Green Topics

Mineral Wool Residential and Commercial Insulation

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Thermafiber’s RainBarrier cavity wall insulation, which provides fire protection, controls noise, and sheds moisture, is used primarily in rainscreen applications.

Photo: Thermafiber

Mineral wool forms naturally when strong winds blow through molten lava to create the thin, gold-colored strands that volcanologists call Pele’s hair. Today’s mineral wool insulation is made in a less dramatic process using basalt and iron-ore slag that is melted, spun into fibers, and held together with a phenolic resin. Adjusting the density of the fibers and the resin mix produces different residential and commercial insulation products, including batts, blankets, and rigid and semi-rigid boards. All of these products provide excellent sound attenuation and flame resistance along with R-values of about 4 per inch.

Though the life-cycle impacts of mineral wool­ production—primarily energy consumption—are significant, some of these are mitigated through the use of pre-consumer recycled slag from iron manufacturing. Thermafiber, for instance, uses a minimum 70% recycled slag and offers products at 75% and 90% recycled content, including a darker colored board for curtain walls at 84%. According to Austin Hess, business development manager for Thermafiber, “the U.S. Government’s [EPA] Comprehensive Procurement Guidelines require 75% recycled content for mineral wool, and we are one of the only companies that can produce that product.”

As with most fiberglass insulation, mineral wool insulation uses a urea-extended phenol formaldehyde binder. Formaldehyde is a known carcinogen, but according to the industry, almost all of the formaldehyde in mineral wool insulation is eliminated in the production process through a chemical reaction and high heat. Roxul’s residential batt insulation, for instance, emits formaldehyde at levels less than 0.0135 parts per million (ppm) and is Greenguard Children and Schools certified. (Typical background formaldehyde levels are estimated at about 0.020 ppm). Fibrex claims similarly low levels, ranging from 0.012 ppm for batt products to 0.05 for its foundation drainage board. Roxul’s Ian Stewart told EBN that its board products have not been tested because it is not required for exterior products. (Tom Lent of the Healthy Building Network has questioned CA 01350 testing protocols for formaldehyde in fiberglass insulation, claiming that some emissions may actually be higher than stated. The industry disagrees with his assertions.)

Selected Mineral Wool Products


Mineral wool is used extensively in Europe, but in the U.S. it is used primarily for industrial and commercial applications. In commercial buildings its natural fire resistance can give it an advantage over fiberglass, which melts at around 1300°F (704°C). “Our unfaced mineral wool can withstand temperatures in excess of 2000°F (1093°C) for over five hours with zero smoke developed,” claims Hess. Rigid polystyrene boards, on the other hand, contain brominated flame retardants (see EBN Aug. 2009), emit toxic smoke when burned, and may require construction of an additional fire break. In some applications, such as the gap between curtain walls and floor assemblies, mineral wool “safing” is the standard material used to meet fire codes. Other commercial mineral wool products, such as sound-attenuating batts and board products specifically formulated for curtain and cavity walls, provide similar fire and smoke protection. Permeability varies among these products, which are available with or without foil facing. Mineral wool is also being used in metal wall panel systems from Kingspan and Metl-Span.

Because mineral wool is used frequently in commercial construction, manufacturers have a distribution network that makes these products more cost-competitive than in residential markets. Prices vary widely depending on many factors, but a random sampling of commercial building suppliers in the New York City area found mineral wool ranged in price from around $0.25 per square foot ($2.70/m2) for 1-inch-thick (2.5 cm) boards (approximately R-4.2 at a density of 2.5 pounds per cubic foot [40 kg/m3]) up to $1.00 per square foot ($11/m2) for 2-inch-thick (5 cm), foil-faced boards (at a density of 8 pounds per cubic foot [128 kg/m3]). Extruded polystyrene (XPS) was about $0.50 per square foot ($5.40/m2) for a 1-inch-thick (2.5 cm) (R-5) board.

