How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

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There's old insulation in the attic labeled rock wool. Is it really dangerous asbestos?

Saturday, October 13, 2018

No, rock wool is not another name for asbestos. It is a type of thermal insulation made from rocks and minerals, unrelated to asbestos. The material is also called mineral wool or slag wool, and some versions of it are actually a recycling of industrial furnace slag.

   The most common types of rock wool are categorized by the International Agency for Research on Cancer to be “not classifiable as carcinogenic to humans.” The material may, however, cause skin irritation, and it is always a good practice to wear gloves and other personal protective equipment while handling it.

   Rock wool is produced naturally during volcanic eruptions, which is how the fiber was first discovered: in the early 1900s, Hawaiian volcanologists found unusual, wool-like clusters of fiber hanging from trees around Mount Kilauea. Experiments with the newly-found fibrous material uncovered it’s exceptional insulation qualities.

  Today the volcanic process is replicated in industrial furnaces, where the minerals are heated to approximately 2900º F, then subjected to a blast of steam or air. Another technique involves spinning the molten rock at high speeds, similar to the way cotton candy is made.

   The finished product is a mass of fine, intertwined fibers that are bound together in blankets or used as a loose fill, and has a wide variety of industrial and residential insulation applications.

   We come across older rock wool insulation occasionally in Gainesville homes, often buried under a top layer of newer insulation. New rock wool insulation is considered an environmentally friendly building material and LEED rated.

   For more information about rock wool and other types of home insulation, we recommend a visit to the North American Insulation Manufacturers Association (NAIMA) website at: To find out more about asbestos, go to our blog post How can I tell if there is asbestos in a house?  

Here’s links to a collection some of our other blog posts about INSULATION: 

What are the common problems with attic insulation?

Why is vermiculite attic insulation a problem for both buyers and sellers of a home? 

 Why is the garage so hot in the summer?

How can I tell if a house has insulation? 

Why is spray foam used for attic insulation?

Should I wrap the water heater with an insulation blanket? 

Should I put some more insulation in the attic?

• What does the "R-Value" of home insulation mean? 

• Is pipe insulation flammable?

• What is minimum requirement for the insulation of a mobile/manufactured home according to the HUD-code? 

How energy efficient is a mobile home?

• Why is insulation not allowed to touch around a gas flue in the attic even if it’s not flammable?

 Is cellulose insulation flammable? 

What does the U-value of insulation mean?  

• Does code require water pipes in the attic to be insulated? 

• Does code require water pipes in a crawl space to be insulated?

What do those numbers on the manufacturer's stickers in new windows mean?

    Visit our INSULATION page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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