Mineral Wool Manufacturing (U.S.) — NAICS 327993

An investor's primer for public-market and private investors

1. Overview

Mineral wool is the fibrous, cotton-candy-like insulation made by melting rock, iron-blast-furnace slag, or glass at roughly 1,300–1,650 °C and spinning the molten stream into fine fibers bound into batts, boards, blowing wool, and pipe coverings [1][2][3]. It is the material stuffed into wall cavities, sprayed over steel beams for fire protection, wrapped around industrial pipe, pressed into acoustic ceiling tile, and — in a fast-growing niche — cut into cubes to grow greenhouse tomatoes and cannabis. The U.S. Census Bureau treats glass wool (fiberglass) and rock/slag ("stone") wool as one industry, NAICS 327993, because they share the same melt-and-spin process and compete for the same jobs [1].

Why an investor cares: this is a capital-intensive, energy-hungry, freight-constrained manufacturing business with strong structural tailwinds. The Energy Information Administration reports fuel consumption of 3,239 million Btu per employee, 11.2 thousand Btu per dollar of value added, and 6.4 thousand Btu per dollar of shipments — confirming how energy-intensive the process is [4]. Demand rides the construction cycle, but it is being ratcheted upward by tightening building-energy codes, decarbonization retrofits, and fire-safety rules that favor noncombustible insulation. Owners make money by running very expensive melting furnaces flat-out, keeping energy and freight costs down, and pushing branded, code-approved product through builder and distributor channels.

Ways in: there is no pure-play, U.S.-listed mineral wool company. Public exposure comes bundled inside diversified building-products firms — Owens Corning (U.S.-listed) and Rockwool A/S (Copenhagen-listed) are the closest — while some of the biggest U.S. plants belong to private or foreign-owned groups (Knauf, Saint-Gobain/CertainTeed) or to a Berkshire Hathaway subsidiary (Johns Manville). Private routes run through plant/JV investment, the horticulture-substrate niche, and the distribution/contractor layer.

2. What it is and how it's structured

Scope of NAICS 327993. The Census definition covers establishments primarily making "mineral wool and mineral wool (i.e., fiberglass) insulation products made of such siliceous materials as rock, slag, and glass, or combinations thereof" [5]. In practice that means three chemistries and their products:

• Glass wool (fiberglass) — melted sand, recycled glass, soda ash, and borates; the lightweight, low-cost workhorse of U.S. residential wall/attic insulation (Owens Corning PINK, Johns Manville, Knauf, CertainTeed). The process melts a glass batch — silica-bearing minerals, soda/lime ingredients, and recycled cullet — before rotary fiberization, binder application, curing, and cutting [3].
• Stone/rock wool — melted basalt or diabase rock; denser, water-repellent, noncombustible to well above 1,000 °C, and priced at a premium (Rockwool, Owens Corning Thermafiber). Traditional cupolas use coke to heat the charge; newer facilities may use gas-assisted or electric melting. The molten material is spun into fibers, sprayed with binder and oil or wetting agent, collected into a blanket, cured, cooled, cut, and packaged [2].
• Slag wool — spun from steel-industry slag, a recycled-content byproduct.

Product forms: batts and rolls, loose "blowing" wool, rigid board, pipe and equipment insulation, spray-applied fireproofing, acoustic panels, and horticultural growing media.

What it excludes (adjacent NAICS codes an investor should not conflate):

• Foam plastic insulation — expanded/extruded polystyrene (EPS/XPS), polyisocyanurate board, and spray polyurethane foam — sits in plastics manufacturing (NAICS 326140 / 326150), not here. Foam is mineral wool's main substitute.
• Textile/reinforcement glass fiber (yarn, chopped strand for composites) is in glass manufacturing (NAICS 3272).
• Steel wool and metallic scouring pads are in NAICS 332999 [5].
• Gypsum wallboard is NAICS 327420; cellulose insulation is elsewhere.
• Some mineral-fiber acoustic ceiling tile blurs the line — it uses mineral wool as an input but a dedicated ceiling-tile plant may be classified in an adjacent product code, so treat ceilings (e.g., Armstrong) as adjacent rather than core.

Ownership mix. This is a concentrated, professionalized industry — not a fragmented trade dominated by tiny operators. The Census counts just 145 firms running 224 establishments [1][6]. The leaders are large multinationals or subsidiaries of bigger conglomerates: one U.S.-listed independent (Owens Corning), one foreign-listed independent (Rockwool), a Berkshire Hathaway unit (Johns Manville), and the U.S. arms of two European giants (Knauf, Saint-Gobain/CertainTeed). TopBuild, a major national purchaser, says it sources most of its fiberglass building products from four primary U.S. manufacturers: CertainTeed, Johns Manville, Knauf, and Owens Corning [7].

