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. Thermal insulation is the method of preventing heat from escaping a container or from entering the container. In other words, thermal insulation can keep an enclosed area such as a building warm, or it can keep the inside of a container cold. Heat is transferred by from one material to another by conduction, convection and/or radiation. Insulators are used to minimize that transfer of heat energy. In home insulation, the R-value is an indication of how well a material insulates. 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.
Combinations of some of these methods are often used, for example the combination of reflective surfaces and vacuum in a vacuum flask.
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. Many organic insulators are made from petrochemicals and recycled plastic. Many inorganic insulators are made from recycled materials such as glass and furnace slag.
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. These insulators can be fibrous (e.g. down feathers and asbestos), cellular (e.g. cork or plastic foam), or granular (e.g. sintered refractory materials).
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. refractory materials used in kilns)
Mechanical properties (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. Astronomic telescope lenses are held in place by solid fiberglass supports, to prevent warping the lens slightly due to heat variations. A ceramic block or tile will keep a kitchen counter from being damaged by a hot pot.
For a list of good and bad insulators, see thermal conductivity.
Choice of insulation
Often, one mode of heat transfer predominates, leading to a specific choice of insulation.
Some materials are good insulators against only one of the heat-transfer mechanisms, but poor insulators against another. 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. When physical contact is made with the layer of metal, the desired thermal resistance is lost and the opposite impact is achieved, as the metal then acts as a thermal conductor and not as an insulator.
Effect of humidity
Damp materials may lose most of their insulative properties. 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. Clothing and building insulation depend on this aspect greatly, 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.
Optimum insulation thickness
For practical and economic reasons, it is undesirable to use too much insulation. Specifications of industrial insulation are usually done following a heat-transfer analysis. In household situations (appliances and building insulation), airtightness is a key in reducing heat transfer due to air leakage (forced or natural convection). Once airtightness is achieved, it is often sufficient to choose the thickness of the insulative layer intuitively. This is based on rules of thumb that account for cost, climate, local building practices, and standards determining comfort.
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. It can be shown that for some systems, there is a minimum insulation thickness required for an improvement to be realized. [1]