##ABOUT BATT AND BLANKET INSULATION
Insulation batts and blankets are sold in bundles or rolls. When the insulation is cut into 4-ft. or 8-ft. lengths and sold in a bundle, each piece is called a batt. When the insulation is sold in a long roll, it is properly called blanket insulation.
In most cases, the width of the batt or blanket corresponds to the size of a framing bay—about 15 in. or 23 in. wide.
Fiberglass batts have R-values ranging from 3.2 to 3.8 per inch, depending on their density. Denser batts have higher R-values because the ratio between the glass fibers and the air pockets has been optimized.
Examples of sloppy fiberglass batt installation, with compressed areas and voids, are all too common. For fiberglass batts to perform well, they must be installed perfectly. It’s essential that the batts be split to accommodate wiring (click photo to enlarge) and cut exactly to fit the framing bay where they are installed. Small voids can significantly impair the performance of fiberglass batts.
Fiberglass batts are air-permeable—they do a poor job of resisting airflow—so it is essential that they be installed in continuous contact with an impeccable barrier to air movement. For the very best performance, fiberglass batts should be installed in a sealed cavity (for example, a stud or joist bay) with an air barrier on all six sides. For more information on fiberglass batt installation, see Installing Fiberglass Right.
FOUR SIGNS OF A GOOD FIBERGLASS JOB:
Continuous, snug, and full: Insulation should be installed without gaps, including at corners. Batts should be trimmed a little oversize so that they fit the cavity snugly. The entire framing cavity should be filled, without any air gaps between the insulation and the drywall. Attic insulation should be…