Rockwool for Insulation

by Nick Gromicko, CMI®

Rockwool refers to a type of thermal insulation made from actual rocks and minerals. A wide range of products can be made from this Rockwool blanketmaterial because of its superior ability to block heat and sound. Rockwool insulation is commonly used in building construction, industrial plants, and in automotive applications.
The term “rockwool” is sometimes used interchangeably with “mineral wool,” although the latter term is, in fact, a larger category of thermal insulators that includes rockwool, slag wool and fiberglass.
Rockwool is produced naturally during volcanic eruptions when high winds flow upon lava streams of basalt or diabase. This was the case when, in the early 1900s, Hawaiian volcanologists found an unusual, wool-like rock fiber hanging from trees near Mount Kilauea, and it wasn’t long before the fiber’s exceptional qualities were discovered.

Today, this process is replicated in commercial furnaces where minerals and other raw materials are heated to roughly 2,910° F (1,600° C) and subjected to a current of steam or air. Oil is also added during production to decrease the formation of dust. More advanced techniques require rotating the molten rock at high speeds in a spinning wheel, resembling the way that cotton candy is made. Finished rockwool is a mass of fine intertwined fibers that are bound together with starch and used as loose fill or assembled into blankets (batts and rolls). The main producers of rockwool in the U.S. are located in North Carolina, Texas, Washington and Indiana.

Rockwool's Performance as an Insulator

The individual fibers that compose rockwool insulation are good conductors of heat on their own, but sheets and rolls of this insulation are efficient at blocking the transfer of heat. They are often used to prevent the spread of fire in buildings, in light of their extremely high melting point of 1,800° F to 2,000° F. With an R-value of 3.10 to 4.0, rockwool can play a significant role in reducing energy consumption in homes and businesses. Problems sometimes arise because rockwool can retain a large amount of water, although gravity will allow it to drain, as long as it has a path to escape.

Rockwool fiber

Common Applications 

  • In loose-fill form, it can be used for insulating equipment, tanks, pipelines, ovens and furnaces.
  • It is used in the manufacture of acoustical ceiling tiles.
  • It is used as for residential, commercial and industrial insulation. Rockwool is very effective for use as insulation behind and around electrical boxes, wires and pipes. It can fill most wall cavities, leaving virtually no voids.
  • It is also used as spray-on fireproofing material.


While many man-made mineral fibers are considered dangerous to humans, the danger is limited mostly to biopersistant materials, such as special-purpose glass wools and refractory ceramic fibers. The more common types of rockwool used as insulation are considered by the International Agency for Research on Cancer to be "not classifiable as carcinogenic in humans." Rockwool can cause skin irritation, although this condition is a temporary mechanical irritation, rather than a more serious chemical irritation. Regardless, it is always good practice for homeowners, inspectors and contractors to wear quality gloves and other personal protective equipment while handling rockwool, or any other insulation.
In summary, rockwool is a type of thermal insulation made from heated, naturally-occurring minerals. It is generally considered to be safe and effective.