enclosed area
such as a building warm, or it can keep the inside of
a container cold. Heat is transferred from one material
to another by conduction, convection and/or radiation.
Insulators are used to minimize the transfer of heat
energy. In home insulation, the R-value is an indication
of how well a material insulates. The major types of
insulation are associated with the major types of heat
transfer:
- Reflectors are used
to reduce radiative heat transfer.
- Foams, fibrous materials
or spaces are used to reduce conductive heat transfer
by reducing physical contact between objects
- Foams, fibrous materials
or evacuated spaces are used to reduce convective
heat transfer by stopping or retarding the movement
of fluids (liquids or gases) around the insulated
object.
Combinations of some
of these methods are often used, for example the combination
of reflective surfaces and vacuum in a vacuum flask.
Understanding heat transfer
is important when planning how to insulate an object
or a person from heat or cold, for example with correct
choice of insulated clothing, or laying insulating
materials beneath in-floor heat cables or pipes in
order to direct as much heat as possible upwards into
the floor surface and reduce heat loss to the ground
underneath.
Materials used for thermal
insulation.
Many
different materials can be used as insulators. Many
organic insulators are made from petrochemicals and
recycled plastic. Many inorganic insulators are made
from recycled materials such as glass and furnace
slag.
Building
insulation.
The goal of insulation used
in building construction is to slow
down heat transfer. The same
materials are required to keep
buildings cooler in hot climates, or
warmer in cold climates; methods may
be different because of the
necessity to manage humidity buildup
differently. Occupied buildings
always need to evacuate humidity. No
substance can stop heat transfer
from occurring.
As more insulation is
installed, more comfort (thermal and
soundproofing) is created, and
operating costs are lowered. There
is no objective test or standard for
thermal insulation, although ASHRAE
Standard 55 defines thermal comfort
goals. The three means of
Heat-transfer resistance reduce
radiative, conductive and convective
losses and gains. Local custom often
determines the methods used to
achieve comfort level goals.
In some climates,
large thermal mass can be used to
damp daily swings in temperature.
Adobe, earth, stone, and concrete
are poor insulators but serve the
purpose of regulating indoor
temperature by damping, evening out
nighttime and daytime lows and
highs. If a house has an attic,
indications that it is poorly
insulated and poorly ventilated
include the attic being extremely
hot in the summer, and dew and frost
forming on cold surfaces in the
attic, such as on the underside of
the roof sheathing, during the
winter.
Trapped air insulators.
Most
insulators in common use rely on the principle of
trapping air to reduce convective and conductive heat
transfer, but not radiative. These insulators can
be fibrous (e.g. down feathers and asbestos), cellular
(e.g. cork or plastic foam), or granular (e.g. sintered
refractory materials).
The quality of such
an insulator depends on:
- The degree to which
air flow is eliminated (large cells of trapped air
will have internal convection currents)
- The amount of solid
material surrounding the air (large percentages
of air are better, as this reduces thermal bridging
within the insulator)
- The degree to which
the properties of the insulator are appropriate
to its use:
-
Stability at
the temperatures encountered (e.g. refractory
materials used in kilns)
-
Mechanical properties
(e.g. softness and flexibility for clothes,
hardness and toughness for steam pipe insulation)
-
Service lifetime
(due to thermal breakdown, water resistance
or resistance to microbial decomposition)
Solid insulators.
Any material with low thermal conductivity
can be used to reduce conductive heat transfer.
Astronomic telescope lenses are held in place
by solid fiberglass supports, to prevent warping
the lens slightly due to heat variations.
A ceramic block or tile will keep a kitchen
counter from being damaged by a hot pot.
Choice
of insulation.
Often, one mode
of heat transfer predominates, leading to
a specific choice of insulation. Some materials
are good insulators against only one of the
heat-transfer mechanisms, but poor insulators
against another. For example, metals are good
radiative insulators, but poor conductive
insulators, so their use as thermal reflective
insulators in buildings is limited to situations
where they can be installed in contact with
air and not with solid material, such as on
metal roofs, in attics (as a radiant barrier)
or in cavity walls when trapped air (as air
pockets, bubbles or foam) is next to the layer
of metal. When physical contact is made with
the layer of metal, the desired thermal resistance
is lost and the opposite impact is achieved,
as the metal then acts as a thermal conductor
and not as an insulator.
Effect of humidity.
Damp materials may lose most of their insulative
properties. The choice of insulation often
depends on the means used to manage humidity
(water vapor) on one side or the other of
the thermal insulator. Clothing and building
insulation depend on this aspect greatly,
to function as expected.
Heat bridging.
Comparatively
more heat flows through a path of least resistance
than through insulated paths. This is known
as a thermal bridge, heat leak, or short-circuiting.
Insulation around a bridge is of little help
in preventing heat loss or gain due to thermal
bridging; the bridging has to be rebuilt with
smaller or more insulative materials. A common
example of this is an insulated wall which has
a layer of rigid insulating material between
the studs and the finish layer. When a thermal
bridge is desired, it can be a heat source,
heat sink or a heat pipe.
Optimum insulation thickness.
Industry standards are often
"rules of thumb" developed over
many years, that offset many conflicting goals:
what people will pay for, manufacturing cost,
local climate, traditional building practices,
and varying standards of comfort. Heat-transfer
analysis can be performed in large industrial
applications, but in household situations
(appliances and building insulation), airtightness
is the key in reducing heat transfer due to
air leakage (forced or natural convection).
Once airtightness is achieved, it has often
been sufficient to choose the thickness of
the insulative layer based on rules of thumb.
Diminishing returns are achieved with each
successive doubling of the insulative layer.
It can be
shown that for some systems, there is a
minimum insulation thickness required for an
improvement to be realized.
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