Insulation, the use of resistant or absorptive materials to retard or prevent the passage of heat, sound, mechanical vibration, electricity, or radiation. The term also is applied to the materials used for this purpose.
Insulation to retard the flow of heat is important for the comfort and, in some cases, the survival, of humans and animals. Insulation also prevents damage to various articles by freezing or high temperatures, and lowers heating and cooling costs.
Contrary to popular belief, insulation is not used to keep cold from entering an area, but only to retard the flow of heat. All substances, whether solid, liquid, or gas, contain some heat, so the condition of cold is merely a condition of relatively less heat. Heat can flow in one direction only—from a warmer body to a less warm body—so insulation serves to retard this flow. In a refrigerator, for example, insulation slows the flow of heat from the room air to the interior of the refrigerator. In a building, insulation keeps heat in during winter and out during summer.
Air is a poor conductor of heat, and when trapped in a hollow area is an excellent insulator. Other insulating materials, some of which depend on air pockets for much of their insulating effect, include mineral wool, fiberglass, asbestos, wood, concrete, vegetable fiber, vermiculite, and foamed plastics such as polystyrene. These substances retard the conduction and convection of heat. Aluminum sheets and aluminum foil, on the other hand, prevent the radiation of heat, by reflecting it back toward its source.
Buildings can be insulated by using building materials that are themselves good insulators or by leaving spaces in walls and ceilings and filling the spaces with an insulating material. Such simple building materials as the snow blocks of Eskimo igloos, the straw of thatched roofs, and the sun-baked clay of houses in northern Africa, the Middle East, and Latin America provide good insulation.
Materials designed exclusively as building insulation commonly come as loose fills or in the form of batts backed with foil or paper. They are installed between the interior and exterior walls and in the floor or ceiling of the attic. Windows and doors are insulated by weather stripping around the frame and by creating a dead-air space through the use of storm windows and storm doors.
Sound and mechanical vibrations travel in waves. Insulation against them therefore makes use of the wave-absorbing and wave-scattering properties of materials that have a somewhat resilient and porous structure.
Sound that travels through the air can be controlled by placing baffles in air passages, by sealing cracks around doors, or by lining walls and ceilings with sound-absorbing materials such as cork, lead, felt, or acoustical tile. Carpeting, overstuffed furniture, and draperies also absorb airborne sound. Fiberglass insulation is frequently used in automobiles and aircraft cabins.
Mechanical vibration is transmitted mainly through solid structures, especially floors. A certain amount of sound also is transmitted in this way. Solid-borne vibration and noise can be controlled by heavy carpeting, felt mats, and cork tiles. Soft materials, such as cotton batting, shredded paper, and foam plastics, readily absorb mechanical vibration and are used for packing. Heavy springs and rubber mountings are used to isolate delicate instruments from outside vibrations, or to help prevent a machine from transmitting its own vibrations.
Substances that insulate against electricity are called dielectrics. They include air, carbon dioxide, nitrogen, paper, varnished cloth, rubber, porcelain, mica, glass, and certain plastics.
Most forms of electrical insulation depend on resistance to conduction of an electric current. Electrical shielding, of telephone cables for example, uses a different principle. The wires, covered with conventional insulation, are enclosed in a conducting material that confines electric signals to a desired path (the wire) while keeping out unwanted signals.
The nuclear radiation and X rays used in industry and medicine are harmful to human beings unless the amounts are carefully limited.