In most aspects of building, the key to quality is to do the basic things
right. With insulation, this usually means installing that most basic of
insulating materials, fiberglass batts, in a way that won't compromise
their performance.
Batts are carefully manufactured to have a specific insulating value, as
indicated by the rated R-value. To keep its R-value, however, a batt
must not be compressed or crushed during installation. Batts should
completely fill the stud cavity, fitting snugly against every piece of
framing, the sheathing, and the drywall. If they don't fill the space
evenly, or if they get crushed, you lose R-value. For example, when a
standard 61/4-inch-thick, R-19