Mineral wool—glass wool, stone wool, slag wool—has many applications, from fireproofing to sound insulation. It resembles fiberglass, but it has no glass. There are two types of mineral wool on the market: rock wool, which is made from stone, such as basalt, or diabase (volcanic rock); and slag wool fibers that are derived from iron-ore waste. The material comes in semi-rigid batts, sheets, and loose-fill, similar to fiberglass. Until the emergence of fiberglass batt insulation, mineral wool was North America’s most widely used insulation.
As a dense, fireproof material, mineral wool has always played an important role in improving the fire performance of buildings. It is not only fireproof but also protects the building by reducing heat transfer and the spread of fire within framing cavities. As a naturally incombustible material, manufacturers do not have to add fire-retardant chemicals that sometimes present a biohazard. And as thermal insulation, mineral wool has additional advantages.
Mineral wool in cavities
When compared to fiberglass batts, mineral wool offers superior insulation. It has about a 22-37% higher R-value per inch. It contains 70% recycled material, making it a greener product than fiberglass at 20-30%. Mineral wool retains its shape better than fiberglass or cellulose. It will not settle within walls, leaving cold gaps along the top plate. If there’s air infiltration, moisture will not compromise or degrade its thermal performance; and the material provides no food to support microorganisms.
With an average of nearly R-5 per inch, within 2×4 walls, the material will deliver a whopping R-17.5, and in a 6-in. cavity, R-27.5. Along with thermal insulation, it offers a true sound barrier. Two-inch-thick panels have an STC (sound transmission class) between 45 and 52 and an NRC (noise reduction coefficient) range from 0.95 to 1.09. Recording studio walls are stuffed with dense, mineral wool batts due to their sound-reducing and deadening properties. “Sound batt” fiberglass insulation does not provide a comparable benefit because this material is not as dense.
Mineral wool on the exterior
As a continuous exterior insulation board, mineral wool averages R-5 per inch, comparable to foam. Yet mineral fiber adds a fire barrier to the assembly that will not create toxic fumes if the walls should catch fire. The material maintains 90% of its insulating qualities for the product’s life and has a vapor permeance rating of roughly 50 perms. Termites don’t like it. And moisture will not compromise its thermal resistance.
Products on the market
Rockwool Comfortboard 80 is a rigid stone wool board designed for continuous insulation applications. A non-structural sheathing product, it provides increased thermal performance in 4 in. and 5 in. thicknesses, providing thermal performance up to R-21 for building envelopes. Retails for $60.
With a minimum of 70% recycled content Thermafiber RainBarrier CI High Compressive (80) from Owens Corning is designed for use behind any cladding type, including combustible and open-joint assemblies. Retails for $26.
Owned by the Oracle of Omaha, Berkshire Hathaway’s Johns Manville makes mineral wool insulation for residential use. The company promotes mineral insulation to prevent spreading fire between interior rooms. Because of its resistance to rot, mold, mildew, and fungi growth, the material serves as suitable friction-fit insulation for crawlspaces. There carry two products: Sound & Fire Block, mineral wool to soundproof and delay the spread of fire between interior rooms, and TempControl, mineral wool for exterior walls, basements, and heated crawlspaces. $36 and $29, respectively for about 50 sq. ft.
Comfortbatt is a semi-rigid stone wool insulation from ROCKWOOL for wood and steel framing. The batts include a flexible edge to compress between framing walls, joists, and rafters. The material comes in various thermal values. Retails for $45 for 60 sq. ft. of R-15.
About 50% more expensive than fiberglass or foam, mineral wool is also unpleasant to work with. You must wear protective gear when installing it because the tiny mineral slivers will lodge into your skin and lungs; the particles inhaled can irritate the alveoli. Some data suggest that persistent inhalation of mineral wool may result in upper respiratory effects. The EPA has not classified rock wool or slag wool for carcinogenicity. But when installing it, wear a dust mask at minimum.
Mineral wool is made from stone melted in furnaces. The manufacturing process involves high-heat combustion using gas and coal in furnaces, boilers, and dryers. Operations run 24 hours a day year-round.
While mineral wool contains recycled content, it is, in turn, difficult to recycle with few reuse applications (although the industry is making an effort to reduce landfilling and improve “take back” services). It does not decompose or break down when disposed of.
Consider the ongoing case of community opposition to mineral wool manufacturing that is unfolding in Jefferson County, West Virginia. Neighbors are asking: “Who bears the pollution costs of manufacturing ‘eco-friendly’ products?” The story was published in March of 2021 in VICE World News. (For a dispassionate, scientific review of the balance between effective insulating characteristics that make the mineral wool green in buildings but an energy-intensive and polluting material to produce, refer to “Environmental assessment of mineral wool manufacturing.” The researchers conclude that “manufacturing optimization should take place within this industry in order to improve the efficiency and mitigate negative connotations of the material production.” Moreover, they recommend the development of new insulation materials with lower carbon footprint and less energy-demanding manufacturing to further minimize the negative environmental implications.)
Life Cycle Assessments (LCAs), which consider the environmental impact of a product over its lifetime—from manufacturing to end-of-life use—provide a clearer assessment of building insulation levels. A Danish study concluded that the production of mineral wool offset the environmental benefits of super-insulated buildings. However, “a shift away from the dominating use of coke to a greener source of energy for thermal energy in the production of mineral wool could change the results of this study entirely, allowing for greater levels of insulation while maintaining climate change impact benefit.”
On the other hand, according to this Rockwool Sustainability Report, the industry claims mineral wool can become “net carbon positive” within 200 days, “meaning the carbon emitted during the production of building insulation is offset by the avoided emissions of the insulation’s use in buildings.” The same report concludes that as an energy-intensive business, Rockwool needs to “develop our melting technologies, which are currently predominantly based on foundry coke.”
Overall, mineral wool is an excellent building material with good performance measures but it has a heavy manufacturing impact.
Fernando Pagés Ruiz is a builder and an ICC-certified residential building inspector active in code development.
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