Mineral wool, also known as glass wool, stone wool, or slag wool, has a range of applications including fireproofing and sound insulation. It is similar in appearance to fiberglass but does not contain glass. The two primary types available on the market are rock wool, made from natural stones like basalt or diabase, and slag wool, derived from waste products from iron ore processing. This insulation material can be found in semi-rigid batts, sheets, and loose-fill formats, similar to fiberglass alternatives. Mineral wool was the leading insulation choice in North America until fiberglass batt insulation became prevalent.
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Mineral Wool Characteristics
As a dense and fireproof material, mineral wool significantly contributes to enhancing a building’s fire performance. Not only is it fire-resistant, but it also aids in reducing heat transfer and limits fire spread within framing cavities. Its natural incombustibility means manufacturers can avoid adding fire-retardant chemicals that may pose biohazards. Besides its thermal insulation properties, the material offers other advantages.
Performance in Cavities
Compared to fiberglass batts, mineral wool boasts superior insulation qualities, demonstrating an R-value per inch that is approximately 22-37% higher. Consisting of about 70% recycled content, it is more environmentally friendly than fiberglass, which contains only 20-30% recycled material. Mineral wool holds its shape more effectively than fiberglass or cellulose, preventing settling inside walls that can create thermal gaps. Furthermore, it is resistant to moisture, which maintains its thermal efficiency, and provides no nutrients to support the growth of microorganisms.
With an average of nearly R-5 per inch, the insulation achieves an impressive R-17.5 within standard 2x4 walls and R-27.5 in a 6-inch cavity. In addition to thermal insulation, it acts as an effective sound barrier. Two-inch-thick panels measure between 45 and 52 on the sound transmission class (STC) scale and range from 0.95 to 1.09 on the noise reduction coefficient (NRC) scale. Studios often utilize dense mineral wool batts for their sound-deadening properties, whereas fiberglass doesn't offer comparable sound insulation due to its lower density.
External Applications
When used as a continuous exterior insulation board, mineral wool achieves an average R-value of R-5 per inch, similar to foam insulation, while also functioning as a fire barrier that produces no toxic fumes in case of fire. It retains around 90% of its insulating properties for its lifespan and has a vapor permeance rating of approximately 50 perms, making it resistant to termite damage and moisture that may hinder thermal resistance.
Available Products
Rockwool Comfortboard 80 is a rigid stone wool board specifically designed for continuous insulation applications, providing thermal performance up to R-21 at thicknesses of 4 and 5 inches. It is available for about $60.
Thermafiber RainBarrier CI High Compressive (80) from Owens Corning features a minimum of 70% recycled content and is suitable for various cladding types, retailing for around $26.
Johns Manville, a brand under Berkshire Hathaway, produces mineral wool insulation aimed at residential use. Their products, Sound & Fire Block and TempControl, enhance fire resistance and provide suitable insulation for various applications, priced at $36 and $29, respectively.
Comfortbatt, offered by ROCKWOOL, is a semi-rigid stone wool insulation designed for wood and steel framing, priced around $45 for 60 square feet with an R-value of R-15.
Challenges
Mineral wool tends to be about 50% more expensive than fiberglass or foam alternatives and can be challenging to work with. Protective gear is required during installation, as the tiny fibers can irritate skin and lungs. Prolonged inhalation of these fibers may lead to respiratory issues, although the EPA has not classified rock or slag wool as carcinogenic. A dust mask is recommended during installation for safety.
The manufacturing process involves melting natural stones in high-heat furnaces using gas and coal, running continuously throughout the year. While mineral wool incorporates recycled content, its recycling process has limitations, with few reuse options available, despite industry efforts to minimize landfill contributions.
In Jefferson County, West Virginia, a community is currently opposing mineral wool production due to concerns about pollution from “eco-friendly” products. Research highlights the need for optimization in material manufacturing to enhance efficiency and mitigate negative environmental impacts. Innovative insulation materials with lower energy demands may further lessen adverse environmental effects.
Life Cycle Assessments (LCAs) assess the environmental impacts of building insulation, indicating that the production of mineral wool may counteract the environmental benefits of highly insulated structures. A transition from coke to greener energy sources could significantly modify these outcomes and maintain insulation levels while benefiting the climate.
Conversely, the Rockwool Sustainability Report argues that mineral wool can achieve “net carbon positive” status within 200 days, implying the carbon emissions from its production can be offset by the reductions in emissions during its use in buildings. The report stresses the need for advancements in melting technologies to reduce reliance on foundry coke.
Overall, mineral wool demonstrates excellent building material characteristics with commendable performance metrics, though its manufacturing processes exert a substantial environmental impact.
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Fernando Pagés Ruiz is a builder and an ICC-certified residential building inspector active in code development.
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