Mineral wool insulation in an atticRock Wool, Stone Wool, Mineral Wool, & Slag Wool Building Insulation Identification
Natural and man-made stone or vitreous fibers

  • MINERAL WOOL - ROCK WOOL INSULATION - CONTENTS: Characteristics of mineral wool, slag wool also commonly referred to as rock wool used for building insulation. Photo guide to identification of different building insulation materials. Properties of old or antique mineral wool as well as modern mineral wool compared with different building insulation products. List of current suppliers of rock wool or mineral wool insulation product. Properties of Johns-Manville Spintex® and other mineral wool products.
  • POST a QUESTION or READ FAQs about Rock Wool mineral fiber insulation

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Rock wool or mineral wool insulation:

This mineral wool or "rock wool" insulation article illustrates and describes mineral wool or "rock wool" and slag wool insulation materials. Rock wool or "rockwool" insulation is also called mineral wool and slag wool though there can be differences among the components of these insulations. We describe old-house or "antique" mineral wool insulation as well as modern mineral wool insulating products still used in buildings.

This document assists building buyers, owners or inspectors who need to identify various insulation materials in buildings by simple visual inspection. We provide photographs and descriptive text of various types of mineral wool insulation and describe its properties, how it is made, health and maintenance concerns, and its insulating values.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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Mineral wool or "rock wool" or "slag wool" building insulation properties

Mineral wool in an attic in Canada  (C) & Carson Dunlop Associates TorontoMineral wool insulation, developed in the 1850's, patented in 1875 in the U.S. and this material, also called rock wool or in some texts slag wool insulation remained in popular use in the U.S. up to the 1950's, and is still in use today in some new construction, in manufactured housing, and in special applications such as the insulation of low-slope roofed cathedral ceilings and scissors-truss roofs.

Mineral wool is produced by melting rock and slag using coke as a fuel. The molten minerals are spun into fine fibers using high-velocity spinning rotor and a stream or air or steam.

A binder is applied to the fibers that are then collected on a wire mesh conveyer. The mineral wool fibers are then cut or granulated and packaged for shipment or they are further cured with additional binder so that the resulting fiber blanket can be cut into insulating batts and in some products, combined with a vapor barrier or facing.

In our photograph of white mineral wool insulation shown here in the attic of a Canadian home, the insulation was delivered and installed in a chopped or "granulated" form that was simply poured and raked level between the floor joists of an attic. Photograph provided courtesy of Carson Dunlop Associates, a Toronto home inspection & education firm.

Here we discuss the properties, appearance, health hazards, and possible asbestos content in mineral wool insulation.

Definitions of Mineral Wool, Mineral Fiber & Fiberglass

Gold Bond brand Mineral Wool in a Haddonfield New Jersey Home in the U.S. (C) Daniel Friedman Gold Bond brand Mineral Wool in a Haddonfield New Jersey Home in the U.S. (C) Daniel Friedman

According to the US EPA,

The term "mineral wool" can be used to describe any fibrous glassy substance made from minerals or mineral products. For the purpose of this study, mineral wool has been defined to include only those fibers made primarily from natural rock or metallurgical slag. 

Shown above: photographs of Gold Bond™ brand mineral wool insulation in a 1960's Haddonfield New Jersey Home, viewed under the stereo microscope and then at higher magnification, about 400x. You can see the gold colored resin binder that gave this insulation its product name.

[Click to enlarge any image]

What's the difference between mineral wool and mineral fiber insulation?

  1. Mineral fiber insulation includes two insulation groups: fiberglass insulation (glass fiber insulation) and stone wool insulation (insulation made from rock and slag).
  2. Mineral wool insulation includes only insulation stone wool insulation (insulation made from rock and slag). Mineral wool insulation will not contain any fiberglass.

Therefore, properly, the term mineral wool insulation is a proper subset of mineral fiber insulation.

