Mineral wool has been around for decades, is widely used in Canada and Europe and is making a comeback in the United States. It’s made by melting down basalt stone and recycled slag from steel mills, then spinning it into fiber that can be formed into batts or boards. We did some research, talked to installers and filled a few walls with the stuff. Here’s our verdict: Mineral wool has some real advantages over the alternatives.
Mineral wool batts are slightly better insulators than the off-the-shelf fiberglass batts sold at most lumberyards and home centers. Mineral wool batts for 2×4 walls have an R-value of 15, while a standard fiberglass batt made to fit in a 2×4 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 2×6 walls have an R-value of 23, compared with R-19 for fiberglass batts. A 2×4 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.