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