  1. Fiberglass, in its most broad use, refers to a reinforced plastic made from glass fibers embedded in a resin matrix, or to a textile woven from glass fibers or filaments. Fiberglass insulation is comprised principally of glass fibers bound together in a loose matrix by small amounts of resin.

    Glass fibers, as in fiberglass insulation, are a flexible but still fragile fiber made by fusing sand with soda, lime and other ingredients. Its base of sand, a granular rock fragment comprised principally of siliceous and other rocks (the beach) that defines glass as a mineral fiber. Our photograph below shows modern fiberglass insulation fragments under the microscope at about 1000x. The yellow-green material is a resin binder.

Lab photo of building insulation

To remind us of sand as the principal ingredient of glass, I include below my photograph of sand at Woodend Beach, Christchurch, New Zealand.

Sand on Woodend Beach, Christchurch New Zealand (C) Daniel Friedman

In the U.S. there were between 80 and 90 mineral wool manufacturers in the 1950's. By 1980 in the United States mineral wool manufacturers included these facilities:

According to Rockwool International1, headquartered in Denmark, in 2016 there remained just five mineral wool / stone wool producers in North America. Rockwool is currently produced in the U.S. in Indiana, North Carolina, Texas, and Washington State as well as being produced in other countries. We list some current producers of rock wool or mineral wool insulation at the REFERENCES for this article.

  1. "Descriptive use of ROCK WOOL at InspectApedia, Fair Use + Research Citations", ROCKWOOL International A/S Hovedgaden 584 DK-2640 Hedehusene Denmark Tel: (+45) 46 56 03 00 Fax: (+45) 46 56 33 11 VAT-number: 54879415 Email: Personal correspondence, MK to DF 2016/11/22
  2. U.S. EPA, "Source Category Survey: Mineral Wool Manufacturing Industry", [PDF] U.S. Environmental Protection Agency, Office of Air, Noise, and Radiation, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina 27711 (March 1980)

Constituents of Rock Wool or Mineral Wool Insulation? (it's not asbestos)

Please see this separate page: MINERAL WOOL / "Rock Wool" COMPOSITION

Appearance of Mineral Wool Products

Mineral wool is generally a dull white but may be fairly homogenous gray (shown earlier on this page) or mineral wool may have black components, especially in older buildings.

Current mineral wool batts are also manufactured in a brown color.


Airborne particle characteristics of Rock Wool / Stone Wool Insulation

Rock wool insulation © Daniel Friedman

Question: Gold Bond Wool Rock Insulation

(May 3, 2012) Gold Bond Rock Wool said:

I've started removing fiberboard from the walls of an old porch-turned-bathroom and found the same Gold Bond Wool Rock as in your photos. Regarding the disintegration factor you mentioned, the product in my walls is very loose. I had trouble removing a section without it coming off the paper and falling apart in my hands.

Since that piece was on the floor and to the open ground to daylight, I would think that particles eventually escaped plus the elements over time (heat in southern climate and/or lack of ventilation) created the deterioration. Ants had made a colony in the insulation and the paper is frail, I am removing it.



No Mike, rock wool is not an asbestos product, at least not as generally would be found in a residential building.

Patent research does, however, disclose some mineral wool or "rock wool" insulation products that were indeed combined with asbestos for special applications. Details are at ASBESTOS IN MINERAL WOOL / STONE WOOL

Our photo above shows the original branding of Gold Bond™ Rock Wool, insulation observed in a 1920's home built in Poughkeepsie, NY. Because it is more dense (about 1.2 pounds /cubic foot) than some other insulating materials such as fiberglass (about 1/2 pound per cubic foot), some writers point out that rockwool is less likely to become airborne.

This is a confusing view unless supported by more specific data. Most air movement in buildings is upwards and outwards through the building attic.

We have identified some conditions in which air moves down from building attics into the occupied space, such as when a whole house fan is inadequately vented to the outdoors (and the attic is both pressurized and its dust stirred up by the fan).

A fiberglass batt provides less total volume of small particles to be stirred by an attic fan than does granular insulation such as vermiculite, rockwool, blown-in cellulose, or even chopped fiberglass (used in blown-in installations).

