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Thermal insulation is the reduction of heat transfer (i.e., 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 specially engineered methods or processes, as well as with suitable object shapes and
materials.

Heat flow is an inevitable consequence of contact between objects of different temperature. Thermal insulation
provides a region of insulation in which thermal conduction is reduced, creating a thermal break or thermal
barrier,[1] or thermal radiation is reflected rather than absorbed by the lower-temperature body.

The insulating capability of a material is measured as the inverse of thermal conductivity (k). Low thermal
conductivity is equivalent to high insulating capability (resistance value).[2] In thermal engineering, other important
properties of insulating materials are product density (ρ) and specific heat capacity (c).

Definition [ edit ]

Main article: Thermal conductivity

Thermal conductivity k is measured in watts-per-meter per kelvin (W·m−1·K−1 or W/mK). This is because heat
transfer, measured as power, has been found to be (approximately) proportional to
difference of temperature
the surface area of thermal contact
the inverse of the thickness of the material

From this, it follows that the power of heat loss is given by

Thermal conductivity depends on the material and for fluids, its temperature and pressure. For comparison
purposes, conductivity under standard conditions (20 °C at 1 atm) is commonly used. For some materials,
thermal conductivity may also depend upon the direction of heat transfer.
Further information: List of thermal conductivities
The act of insulation is accomplished by encasing an object in material with low thermal conductivity in high
thickness. Decreasing the exposed surface area could also lower heat transfer, but this quantity is usually fixed
by the geometry of the object to be insulated.

Multi-layer insulation is used where radiative loss dominates, or when the user is restricted in volume and weight
of the insulation (e.g. emergency blanket, radiant barrier)

Insulation of cylinders [ edit ]

For insulated cylinders, a critical radius blanket must be reached. Before

the critical radius is reached, any added insulation increases heat
transfer.[3] The convective thermal resistance is inversely proportional to
the surface area and therefore the radius of the cylinder, while
the thermal resistance of a cylindrical shell (the insulation layer) depends
on the ratio between outside and inside radius, not on the radius itself. If
the outside radius of a cylinder is increased by applying insulation, a
fixed amount of conductive resistance (equal to 2×π×k×L(Tin-
Tout)/ln(Rout/Rin)) is added. However, at the same time, the convective
resistance is reduced. This implies that adding insulation below a
Car exhausts usually require some
certain critical radius actually increases the heat transfer. For insulated
form of heat barrier, especially high-
cylinders, the critical radius is given by the equation[4]
performance exhausts, where
a ceramic coating is often applied.

This equation shows that the critical radius depends only on the heat transfer coefficient and the thermal
conductivity of the insulation. If the radius of the insulated cylinder is smaller than the critical radius for insulation,
the addition of any amount of insulation will increase heat transfer.

Applications [ edit ]

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Clothing and natural animal insulation in birds and mammals [ edit ]

Main article: Clothing insulation

Gases possess poor thermal conduction properties compared to liquids and solids and thus make good insulation
material if they can be trapped. In order to further augment the effectiveness of a gas (such as air), it may be
disrupted into small cells, which cannot effectively transfer heat by natural convection. Convection involves a
larger bulk flow of gas driven by buoyancy and temperature differences, and it does not work well in small cells
where there is little density difference to drive it, and the high surface-to-volume ratios of the small cells retards
gas flow in them by means of viscous drag.

In order to accomplish small gas cell formation in man-made thermal insulation, glass and polymer materials can
be used to trap air in a foam-like structure. This principle is used industrially in building and piping insulation such
as (glass wool), cellulose, rock wool, polystyrene foam (styrofoam), urethane foam, vermiculite, perlite, and cork.
Trapping air is also the principle in all highly insulating clothing materials such as wool, down feathers and fleece.

The air-trapping property is also the insulation principle employed by homeothermic animals to stay warm, for
example down feathers, and insulating hair such as natural sheep's wool. In both cases the primary insulating
material is air, and the polymer used for trapping the air is natural keratin protein.

Buildings [ edit ]

Main article: Building insulation

Maintaining acceptable temperatures in buildings (by heating and
cooling) uses a large proportion of global energy consumption. Building
insulations also commonly use the principle of small trapped air-cells as
explained above, e.g. fiberglass (specifically glass wool), cellulose, rock
wool, polystyrene foam, urethane foam, vermiculite, perlite, cork, etc. For
a period of time, asbestos was also used, however, it caused health
problems.

Window insulation film can be applied in weatherization applications to

reduce incoming thermal radiation in summer and loss in winter.

