出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/09/06 12:58:45」(JST)
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Thermal insulation is the reduction of heat transfer (the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with especially engineered methods or processes, as well as with suitable object shapes and materials.
Heat flow is an inevitable consequence of contact between objects of differing temperature. Thermal insulation provides a region of insulation in which thermal conduction is reduced or thermal radiation is reflected rather than absorbed by the lower-temperature body.
The insulating capability of a material is measured with thermal conductivity (k). Low thermal conductivity is equivalent to high insulating capability (R-value). In thermal engineering, other important properties of insulting materials are product density (ρ) and specific heat capacity (c).
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Clothing can help control the temperature of the human body.
To offset high ambient temperature, clothing can enable sweat to evaporate (thus permitting cooling by evaporation). The billowing of fabric during movement can create air currents that increase evaporation and cooling. ‹The template Reference necessary is being considered for merging.› A layer of fabric then insulates slightly and can help keep skin temperatures to a cooler level[citation needed].
To combat low ambient temperatures, a thick insulation is desirable to reduce conductive heat loss. Other things being equal, a thick sleeping bag is warmer than a thin one. At the same time, evacuating skin humidity ‹The template Reference necessary is being considered for merging.› remains important[citation needed]: several layers of materials with different properties may be used to achieve this goal while lowering heat losses so they match the body’s internal heat production. Clothing heat loss occurs due to wind, radiation of heat into space, and conductive bridging. The latter is most apparent in footwear where insulation against conductive heat loss to the ground is most important.
Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion of global energy consumption. When well insulated, a building:
Many forms of thermal insulation also reduce noise and vibration, both coming from the outside and from other rooms inside a building, thus producing a more comfortable environment.
Window insulation film can be applied in weatherization applications to reduce incoming thermal radiation in summer and loss in winter.
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 energy requirements of a process, and therefore the cost and environmental impact.
Space heating and cooling systems distribute heat throughout buildings by means of pipe or ductwork. Insulating these pipes using pipe insulation reduces energy into unoccupied rooms and prevents condensation from occurring on cold and chilled pipework.
Pipe insulation is also used on water supply pipework to help delay pipe freezing for an acceptable length time.[1]
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 due to compression of the air at high speeds. Insulators must meet demanding physical properties beyond their thermal transfer retardant properties. E.g. reinforced carbon-carbon composite nose cone and silica fiber tiles of the Space Shuttle. See also Insulative paint.
Internal combustion engines produce a lot of heat during their combustion cycle. This can have a negative effect when it reaches various heat-sensitive components such as sensors, batteries and starter motors. As a result, thermal insulation is necessary to prevent the heat from the exhaust reaching these components.[2]
High performance cars often use thermal insulation as a means to increase engine performance.
Insulation performance is influenced by many factors the most prominent of which include:
It is important to note that the factors influencing performance may vary over time as material ages or environmental conditions change.
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. Both heat transfer and layer analysis may be performed in large industrial applications, but in household situations (appliances and building insulation), air tightness is the key in reducing heat transfer due to air leakage (forced or natural convection). Once air tightness is achieved, it has often been sufficient to choose the thickness of the insulating layer based on rules of thumb. Diminishing returns are achieved with each successive doubling of the insulating layer. It can be shown that for some systems, there is a minimum insulation thickness required for an improvement to be realized.[3]
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| リンク元 | 「保温材」「heat insulating material」 |
| 関連記事 | 「insulator」「heat」「heating」 |