Early uses of the rock wool insulation trace back to the Hawaiian Islands hundreds of years ago – where huts were blanketed with fibrous wool collected from volcanic deposits where steam had broken the molten lava into fluffy fibers. And although rock wool and slag wool insulations are commonly perceived to be solely for commercial or industrial use by many people today, in reality, they are ideal choices for residential buildings because of their thermal performance, as well as mold and fire resistance.
Architect Gregory La Vardera says his experience has taught him that home builders are missing out on an opportunity to use a “fantastic” product because they have not seen it in homes. “Builders I’ve worked with who tried it for the first time were surprised by how easy it was to install and the quality of the resulting work,” he said.
Mineral wool insulation is available in high density batts with R-values of R-23 in 2×6 construction and R-30 in 2×8. In addition to its physical properties, including its density and firmness, mineral wool is easy to handle, cut, install and fit into stud spaces. It holds its form and literally fits like a block into a wall cavity – and it’s easy to push into place securely with no sagging or gaps. Moreover, installation of mineral wool requires little equipment – typically just a table and large serrated knives for cutting.
“Mineral wool is easy for builders to use when they have not had any prior experience trying to improve energy performance with insulation packages,” La Vardera says. “Our customers don’t want to waste money on energy bills, so improving energy performance is the easiest way to offer them more value. Similarly, builders don’t want to waste money and time learning how to install new insulation products, so the fact that they can easily install mineral wool, which comes in batts, makes it a very easy home performance improvement.”
Because mineral wool batts come unfaced, an independent vapor retarder may be required depending on the climate zone, according to La Varda, who recommends a permeability or “smart” membrane because it self-adjusts to suit conditions. Like all insulation materials, mineral wool requires an air barrier to assure an airtight envelope.
La Vardera said he also prefers to specify semi-rigid mineral wool for exterior insulation. These extremely dense insulation panels provide high R-value, but they also breathe, allowing water vapor to pass out of the wall system and help resist mold growth.
Fire resistance also makes mineral wool an attractive option. It will not melt until reaching 2150°F (1177°C). The International Building Code (IBC) and the International Residential Code (IRC) both contain lists of materials approved for use as fireblocking materials. Those lists include the following item:
Batts or blankets of mineral wool or glass fiber or other approved materials installed in such a manner as to be securely retained in place.
“Mineral wool allows you to get high performance, quality insulation and great fire protection,” La Varda says. “It’s a great product that more builders should consider.”
Want to learn more about mineral wool insulation? Check out our product comparison page.
 International Code Council, International Building Code Section 717.2.1