For residential construction, low-cost fiberglass batts and readily available rigid foam boards dominate the market, but Roxul is now distributing mineral wool batt insulation in the U.S., and Thermafiber’s batt products are being sold through Certainteed distributors, through some Home Depot outlets, and at select stores on the East Coast—and a motivated home builder can purchase the company’s board products through commercial suppliers. Drainage boards made by Roxul and Fibrex, geared largely toward use on residential foundation walls, are ironically hard to find in the U.S., which is too bad because they are hydrophobic, don’t need HBCD flame retardants, and are termite resistant, making them an attractive alternative to extruded polystyrene (XPS). Because of the limited availability, in most cases you pay a premium for residential mineral wool insulation. Roxul’s 3.5-inch (9 cm) R-19 and 5.5-inch (14 cm) R-23 residential batts, for instance, are selling for about $0.60 and $0.85 per square foot ($6.50/m2 and $8.60/m2), respectively, about twice the cost of fiberglass. But mineral wool products won’t sag in wall cavities and resist insects and rodents, and when you combine these traits with excellent fire, acoustic, and thermal properties, this little-known insulation should deserve consideration in your next green building project.

For more information:


Industrial Insulation Group,


Comments (4)

1 "Green" rock wool posted by Francois Theriault on 10/01/2009 at 08:17 pm

The MSDS for Roxul rockwool insulation reports 1-6% formaldehyde content. That is 10,000 to 60,000 ppm. I'm more inclined to believe an independent third party than the manufacturer's rep. Knauf has launched a new rock wool product in Europe that uses a non-petroleum based binder with no phenol, no formaldehyde, and lower embodied energy (ECOSE). Too good to be true? Maybe so. Unfortunatly no details are provided regarding this patented treatment. The perfect insulator remains elusive.

2 "Green" rock wool posted by Brent Ehrlich on 10/02/2009 at 06:34 am

Hi Francois. Yes, the MSDS does report those formaldehyde levels, but as required that is the raw amount added at the start of manufacturing. Through a chemical reaction and heat curing, there is little formaldehyde left in the final product. Roxul's batt products, for instance, are Greenguard Children and Schools certified to levels less than 0.0135 ppm in the final product. The other manufacturers use similar production methods so should have comparable formaldehyde levels. Getting third-party certification can be expensive and companies may not feel its worth the money since much of their sales comes from exterior board products where indoor emissions have been less of a concern (Roxul has not had its board products certified, either). The industry should do a better job explaining the discrepancy between MSDS formaldehyde levels and final emissions, but in the end, you're right. While Greenguard C&S is impressive, ideally a no-formaldehyde product would be preferred. Knauf is slowly rolling out its Ecose binder across its entire fiberglass line in the U.S., and its rigid fiberglass board products will be made with Ecose by 2010. I hope Knauf introduces a rockwool using Ecose in the U.S. That would be a game changer (though I wish Knauf would tell us what's in its proprietary resin). Thanks for the feedback and for calling attention to the Ecose mineral wool. We'll keep our eyes open for that!

3 emissions versus concentratio posted by Greta Eckhardt on 01/25/2013 at 09:36 am

The following may be a technicality but I think it is important for people to understand what is meant by emissions data.

A CONCENTRATION of formaldehyde is an amount of airborne formaldehyde present in a space, measured per unit volume. It is the result of EMISSIONS which are rates at which formaldehyde is emitted.

When standards indicate values of airborne contaminants in ppm, these are measurements of CONCENTRATIONS in the air.

Measurements of EMISSIONS are indicated in micrograms of contaminant emitted per square meter of exposed surface.

Emissions measurements can be used to estimate the concentrations in a room of a given volume if the total exposed surface and air changes per hour are taken into account.

Greenguard and other certification programs measure concentrations under a standardized laboratory condition.

4 emissions versus concentratio posted by Brent Ehrlich on 01/28/2013 at 11:27 am

Hi Greta. That’s not a technicality; it’s an important distinction that, while touched on in the article, should have been clarified in my comment response a few years ago. Thanks!

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September 25, 2009