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. Only the first three mechanisms need to be considered here. The flow of heat can be delayed by one or more of these mechanisms; this would depend on the physical properties of the material used to do this.
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Thermal Radiation and Radiant Barriers
Thermal radiation is composed of all wavelengths of light. Most of the energy of the thermal radiation of objects at room temperature is in the infrared part of the spectrum, however, 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 toward one another, the important consideration is the net direction of energy flow.
Thermal radiant barriers have the characteristics of low emissivity, low absorptivity, and high reflectivity in the infrared spectrum. They may also exhibit this for other wavelengths, including visible light; however, this is not necessary to function as a thermal barrier. Only a small fraction of radiant energy is absorbed by such a material (most being reflected back away); therefore, only a small fraction is reemitted. 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.
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
- its thermal conductivity
Most gases, including air, are poor conductors but 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 have low thermal conductivity) rather than by the material itself. Metals 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 an adequate air gap to function adequately.
Convective transfer and convective barriers
Convective heat transfer occurs 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 that are often used to reduce conduction also decrease convection. The small air spaces retard convective movement. There is an ideal density of the material that maximizes both effects simultaneously.
Another example in which 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. You can do this, for example, by correctly choosing insulated clothing, or laying insulating materials beneath in-floor heat cables or pipes. In the latter case, you can 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 that has a layer of rigid insulating material between the studs and the finish layer. A thermal bridge 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. These rules 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; however, in household situations (appliances and building insulation), airtightness is the key in reducing heat transfer caused by 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.
For some systems, there is a minimum insulation thickness required for an improvement to be realized.
Applications
Clothing
Clothing is chosen to maintain the temperature of the human body.
To combat heat, clothing must enable sweat to evaporate (cooling by evaporation). When we anticipate high temperatures and physical exertion, we should choose fabric that billows during movement and creates air currents that increase evaporation and cooling. A layer of fabric insulates slightly and keeps skin temperatures cooler.
To combat cold, evacuating skin humidity is still essential. Several layers may be necessary to achieve this goal while matching one’s internal heat production to heat losses caused by wind, ambient temperature, and radiation of heat into space. Also, insulation against conduction of heat into solid materials is crucial for footwear .
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 insulation also absorb noise and vibration coming from both the outside and the inside of the house, thus producing a more comfortable occupant environment.
Pipe insulation is also important in buildings for pipes that carry heated or cooled fluids.
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 reentry 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). Reentry 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).
External Links
- Home and Industrial Insulating Paint Additive
- Industrial Insulation Basics
- R-value Recommendations from DOE/CE
- Natural Handyman, insulation article
- Article on insulation for older properties
- New and alternative insulation materials
- Home Insulation
- Removable Insulation Photographs
- Thermocouples placed in-situ into a wall, with insulation monitored in real-time