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.
“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.
“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.
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.