Thermal insulation

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Thermal insulation on the Huygens probe
Thermal insulation on the Huygens probe

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 the71.236.223.204 23:16, 27 March 2007 (UTC) 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.

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[edit] 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.

[edit] 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)

[edit] 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 thermal conductivity.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.[1]

[edit] Personal insulation

Clothing is chosen to maintain the temperature of the human body.

To offset high ambient heat, clothing must enable sweat to evaporate (cooling by evaporation). When we anticipate high temperatures and physical exertion, the billowing of fabric during movement creates air currents that increase evaporation and cooling. A layer of fabric insulates slightly and keeps skin temperatures cooler than otherwise.

To combat cold, evacuating skin humidity is still essential while several layers may be necessary to simultaneously achieve this goal while matching one's internal heat production to heat losses due to wind, ambient temperature, and radiation of heat into space. Also, crucial for footwear, is insulation against conduction of heat into solid materials.

[edit] Building insulation

Main article: Building insulation

Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion of total energy consumption worldwide[citation needed]. When well insulated, a building:

  • is energy-efficient, thus saving the owner money constantly.
  • absorbs noise and vibration, both coming from the outside and from other rooms inside the house, thus producing a more comfortable occupant environment.
  • provide more uniform temperatures throughout the space. There is less temperature gradient both vertically (between ankle height and heat height) and horizontaly from exterior walls, ceilings and windows to the interior walls, thus producing a more comfortable occupant environment when outside temperatures are extremely cold or hot.
  • does not need extra effort and expense. Insulation is permanent and does not require maintenance, upkeep, or adjustment.

See also weatherization and thermal mass; both describe important methods of saving energy and creating comfort.

[edit] Industrial insulation

In industry, energy has to be expended to raise, lower, or maintain the temperature of objects or process fluids. If these are not insulated, this increases the heat energy requirements of a process, and therefore the cost and environmental impact.

[edit] Insulation in space travel

Spacecraft have very demanding insulation requirements. Lightweight insulators are a strong requirement, as extra mass on a vehicle to be launched into earth orbit or beyond is extremely expensive. In space, there is no atmosphere to attenuate the sun's radiated energy, so the surface of objects in space heats up very quickly. In space, heat cannot be given off by convective heat transfer, nor conducted to another object. Multi-layer insulation, the gold foil often seen covering satellites and space probes, is used to control thermal radiation, as are specialty paints.

Launch and re-entry place severe mechanical stresses on spacecraft, so the strength of an insulator is critically important (as seen by the failure of insulating foam on the Space Shuttle Columbia). Re-entry through the atmosphere generates very high temperatures, requiring insulators with excellent thermal properties, for example the reinforced carbon-carbon composite nose cone and silica fiber tiles of the Space Shuttle.

[edit] References

  1. ^ Frank P. Incropera; David P. De Witt (1990). Fundamentals of Heat and Mass Transfer, 3rd Ed., John Wiley & Sons, 100 - 103. ISBN 0-471-51729-1. 
  • U.S. Environmental Protection Agency and the U.S. Department of Energy's Office of Building Technologies.
  • Loose-Fill Insulations, DOE/GO-10095-060, FS 140, Energy Efficiency and Renewable Energy Clearinghouse (EREC), May 1995.
  • Insulation Fact Sheet, U.S. Department of Energy, update to be published 1996. Also available from EREC.
  • Lowe, Allen. "Insulation Update," The Southface Journal, 1995, No. 3. Southface Energy Institute, Atlanta, GA.
  • ICAA Directory of Professional Insulation Contractors, 1996, and A Plan to Stop Fluffing and Cheating of Loose-Fill Insulation in Attics, Insulation Contractors Association of America, 1321 Duke St., #303, Alexandria, VA 22314, (703)739-0356.
  • US DOE Consumer Energy Information.
  • Insulation Information for Nebraska Homeowners, NF 91-40.
  • Article in Daily Freeman, Thursday, 8 September 2005, Kingston, NY.
  • TM 5-852-6 AFR 88-19, Volume 6 (Army Corp of Engineers publication).
  • CenterPoint Energy Customer Relations.
  • US DOE publication, Residential Insulation
  • US DOE publication, Energy Efficient Windows
  • US EPA publication on home sealing
  • DOE/CE 2002
  • University of North Carolina at Chapel Hill
  • Alaska Science Forum, May 7, 1981, Rigid Insulation, Article #484, by T. Neil Davis, provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community.
  • Guide raisonné de la construction écologique (Guide to products /fabricants of green building materials mainly in France but also surrounding countries), Batir-Sain 2007

[edit] See also

[edit] External links

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