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This page illustrates and describes mineral wool or rock wool and slag wool insulation materials. Rock wool insulation is also called minreral wool and slag wool though there can be differences among the components of these insulations. 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.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

MINERAL WOOL or ROCK WOOL - Mineral wool or "rock wool" or "slag wool" building insulation is not asbestos

Mineral wool insulation, developed in the 1850's, patented in 1875 in the U.S. and this material, also called rock wool insulation remained in popular use in the U.S. up to the 1950's, and is still in use today (2008) 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.

What is Rock Wool Insulation?

Just to add to the confusion about what to call this insulation, the insulation industry calls a range of products "mineral wool" and includes under that name: fiberglass, slag wool, and rock wool.

Rock wool is a naturally-occurring mineral fiber that looks like chopped white cottony material. (It may have originally been formed by wind blowing across streams of lava from erupting volcanoes.)

Rockwool is currently produced in the U.S. in Indiana, North Carolina, Texas, and Washington State.

As you can see in our photo at left, mineral wool insulation is not necessarily stark white. In an attic rockwool is typically about 1 1/2 pounds per cubic foot in density. In building walls this material is installed in more dense sprays of four to five pounds of material per cubic foot.

At roughly 1.2 pounds per cubic foot, rockwool or mineral wool has an R-value of R-30 (heavier than fiberglass).

Rock wool as a building insulating material remains in active use in the U.S. and has been reported in application in the Southern U.S. in the mid 1980's and continuing at least into the mid 1990's. [Home Energy Magazine Online, July/August 1997]

Modern rock wool is a manufactured product comprised of a mix of limestone, slag waste from steel blast furnaces, and basalt or diabase. Rockwool is about 3/4 steel slag and 25% basalt rock. Some rockwool producers use nearly pure recycled steel slag.

Similar to the production of fiberglass, rock wool is manufactured by heating these materials and spinning them into fine fibers that are then often applied in a sprayed-on chopped fiber form.

Airborne particle characteristics of Rock Wool Insulation

Our photo (left) shows the original branding of Gold Bond™ Rock Wool, insulation bserved 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.

What is Slag Wool Insulation?

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

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 - We have been unable to find reports indicating otherwise.

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. ]

What is the insulating 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 Properties of Insulating Materials

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

ASBESTOS IDENTIFICATION IN BUILDINGS
Asbestos Risk Assessment
INTERIORS
INSULATION & VENTILATION INSPECTION & IMPROVEMENT
Insulation Material Identification Guide
  Asbestos Identification in Buildings
  Asbestos Pipe Insulation
  Asbestos-Free Insulation Materials
  Balsam Wool Batt Insulation
  Cotton Insulating Batts
  Cellulose loose fill insulation
  Concrete insulation, light-weight
  Fiberglass Insulation
  Foam Board Insulation
  Foam Insulation Types - Visual Id
  Homasote & Other Insulating Board
  Icynene Foam Spray Insulation
  Mineral Wool/Rock Wool Insulation
  Paper Duct Insulation
  Perlite Insulation
  POLYISOCYANURATE FOAM INSULATION
  POLYSTYRENE FOAM INSULATION
  RADIANT BARRIERS
  RIGID FOAM USE INDOORS
  Urea Formaldehyde Foam Insulation UFFI
  URETHANE FOAM Deterioration, Outgassing
  Vermiculite Insulation
HEAT LOSS CALCULATIONS
INSULATION LOCATION for BASEMENTS
INSULATION LOCATION for CAPES, CRAWLSPACES
INSULATION for GREENHOUSE or SOLARIUM
INSULATION MATERIAL IDENTIFICATION GUIDE
INSULATION R-Values & Properties
ASBESTOS: Photo Guide to Materials / Products
Mold Growth Resistance of Foam Insulation

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• 06/07: thanks for photographs of transite asbestos heating ducts, courtesy of Thomas Hauswirth, Managing Member of Beacon Fine Home Inspections, LLC and (in 2007) Vice President, Connecticut Association of Home Inspectors Ph. 860-526-3355 Fax 860-526-2942 beaconinspections@sbcglobal.net
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• Asbestos Identification, Walter C.McCrone, McCrone Research Institute, Chicago, IL.1987 ISBN 0-904962-11-3. Dr. McCrone literally "wrote the book" on asbestos identification procedures which formed the basis for current work by asbestos identification laboratories.
• Stanton, .F., et al., National Bureau of Standards Special Publication 506: 143-151
• Pott, F., Staub-Reinhalf Luft 38, 486-490 (1978) cited by McCrone

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