Rock Wool

By George Harvey

When rock wool was invented in the1840’s no one was especially interested. It was re-introduced in the 1870’s, but mostly passed from use. Now it is being brought back to market, with a new set of “green” credentials, and people who understand it are paying attention.

Rock wool, also called mineral wool or stone wool, is a man-made product. It is spun from various sources, including rocks and slag. It is similar to the fiberglass used in insulation, but has some advantages of its own.

It is used mostly for thermal insulation. While it is good at this, it also is effective to dampen sound. Its other attributes make it more attractive than other types of insulation for a number of applications. It is not attractive to vermin, and does not support growth of mold. It can dry fairly efficiently, if it gets wet. One really important characteristic is that it is a good barrier to fire. It can provide an insulation value of R4 per inch, which is similar to dense fiberglass and blown cellulose.

It can be used in a number of ways. It can be blown into hollows in walls and similar places. It is used to provide insulation in modular products, such as structural insulated panels. It can be applied in batts. These forms of insulation are now becoming more available, as large companies are making them. Owens Corning produces the Thermafiber brand. Roxul, a subsidiary of the Danish company, Rockwool International, may be the largest North American producer, selling its product under its own name. There are a number of smaller producers as well.

Rock wool may not be the first thing to come to mind, when insulation is considered, but it deserves some thought. Eric Solsaa, of Solsaa Energy Solutions in Rutland, Vermont, specializes in insulation. Several years back he faced a somewhat unusual problem of having to fill architectural spaces as large as 14 inches. “I was not comfortable with the idea of filling this space with blow-in cellulose,” he said, “so I started doing a web search for alternatives.”

Eventually, Eric found a company in Texas that could provide him with what he wanted. “The rock wool was mixed with a binder for this,” he said. “It came in a five-gallon tub, to be mixed with water. The rock wool was then blown into the spaces.”

Eric says not everyone he knows in the insulation industry likes blowing rock wool with a binder, because it is hard on some types of machines. Nevertheless, he uses it and is happy with the results.

He is particularly happy with two points. One is that damp rock wool seems to lose its moisture faster than cellulose would. This is very important in frame buildings. Another thing is that rock wool is a good fire barrier, and so contributes to the safety of a building where it is used.

Though a number of insulation specialists who blow in rock wool, not all do. Solsaa Energy Solutions in Rutland, Vermont can be found at http://www.rutlandinsulation.com. Another is Green Cocoon, in Salisbury, MA, whose web site is http://www.thegreencocoon.com.