Although the reason for sealing cracks and gaps in a house frame may be fairly obvious—you don’t want cold breezes (or hot air, depending on where you live) blowing through the house—the function of insulation may not be as evident. It is not to block airflow but rather to create pockets of dead air. Air pockets do the actual insulating work, whereas the insulation fibers or beads simply hold the air in place. That’s why jamming fiberglass insulation into a too-small space isn’t very helpful. In fact, doing so just eliminates much of the air space within the material, effectively reducing its value as insulation.
There are a number of materials used for insulation, but the three most common ones are cellulose, rigid foam, and fiberglass.
Subcontractors most often install cellulose insulation by blowing the loose material into attics with special equipment (see the sidebar at left). When binders are added to cellulose insulation, it can also be sprayed in dampened form between studs. When it is properly applied, the insulation stays in place after the moisture evaporates. Unlike fiberglass insulation, which demands