When well insulated, a building is: Common insulation applications

energy efficient and cheaper to keep warm in the winter, or cool in the in apartment building in Ontario,
summer. Energy efficiency will lead to a reduced carbon footprint. Canada.
more comfortable because there is uniform temperatures throughout
the space. There is less temperature gradient both vertically
(between ankle height and head height) and horizontally 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.
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.

Mechanical systems [ edit ]

Main article: Pipe insulation

Space heating and cooling systems distribute heat throughout buildings by means of pipes 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 of time.

Mechanical insulation is commonly installed in industrial and commercial facilities.

Passive radiative cooling surfaces [ edit ]

Thermal insulation has been found to improve the thermal emittance of passive radiative cooling surfaces by
increasing the surface's ability to lower temperatures below ambient under direct solar intensity.[5] Different
materials may be used for thermal insulation, including polyethylene aerogels that reduce solar absorption and
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parasitic heat gain which may improve the emitter's performance by over
20%.[5] Other aerogels also exhibited strong thermal insulation
performance for radiative cooling surfaces, including a silica-
alumina nanofibrous aerogel.[6]

Refrigeration [ edit ]

A refrigerator consists of a heat pump and a thermally insulated

compartment.[7]
Insulated hot water supply and return
hydronic piping on a gas-fired boiler
Spacecraft [ edit ]

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 tiles on the Space Shuttle Columbia, which caused the shuttle
airframe to overheat and break apart during reentry, killing the astronauts
on board). 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. Examples of insulation used on spacecraft include
reinforced carbon-carbon composite nose cone and silica fiber tiles of
the Space Shuttle. See also Insulative paint.
Thermal insulation applied to exhaust
component by means of plasma
Automotive [ edit ] spraying
Main article: Exhaust Heat Management
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 from reaching these components.

High performance cars often use thermal insulation as a means to

increase engine performance.

Factors influencing performance [ edit ] Thermal insulation on the Huygens

probe
Insulation performance is influenced by many factors, the most
prominent of which include:
Thermal conductivity ("k" or "λ" value)
Surface emissivity ("ε" value)
Insulation thickness
Density
Specific heat capacity
Thermal bridging
It is important to note that the factors influencing performance may vary
over time as material ages or environmental conditions change.
Cabin insulation of a Boeing 747-

Calculating requirements [ edit ]

8 airliner

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), 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 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.[8]

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See also [ edit ]

Thermal mass – Use of thermal energy storage in building design

List of thermal conductivities
Insulative paint – Type of paint in which can be used to coat a surface to reduce heat transfer
Heat trap – Valves or loops of pipe on water heaters
Removable insulation blanket – Cover fastened onto a mechanical component
Thermal pad – pad on a printed circuit board connected to surrounding copper with a thermal connection
Thermal envelope – Concept in architectural and engineering practice

References [ edit ]

1. ^ "Thermal Break Technology - IQ Technical" . IQ Glass Technical. 28 July 2017. Retrieved 2019-10-16.
2. ^ Ashley, Jake (26 December 2022). "Choosing the Correct Insulation for Your Home" . Homaphy.
3. ^ "17.2 Combined Conduction and Convection" . web.mit.edu. Archived from the original on 19 October 2017.
Retrieved 29 April 2018.
4. ^ Bergman, Lavine, Incropera and DeWitt, Introduction to Heat Transfer (sixth edition), Wiley, 2011.
5. ^ a b Leroy, A.; Bhatia, B.; Kelsall, C.C.; Castillejo-Cuberos, A.M.; Capua H., Di; Zhang, L.; Guzman, A.M.; Wang, E.N.
(October 2019). "High-performance subambient radiative cooling enabled by optically selective and thermally
insulating polyethylene aerogel" . Materials Science. 5 (10):
eaat9480. doi:10.1126/sciadv.aat9480 . PMC 6821464 . PMID 31692957 .
6. ^ Li, Tao; Sun, Haoyang; Yang, Meng; Zhang, Chentao; Lv, Sha; Li, Bin; Chen, Longhao; Sun, Dazhi (2023). "All-
Ceramic, Compressible and Scalable Nanofibrous Aerogels for Subambient Daytime Radiative Cooling" . Chemical
Engineering Journal. 452: 139518. doi:10.1016/j.cej.2022.139518 – via Elsevier Science Direct.
7. ^ Keep your fridge-freezer clean and ice-free . BBC. 30 April 2008
8. ^ Frank P. Incroperation; David P. De Witt (1990). Fundamentals of Heat and Mass Transfer (3rd ed.). John Wiley &
Sons. pp. 100–103 . ISBN 0-471-51729-1.

Further reading

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