Does Some Attic Insulation Contain Asbestos?

By: Bill Kibbel , Contributing Writer
In: Old House Inspection

I received an e-mail from a homeowner concerned that his attic might have asbestos insulation.  A home inspector pointed out that the insulation behind the upstairs knee-walls is called “something wool”.  It’s old, dirty and fibrous and the inspector told him that it should be tested for asbestos fibers.

Mineral Wool Insulation

“Rock wool” is made from mineral fibers manufactured from stone and waste from mining . It consists of aluminum silicate rock (basalt), furnace slag and limestone or dolomite.

Rock Wool

“Slag wool” is produced mainly from blast furnace slag with some natural stone.

The term for both types of this fibrous insulation is mineral wool.  Mineral wool was the most common thermal insulation for residential use until the 1960’s, when fiberglass insulation become the standard.  The raw materials are melted in furnaces and blown with air or steam over spinning drums or a centrifuge to create the fibers (picture making cotton candy).

Asbestos is a naturally occurring fiber that is mined.  Since mineral wool is a man-made fiber, it does not contain asbestos.  I’ve read about mineral wool, asbestos and resins mixed together to manufacture a couple specific industrial insulating products, but it’s not likely the kind of stuff used for residential insulation.

Other Insulation

“Balsam wool” is a shredded wood product, treated with borax as a fire-retardant.  It’s considered a very natural product and does not contain asbestos.

Balsam Wool

There is one type of  older insulation that has a significant possibility of being contaminated with asbestos.  That is the loose, granular insulation called vermiculite. See this article on vermiculite and asbestos for more information.

Fiberglass and cellulose are the most common insulation installed in residential attics today.  I have never read or heard of either containing any asbestos fibers.

Other than vermiculite, it’s actually quite rare to find thermal insulation in residential attics that contains asbestos.  In those rare cases, the asbestos containing materials used for insulation were manufactured for some other purpose.  It was likely brought home from work at a factory or salvaged from some other type of building.  I heard one story of a worker at a ship-yard bringing the stuff home regularly and stuffing his attic full.


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  1. 27 Responses  to “Does Some Attic Insulation Contain Asbestos?”

  2. Ruben
    Jan 23, 2015
    I need to see a picture of an attic with new insulation. Can you send me one?
  3. Andre
    Oct 4, 2014
    I've been working on a home that has had water damage although the insulation has turned grayish and looks like gray pebbles but in this attic only in the attic.
  4. Sep 22, 2014
    I looked at a house recently and in the attic the insulation looked like little clay pebbles reddish orange in color. Also had 1" round disks in the mix. It looks like the field conditioner I use on my son's baseball infield for those that have this experience. The house is 100+ years old. Anyone have any idea what this may be? Thanks
  5. Dafi
    Aug 13, 2014
    I like to know more about how to produce slag wool in a laboratory scale for my pilot project. what equipment that i ned to produced slag wool? You can reply to my email : marsa_boy89@yahoo.com thank you
  6. Steve Delzell
    Nov 7, 2013
    I was inspecting a home built in the 1970s and discovered insulation in the attic that appeared to be similar to fiberglass but was completely black in color. Not just like mold or mildew but completely black. It had paper on one side but has been installed between the ceiling joists backwards with the paper towards the attic. There is also a fireplace in the home which is now gas burning, but I don't imagine that that much discoloration could be be from that. I was wondering if anybody else has ever seen this type of insulation before. Is there rolled rockwool that was made it was completely black and color?
  7. Earl Finnegan
    Aug 15, 2013
    If you know a good company and you think this might be the case I'd recommend getting them to look at it for you. I know a place hat does insulation in Calgary and they helped me out with a similar situation.
  8. Jayne Mather
    Mar 12, 2013
    Our attic is insulated with the Balsam-Wool blanket insulation. After the last roof repair the pitch forks broke through some of the insulation and it is hanging in the attic. Can you recommend a type of tape or other material to repair these cuts.
  9. Eugene
    Aug 29, 2011
    The insulation in our attic was exposed because of leaking during the winter rains. The date of construction was about 1970 and the brand of the insulation is PREMIUM BRAND High Density BUILDING INSULATION. It is in large bags with an Indian Head Symbol on them. The actual material inside is dark grey or black in color and looks like cotton in consistency and texture. Do you or anyone reading this article know if this insulation is dangerous in anyway as it has been exposed by the rains.
  10. Aug 29, 2011
    asbestos is a lung killer but some countries are still using asbestos as a heat insulator .,;
  11. Aug 29, 2011
    Those two pictures alone are worth reading this article! Finding information or good pictures of old slagwool insulation is difficult, and the brands are nolonger around. I have the exact insulation in my home (1946) and was looking for information on asbestos for that brand. Im relieved to find that there isn't any added asbestos in that type of insulation. Now if only i would have been so lucky with my floor tiles, ceiling tiles, mastic, lead paint, etc... By the way Asbestos has been banned for the use in North America, however there is a dirty little secret that we (Canada), im not sure of the U.S. is STILL selling asbestos by the boat load to third world countries (Africa, India, Asia, etc). Politicians will not address this issue because its a very HOT BUTTON political issue in the province of Quebec (french canadians). They are so proud of their asbestos industry that they renamed a town after it http://www.ville.asbestos.qc.ca/ (its in french).