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.

Heat energy can be transferred by conduction, convection, radiation or when undergoing a phase change. For the purposes of this discussion only the first three mechanisms need to be considered.

The flow of heat can be delayed by addressing one or more of these mechanisms and is dependent on the physical properties of the material employed to do this.

Thermal radiation and radiant barriers

Thermal radiation is composed of all wavelengths of light, however most of the energy of the thermal radiation of objects at room temperature is in the infrared part of the spectrum according to Wien's displacement law. As with all electromagnetic radiation, it requires no medium in which to travel. The amount of energy radiated by an object is proportional to its surface temperature and its emissivity. Any object above Absolute Zero radiates thermal radiation. As all objects radiate energy towards one another, the important consideration is the net direction of energy flow.

Thermal radiant barriers possess the characteristics of low emissivity, low absorptivity and high reflectivity in the infra-red spectrum. They may also exhibit this for other wavelengths including visible light but this is not necessary to function as thermal barrier. Only a small fraction of radiant energy is absorbed by such a material (most being reflected back away) and therefore only a small fraction is re-emitted. Highly polished metals are one example. Conversely, dark materials with low reflectivity will absorb a large fraction of energy, and similarly emit a large fraction. (see Black Body, Grey Body)

Thermal conduction and conductive barriers

Conduction occurs when heat travels through a medium. The rate at which this occurs is proportional to the thickness of the material, the cross-sectional area over which it travels, the temperature gradients between its surfaces and its thermal conductivity.

Most gases including air are poor conductors, good insulators. Conductive barriers often incorporate a layer or pockets of air to reduce heat transfer. Examples include styrofoam and double glazed windows. Conductive heat transfer is largely reduced by the presence of the air-filled spaces (which has low thermal conductivity) rather than by the material itself. Metals exhibit high thermal conductivity and allow heat conduction to occur readily.

The effectiveness of an aluminum foil radiant barrier in preventing conduction is negated if it abuts any material with high thermal conductivity. Reflective foil needs an adequate air gap to function adequately as a conduction insulation material. A radiant barrier system is defined as a reflective material facing an air space. When the radiant barrier faces an enclosed air space it becomes a reflective insulation with a measurable R-value. Reflective insulation traps air within layers of foam, or plastic bubbles.

Convective transfer and convective barriers

Convective heat transfer occur between two objects separated by a moving interface of liquid or gas. Convective currents driven by heat energy occur between the objects. The physical properties of the fluid or gas and the velocity at which the molecules travel influence the rate of transfer. Convection can be reduced by dividing the convective medium into small compartments to prevent large currents from forming.

Combined barriers

Materials which are often used to reduce conduction also decrease convection. The small air spaces retard convective movement. There is an ideal density of the material which maximises both effects simultaneously.

Another example where different systems are combined are the reflective surfaces and vacuum in a vacuum flask, or Dewar vessel.

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 heating of the ground beneath.

Factors that compromise insulation

Moisture

Damp materials may lose most of their insulating properties. The choice of insulation often depends on the means used to manage moisture and condensation on one side or the other of the thermal insulator. Clothing and building insulation depend on this aspect 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 conductive material, a heat pipe, or a radiative path.

Calculating requirements

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. [cite book
author = Frank P. Incropera
coauthors = David P. De Witt
title = Fundamentals of Heat and Mass Transfer
pages = 100 - 103
edition = 3rd Ed.
publisher = John Wiley & Sons
year = 1990
id = ISBN 0-471-51729-1 ]

Applications

Clothing

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.

Buildings

Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion of total energy consumption worldwide. When well insulated, a building:
* is energy-efficient, thus saving the owner money.
* provides more 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.
* has minimal recurring expense. Unlike heating and cooling equipment, insulation is permanent and does not require maintenance, upkeep, or adjustment.

Many forms of thermal insulations also absorb noise and vibration, both coming from the outside and from other rooms inside the house, thus producing a more comfortable occupant environment.

Pipe insulation is also important in buildings for pipes that carry heated or cooled fluids.

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

Industry

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.

pace 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 surfaces of objects in space heat 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.

References

* 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, [phone].
* 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.
* [http://www.energycodes.gov/implement/pdfs/lib_ks_residential_insulation.pdf US DOE publication, Residential Insulation]
* [http://www.energycodes.gov/implement/pdfs/lib_ks_energy-efficient_windows.pdf US DOE publication, Energy Efficient Windows]
* [http://www.energystar.gov/ia/home_improvement/home_sealing/DIY_COLOR_100_dpi.pdf US EPA publication on home sealing]
* [http://www.ornl.gov/sci/roofs+walls/insulation/ins_02.html DOE/CE 2002]
* 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), [http://www.batirsain.org Batir-Sain] 2007
* [http://www..insulation-r-values.com Comparing Insulation R-Values]

ee also

*Heat transfer
*Building insulation
*Building construction
*Dewar flask
*Superinsulation
*Weatherization
*Thermal mass
*Fireproofing
*Firestop
*Mineral wool
*Asbestos
*Polystyrene
*Calcium silicate
*Vermiculite
*Perlite
*Fiberglass
*Thinsulate
*Insulated shipping container

External links

* [http://www.thermilate.com Home and Industrial Insulating Paint Additive]
* [http://www.cheresources.com/insulationzz.shtml Industrial Insulation Basics]
* [http://www.processheat.ltd.uk/thermal-insulation.html Industrial Thermal Insulation]
* [http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html R-value Recommendations from DOE/CE]
* [http://www.naturalhandyman.com/iip/inf/inf/infxtra/infinsul.shtm Natural Handyman, insulation article]
* [http://www.periodhomeandgarden.co.uk/Features/tabid/63/articleType/ArticleView/articleId/74/Insulate-your-home-and-let-it-breathe.aspx Article on insulation for older properties]
* [http://doityourself.com/insulate/newalternativeinsulatematerials.htm New and alternative insulation materials]
* [http://www.cus.net/insulation/insulation.html Home Insulation]
* [http://www.thermalpipeinsulation.com/ Removable Insulation Photographs]
* [http://livebuilding.queensu.ca/structural/materials/wall.php Thermocouples placed in-situ into a wall, with insulation monitored in real-time]
* [http://www.radiant-barrier.org Radiant Barrier Information]
* [http://www.activeinsulation.com.au Australian Thermal Insulation Contractors]
* [http://www.roymech.co.uk/Related/Thermos/Thermos_insulation.html#k RoyMech]