Mark Kandrick, age 14, of Erie, Penn., for his question:

WHY ARE HOUSES INSULATED?

Insulation is a material that protects against heat, cold or sound. House insulation is very important to comfortable, economical living. Houses are usually insulated because the loss of heat from uninsulated houses is very high.

Building experts tell us that insulation in the outside walls and in the ceiling of the top story of a house pays for itself in a short time in lower fuel bills.

The hollow spaces between the studs in the walls and in the spaces between the ceiling joists of houses are often filled with insulation. The most common types of insulation include batt, blanket, loose fill, rigid and reflective insulation.

Batt insulation is made in soft, flexible units that fit between rafters and joists. Batts are made of fireproof fibrous material such as treated wood fiber, mineral wool, hair felt, flax fiber, eel grass or shredded paper. This material is stitched between two sheets of waterproof paper. Batts are available in thicknesses up to one inch.

Blanket insulation is made in the same way and of the same materials as batt insulation. However, blanket insulation comes in long rolls rather than pieces. The rolls usually contain about 100 square feet.

Loose fill insulation include those made of granulated cork, expanded mica, mineral wool in the form of pellets and other materials. Loose fill insulation is poured or blown into place.

Rigid insulation consists of thick sheets of fiberboard nailed to the outside walls in place of wood sheathing. The sheathing is then covered with wood siding or brick or stone veneers.

Reflective insulation consists of thin copper or aluminum sheets, or of aluminum or copper foil. It also includes coatings applied to the surfaces of rigid insulation, plasterboard or even heavy paper. The bright metallic surfaces reflect heat waves.

Soundproofing is another type of insulation. There are two different types of soundproofing. One consists of sound absorbing materials placed on walls and ceilings to reduce echoes. These materials make a speaker's voice more distinct. They also improve the quality of music by reducing objectionable echoes.

Materials used for this type of insulation are hair felt covered with burlap, perforated fiberboard, corkboard and special types of acoustical plaster and tiles. Heavy cloth curtains and upholstered furniture are also effective.

There materials work to reduce echoes because they contain a number of small air passages. As sound waves penetrate these passages, a part of their energy is converted by friction into heat energy.

Another type of soundproofing reduces the sound transmitted from room to room by vibration of the walls and floors. In general, heavier building materials, such as brick, stone or concrete, produce less vibration than lighter materials such as wood and fiberboard.