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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.

Heat is transferred from one material to another by conduction, convection and/or radiation. Insulators 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 reduce Radiative heat transfer.

  • Foams, fibrous materials or spaces reduce Conductive heat transfer by reducing physical contact between objects

  • Foams, fibrous materials or Evacuated Spaces 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.


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 Kiln s)

  • --- 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.

For a list of good and bad insulators, see List Of Thermal Conductivities .


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.