Mineral wool insulation in an atticVisual Identification of Rock Wool, Stone Wool, Mineral Wool, & Slag Wool Building Insulation

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Rock wool or mineral wool insulation appearance:

This article describes what mineral wool insulation looks like. Mineral wool or "rock wool" or slag wool may be installed as chopped or stranded material, usually white but possibly gray or with yellow, gold or other colors imparted by a resin binder.

This mineral wool or "rock wool" insulation article series illustrates and describes mineral wool or "rock wool" and slag wool insulation materials. 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.

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Variations in Appearance of Mineral Wool Products

Mineral wool insulation in an attic 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.

The most-common appearance of mineral wool insulation is shown in our photo of the white material in the attic eaves of our photograph.

Notice the dark gray and white areas on this insulation? No it's not mold.

Often the black or gray discoloration is due to particle deposition from air bypass leaks. You'll see that some of the mineral wool photos in this article series also include newer yellowish product, typically a resin binder.

Mineral wool, that some folks call slag wool, rock wool, stone wool, or "rockwool" (a trademark discussed at MINERAL WOOL - ROCK WOOL INSULATION ) may appear in chopped form, in batts, in faced batts, and in chopped stringy material that we illustrate later in this article.

Mineral wool also appears in a range of colours, principally whitish but also gray, yellow and white, pink and white, or dull white with colored flecks of included materials.

Some mineral wool insulation was produced to include additinoal fibers of flax, jute or other materials to resist settling.


As you can see in our first photo below, mineral wool insulation is not necessarily stark white.

Brownish gray slag wool Mineral wool with cellulose inclusions from a 2002 home  (C) Daniel Friedman

Our second photo of mineral wool insulation, seen close-up, illustrates a contemporary product installed in a New York home in 2002. This mineral wool insulation is mostly white but contains darker areas of yellowish resin and colored fragments of cellulose (paper) insulation.

Below: a close-up of a dense-packed gray-brown mineral-wool insulating batt at a Vassar College construction site in Poughkeepsie, New York in 2016.

Brown dense packed mineral wool insulating batt (C) Daniel Friedman

Question: is this white stringy insulation mineral wool?

This insulation looks like balls of string and feels like a type of mineral wool. Please let me know if you can identify it.

Apparent mineral wool in thick strands  (C) John Lenig Apparent mineral wool in thick strands  (C) John Lenig

[Click to enlarge any image]

The house where this insulation is installed is located in Hackettstown, NJ, and was built in 1930. But many changes and additions to the home have occurred since then. Some areas lack any insulation at all. The insulation pictured is loose fill with no identifying facing or bags found. [The buyer ]is in construction and has also never seen an insulation material like this.

The only question is whether it could possibly be asbestos containing. [We think ] that, although it does not look typical of an asbestos containing material, the only way we could rule it out is to send a sample to a lab for analysis.

- Anonymous home inspector, by private email, 2016/11/15


White mineral wool insulation in a 2002 home (C) Daniel Friedman To me too it looks like mineral wool insulation, though I've not seen this specific twisted pattern before. As you probably saw, there are also some scraps of yellow fiberglass insulation mixed in with what is probably antique mineral wool insulation loose-filled in place.

Other readers are welcome to help out with a comment about this insulation using the page bottom COMMENTS BOX or you can send us email and photos using the page top or bottom CONTACT link .

Our photo showing a close look at white mineral wool insulation was taken in a home built in 2002. To compare these mineral wool insulation pictures with fiberglass insulation, see FIBERGLASS INSULATION IDENTIFICATION.

The age and location of the home are consistent with use of mineral wool insulation, and as we discussed by email, it would be not impossible but quite unusual to find asbestos attic insulation in any residential home much less in this form.

I speculate that this mineral wool product was produced in longer wound strands for ease of handling during manual placement in the attic floor. Later alternative "blown-in" and "poured-in" insulation products had to be chopped for easier placement requiring less hand labor.

If you want to send me a sample to examine in our lab - if you have some - let me know. I should at least be able to rule cotton and asbestos out and probably can rule mineral-fiber or slag wool insulation "in".

Researching for images of "rope-like mineral wool" it is interesting to note that there are current manufacturers of thermal insulation mineral wool rope and braid, such as the Zibo Jiuqiang refractory Co., a Chinese firm.

Article Series Contents


Continue reading at MINERAL / SLAG WOOL AIRBORNE PARTICLES 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



Suggested citation for this web page

MINERAL WOOL APPEARANCE VARIATIONS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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