THE PROS AND CONS
Mineral wool batts are slightly better insulators than the off-the self fiberglass batts sold at most lumberyards and home centers. Mineral wool batts for 2x4 walls have an R-value of 15, while a standard fiberglass batt made to fit in a 2x4 wall has an R-value of 11 or 13. However, you can special-order R-15 high-density fiberglass batts. (The larger the R-value, the better insulator the material is.)
Mineral wool batts for 2x6 walls have an R-value of 23, compared with R-19 for fiberglass batts. A 2x4 stud cavity filled with blown-in cellulose has an R-value of about 13. Spray foam insulates better than all of these, but it’s far more expensive.
It’s much easier to do a top-notch insulation job if you use mineral wool instead of fiberglass. Because of its greater rigidity, mineral wool can be cut more exactly, allowing precise trimming around outlets and other obstructions. By contrast, fiberglass batts simply aren’t easy to install accurately. They’re floppy, hard to cut precisely, and prone to overstuffing.
Cost and availability
On a square-foot basis, mi