Thermal insulating material is placed between a warm region and a cooler region to reduce the rate of heat flow to the cooler region. In a warm house during thee winter, insulation in thee walls, ceilings, and floors reduces thee loss of heat from the warm interior to the colder outdoor air; in a cool house during the summer, the insulation reduces the entry of heat from the warmer outdoor air. The thermal insulation is useful not only because it helps to provide comfortable living conditions but also because it helps to conserve fuel or electric power used for heating or cooling the house.
In industry, thermal insulation is used for purposes such as enclosing heating equipment, pipes that carry steam, and cold storage spaces. The insulation helps to conserve fuel or power or to maintain a uniform temperature in an enclosure.
Heat is energy that flows from one region to another because of a difference in temperature between the two regions. The heat is transferred by conduction, convention, or radiation, or any combination of them; for example, the transfer of heat to or from a wall surface involves all three modes of heat transfer. Thermal insulating material is useful because it reduces the contribution made by one or more of these modes of heat transfer.
Reducing Heat Transfer by Conduction
In a flat wall made of any solid material, if one face is at a higher temperature than the other, heat will flow through the wall by conduction. The flow will be perpendicular to the faces. The rate at which heat will flow through the wall is directly proportional to the area of the wall and to the temperature differences between the two sides of the wall, and it is inversely proportional to the tl1ickness of the wall. Thus, if a wall is the insulation on a flat surface, doubling the insulation thickness will halve the rate of heat flow if the temperatures at the two faces of the insulation remain unchanged. (Although the rate of heat flow can be reduced by increasing the insulation thickness, the rate cannot be made zero no matter how much insulating material is used. )
The rate at which heat will flow through the wall also depends on the thermal conductivity of the wall material. The thermal conductivity of the wall is defined as the rate at which heat flows through it per unit area perpendicular to the direction of heat flow and per unit temperature gradient (temperature drop per unit of length) in the direction of heat flow.
The higher the thermal conductivity of a material, the poorer it is for use as thermal insulation.
The thermal conductivities of different materials vary over a wide range; for example, under room temperature conditions the thermal conductivity of copper is approximately 6 times that of iron, 2,300 times that of asbestos sheets, and 15,000 times that of still air.
Reducing Heat Transfer by Convection
Because the thermal conductivity of air is so low, it might appear that an excellent way to insulate any body would be simply to surround it with air. This is not the case, however, as can be seen by considering the air space within the outside walls of a frame house. In winter the air trapped in this space is in contact with a cold outside surface and a warm inside surface. The air in contact with the cold surface becomes more dense as it is cooled, and this causes it to< fall; the air in contact with the warm surface becomes less dense as it is heated, and this causes it to rise. Thus, a continual flow of air is set up-the air moves upward over the warm surface and picks up heat and then flows downward over the cold surface and gives up heat. This mode of carrying heat from the warm to the cold side of the air space is called convection, and the air flows are called convection currents. Because of the convection currents, the rate of heat transfer through the wall is much greater than it would be if the air could be kept still.
If the air space within the outside walls of a house is filled with a porous material, the air circulation will be impeded, and the rate of heat transfer due to convection will be greatly reduced. The porous material will introduce some paths along which heat can flow by conduction, but the heat flow due to conduction will be small if the material has a low thermal conductivity and it is packed loosely.
Porous materials used for insulation in house walls include rock wool, glass wool, and expanded vermiculite. These materials are chemically inert and incombustible, properties not possessed by some materials that otherwise would be suitable.
For example, wood shavings or crumpled paper could be used to insulate the walls of a house except that they would increase tl1e fire hazard, would not be vermin-proof, and might deteriorate in time.
Reducing Heat Transfer by Radiation
The transfer of heat by radiation differs from the other two modes in that it does not require the presence of any matter; for instance, the earth receives radiant energy from the sun even though separated from it by an almost perfect vacuum. The rate at which heat is transmitted by radiation depends on various factors, including the temperatures of the surfaces and the kinds of surfaces involved. A highly polished metal surface, for example, is likely to emit only 1/25 as much radiation as does a dull, oxidized metal surface at the same temperature.
The effectiveness of thermal insulation in the form of sheets of crumpled aluminum foil partly depends on the fact that shiny metal surfaces are poor radiators of heat. When this foil is used in an air space within the walls of a house, heat flow by radiation is low because of the poor emissivity of the shiny metal surfaces, heat flow by conduction is low because of the poor contact between the layers of foil, and heat flow by convection is low because the air is trapped in a number of small pockets.
Protection of Insulation from Moisture
Besides heat transfer, another factor that must be considered when insulating a house is the effect of moisture on the insulation. In winter, the air inside the house is likely to be more humid than the outdoor air. If any of the warm humid air from inside the house flows into the insulated space within the outside walls, it is likely to be cooled below its dewpoint, and condensation will occur. Water in the insulation not only will damage the house but also will destroy the effectiveness of the insulation because the thermal conductivity of water is almost 25 times that of air.
Air from inside the house will flow slowly even through plaster walls because the plaster actually is porous. This flow of air results from slight differences in pressure caused by the wind and by the difference in the density of the air inside and outside the house. To stop the flow of warm humid air from the interior of the house, a vapor barrier consisting of a sheet of plastic or of metal foil is placed under the plaster. This barrier should be placed on the warm side of the air space rather than on the cold side because its function is to prevent the warm humid air from entering the insulation.
Selection of Insulating Materials
Any material having a low thermal conductivity can be used as insulation, but to be completely satisfactory it usually must also have certain other properties. If it is to be used at high temperatures, it must be able to withstand these temperatures without deteriorating. If it is to be used at low temperatures, it must either be protected from or unaffected by moisture. For some applications the insulation may need to have sufficient structural strength to be self supporting; it may need to be tough enough to resist rough usage and vibration; it may need to be fireproof, odorless, and vermin-proof; and it may need to be light in weight. In addition to these factors, the choice of the kind of insulation, as well as the thickness to be used, depends on the cost of the insulation and the cost of the heat or of the refrigeration that the insulation will save.
Insulation in new homes is installed as the home is being built. Many manufacturers make batts of glass or rock wool for insulation in new homes; they size the batts so that they will fit into standard stud spaces. The insulating material is usually fastened to one side of a sheet of asphalted paper that acts as a vapor barrier. Some manufacturers place the insulation between two sheets of paper, only one of which is treated to act as a vapor barrier. The batts should be installed with the treated paper on the warm side of the wall.
If insulation is to be added to an older home, the insulation usually must be blown or poured into the spaces between the studs. Glass or rock is changed by the manufacturer into a granulated form so that pneumatic machines can readily blow it into the house walls. It is important that the material not be allowed to "bridge" over, as this would leave empty pockets within the wall. It is also necessary to guard against settling, as this would leave voids at the top of the wall. If expanded vermiculite is used, it is poured into place.
Since the materials that are commonly used to insulate homes have essentially the same thermal conductivity, the cost of the installation can be used in deciding which one to choose.
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