We'd also need to consider the average particle size and weight when evaluating the ease with which a material becomes airborne. So particle density or density of an insulation material per cubic foot is not the whole story.

How to Avoid Moisture & Settling Problems when Spray Applying Mineral Wool, Rock Wool, Slag Wool Insulation

Because these mineral fiber insulation products are often applied by spraying as a wet slurry onto building surfaces or into building cavities, they should not be covered by a vapor retarder barrier until the insulation has dried.

We recommend using a long-probe moisture meter such as models made by Delmhorst™ to assure that the rock wool sprayed into building cavities has dried to ambient indoor humidity before the wall or ceiling is covered with a vapor retarder or enclosed.

Mineral wool insulation is used by some manufactured home and possibly some mobile home manufacturers. Because these structures are later transported to their ultimate building site the insulation is exposed to vibration that can cause settling of the insulation within building cavities.

Wet-process blown-in mineral wool insulation thickness can decrease by up to 16% in this case, according to Graves and Yarborough. What the study did not examine, and which may be still more important, would be the development of gaps at the tops or sides of vertical wall sections if settlement moves the wall cavity insulation. ["An Evaluation of the Settling of Loose-Fill Rock Wool Insulation in the Attics of Two Manufactured Home Units", ASTM, Graves RS, Yarbrough DW, January 1990. ]

A review of patent disclosures shows that some mineral wool insulation products added cellulose, flax, or other products to reduce settlement when this material was installed in walls.

What is the insulating R-value of mineral wool?

This is a good insulating material and has a better "R" value and more sound-reduction ability per inch than some fiberglass. Rock Wool insulating Batts have an R value of about 3.14 to 4.00 per inch. Blown-in rock wool or mineral wool insulation such as that shown in our attic photo above, has an R value of about 3.10-4.00 per inch.

Other sources we researched indicated that slag wool loose-fill insulation had an R value of about R-2 to R 3.3 and one source claimed an R value of 4.1 per inch for rock wool. [Home Energy Magazine Online, July/August 1997]

A safe rough estimate is to assume mineral wool made of rock or slag has an R value of about 3.35 per inch in batt form and an R value of about 2.25 in loose fill installations.

To compare insulating material R-values see our table of INSULATION R-VALUES & PROPERTIES

Question: Does the R-value of mineral fiber insulation deteriorate with age?

What about R-value deterioration of rock wool as it ages? - Fabian Jennings

Reply: No, except insofar as the loose fill insulation may become compacted or unevenly distributed due to disturbance or settle over time

Rock wool does not deteriorate in R-value except, as would be the case with any loose fill insulation, if it is disturbed so as to no longer be uniformly distributed, or if it is packed by being compressed from storing items on top of it or walking on it or something similar.

However in vertical applications such as wall cavities, there were problems with rock wool settling. If settlement leaves voids at the top or sides of an insulated wall cavity, that'd certainly reduce its effectiveness. I don't have data on the time-rate of settlement of mineral wool insulation - something that will be interesting to research. I have read of efforts to mix other fibres in with mineral wool to reduce the settlement problem.

Slag Used in Rock Wool / Mineral Wool Insulation Products: Health Effects?

Slag wool is a manmade vitreous fiber made by spinning slag into insulating fibers. Some rockwool producers use nearly pure recycled steel slag.

This topic has moved to MINERAL or SLAG WOOL HEALTH EFFECTS where you can read details.

Do Rock Wool, Slag Wool, or Mineral Wool Insulation contain asbestos fibers?

Mineral wool insulation, slag wool insulation, and "rock wool" insulation would not be expected to contain asbestos fibers. There are a few exceptions.


Properties of Johns Manville Spintex® Insulating Batts



Continue reading at MINERAL WOOL / "Rock Wool" COMPOSITION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see BALSAM WOOL BATT INSULATION - a cellulose product, not a mineral fiber




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