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 retarded 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 comprised of the infra-red wavelength of the electromagnetic spectrum. And like all electromagnetic radiation requires no medium in which to travel. The amount of energy radiated by an object is proportional its surface temperature and its emissivity. Any object above Absolute Zero radiates some degree of 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 for its 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 such an 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. Conductive barriers often incorporate a layer or pockets of air to retard heat transfer. Examples include styrofoam or 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. On the other hand, metal exhibit high thermal conductivity and allow heat conduction to occur readily.

The effectiveness of a radiant barrier is negated if it abuts any material with high thermal conductivity. For instance reflective foil needs to be provided an adequate air gap to function adequately.

Convective transfer and convective barriers
Convective heat transfer occur between two objects separated by a moving interface of fluid 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 heat loss to the ground underneath.

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 heat source, heat sink or a heat pipe.

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.

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.

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 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.