Shielding whose function is to reduce heat flow. Heat loss by conduction is stemmed using layers of material having low thermal conductivity. Loss by convection is reduced by preventing the movement of fluids around the object. Loss by heat radiation is reduced using reflective coatings.
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.
The major types of insulation are associated with the major types of heat transfer:
Reflectors are used to reduce radiative heat transfer. Foams, fibrous materials or spaces are used to reduce conductive heat transfer by reducing physical contact between objects Foams, fibrous materials or evacuated spaces are used to reduce convective heat transfer by stopping or retarding the movement of fluids (liquids or gases) around the insulated object.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.
Materials used for thermal insulation
Many different materials can be used as insulators.
Trapped air insulators
Most insulators in common use rely on the principle of trapping air to reduce convective and conductive heat transfer, but not radiative.
The quality of such an insulator depends on:
The degree to which air flow is eliminated (large cells of trapped air will have internal convection currents) The amount of solid material surrounding the air (large percentages of air are better, as this reduces thermal bridging within the insulator) The degree to which the properties of the insulator are appropriate to its use: Stability at the temperatures encountered (e.g. softness and flexibility for clothes, hardness and toughness for steam pipe insulation) Service lifetime (due to thermal breakdown, water resistance or resistance to microbial decomposition)Solid insulators
Any material with low thermal conductivity can be used to reduce conductive heat transfer.
Choice of insulation
Often, one mode of heat transfer predominates, leading to a specific choice of insulation. For example, metals are good radiative insulators, but poor conductive insulators, so their use as thermal reflective insulators in buildings is limited to situations where they can be installed in contact with air and not with solid material, such as on metal roofs, in attics (as a radiant barrier) or in cavity walls when trapped air (as air pockets, bubbles or foam) is next to the layer of metal. The choice of insulation often depends on the means used to manage humidity (water vapor) on one side or the other of the thermal insulator.
Heat Bridging
Comparatively more heat flows through a path of least resistance than through insulated paths. Insulation around a bridge is of little help in preventing heat loss or gain due to thermal bridging; When a thermal bridge is desired, it can be a heat source, heat sink or a heat pipe.
Optimum insulation thickness
For practical and economic reasons, it is undesirable to use too much insulation. In household situations (appliances and building insulation), airtightness is a key in reducing heat transfer due to air leakage (forced or natural convection).
As the rate of heat transfer depends on the surface area of the object being insulated, adding a thin layer of poor quality insulation material to a small object can actually increase heat transfer.
Personal insulation
Clothing is chosen to maintain the temperature of the human body by matching the degree of insulation to the environmental temperature and rate of heat production: We choose light clothes when we anticipate high temperatures and physical exertion.
Building insulation
Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion of total energy consumption worldwide.
Industrial insulation
In industry, energy has to be expended to raise, lower, or maintain the temperature of objects or process fluids.
Insulation in space travel
Spacecraft have very demanding insulation requirements. In space, heat cannot be given off by convective heat transfer.
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).
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