Thermal 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. i am going to bitch slap that shit bagg.

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.

Trapped air insulators

Most insulators in common use rely on the principle of trapping air to reduce convective heat transfer. 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.

For a list of good and bad insulators, see thermal conductivity.

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" or "heat leak". 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. When a thermal bridge is desired, it can be a heat source, heat sink or a heat pipe.

Optimum insulation thickness

For practical and economic reasons, it is undesirable to use too much insulation. Specifications of industrial insulation are usually done following a heat-transfer analysis. In household situations (appliances and building insulation), airtightness is a key in reducing heat transfer due to air leakage (forced or natural convection). Once airtightness is achieved, it is often sufficient to choose the thickness of the insulative layer intuitively, based on rules of thumb that account for cost, climate, local building practices and standards determining comfort.

As the rate of heat transfer depends on the surface area of the object being insulated, adding a thin layer of poor quality insulation material to a small object can actually increase heat transfer. It can be shown that for some systems, there is a minimum insulation thickness required for an improvement to be realized. ^[1]

Personal insulation

Clothing is chosen to maintain the temperature of the human body by matching the degree of insulation to the environmental temperature and rate of heat production: We chose light clothes when we anticipate high temperatures and physical exertion.

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:

being more energy-efficient, saves the owner money.
absorbs noise and vibration, both coming from the outside and from other rooms inside the house, therefore being quieter.
is more comfortable, temperatures being more uniform throughout the house. There is less temperature gradient between exterior shell (walls, ceiling, ground floor) and the interior.
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.

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.

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.

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.

References

^ Frank P. Incropera (1990). Fundamentals of Heat and Mass Transfer (3rd Ed. ed.). John Wiley & Sons. pp. 100–103. ISBN 0-471-51729-1. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

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 2004
Thermal Insulation

External links

Applications of these principles on super-insulated houses (translated from french).

[1] Frank P. Incropera (1990). Fundamentals of Heat and Mass Transfer (3rd Ed. ed.). John Wiley & Sons. pp. 100–103. ISBN 0-471-51729-1. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

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