3. How big it is

Ground-truth federal figures for NAICS 327993:

Reading the numbers. At ~$7.9 billion of shipments from about 15,000 workers and only 224 plants, this is a high-output, low-headcount, plant-scale business — roughly $35 million of shipments per establishment. Pay averages about $73,000, well above the manufacturing norm, reflecting skilled furnace and process operators. The four largest firms make 61% of shipments and an HHI of 1,106 puts the industry in the "moderately concentrated" band [1].

A scope caveat (not an undercount). Unlike industries dominated by government or by tiny sole proprietors, mineral wool is fully captured by the manufacturing census. The trap runs the other way: private market-research reports that value the "U.S. mineral wool insulation market" at roughly $2.7 billion [9] are usually counting only rock/slag wool. The Census $7.9 billion figure is larger because NAICS 327993 also folds in fiberglass insulation [1][5]. When you compare a headline number, check whether it means "stone wool only" or "all mineral/glass wool." Similarly, customs statistics split rock/slag wool (HTS 6806) from glass wool (chapter 70), so market reports built from only one tariff heading do not measure the full NAICS industry [10][11].

4. The investable universe

No U.S.-listed company is a mineral wool pure-play. The cleanest exposure and the largest owners:

Major private / foreign-owned owners with no direct ticker:

• Knauf Insulation — private German group; U.S. fiberglass plants (Albion MI, Inwood WV, Lanett AL, Shasta Lake CA, Shelbyville IN); acquired USG in 2019, adding mineral-fiber ceilings [19]. Knauf North America employs more than 1,900 people in multiple U.S. facilities; its newer McGregor, TX facility was designed for 500 million square feet of annual insulation output [20][21].
• Johns Manville — see Berkshire above; nine-plus U.S. insulation plants, with a new Georgia blowing-wool line planned for 2027 [18][22].
• Paroc — European stone wool brand, owned by Owens Corning [12].

Downstream exposure. Manufacturing concentration should not be confused with downstream structure. Distribution, fabrication, and installation are much more fragmented and local. TopBuild (NYSE: BLD) reported $4.3B of 2025 insulation-and-accessories revenue across more than 200 installation branches and 250+ distribution centers [7]. Installed Building Products (NYSE: IBP) operates approximately 250 installation branches [23]. These downstream companies substitute branch density, labor execution, purchasing scale, and acquisition integration for furnace and energy risk — they are not NAICS 327993 manufacturers but offer exposure to insulation demand.

Bottom line: to own this industry through public markets you buy a diversified building-products company (OC, Saint-Gobain) or a foreign-listed stone wool leader (Rockwool); there is no small, focused U.S. equity to buy.

5. How the money works

Mineral wool is a volume-and-utilization manufacturing business, priced like a branded commodity. The basic earnings algorithm is volume × price/mix, less energy, mineral/glass feed, binder and chemicals, labor, packaging, freight, maintenance, and furnace depreciation. The economics that matter:

• Capacity utilization. A melt line (a coke-fired cupola for stone wool, or a gas/electric glass furnace for fiberglass) runs continuously, 24/7, with enormous fixed cost. A single new stone wool plant runs $150–175 million-plus [24][25]. Profit is made by keeping lines full: small swings in utilization move margins hard, and the biggest operating risk is a demand air-pocket that leaves an expensive furnace half-idle. Owens Corning's 2025 results illustrate: approximately 5% lower volume, $50 million of production-downtime cost, and $42 million of input-cost inflation outweighed $27 million of higher selling prices and $30 million of manufacturing-cost improvement [12].
• Energy is the swing input. Melting rock or glass to ~1,500 °C is energy-intensive; natural gas, coke, and electricity are a large share of cash cost [1][4]. Energy-price spikes compress margins directly, and carbon pricing is a forward risk. Electrifying furnaces (Rockwool, Owens Corning pilots) is a decarbonization and cost lever.
• Freight defines the map. The product is mostly air — very low density — so shipping is expensive relative to value. That caps the economic delivery radius at a few hundred miles, gives incumbents regional moats, keeps import competition modest, and is why foreign leaders (Rockwool, Knauf) build U.S. plants rather than export to the U.S. [24][25].
• Pricing and mix. Product is spec-driven and code-approved, so makers hold pricing power in inflationary periods and can push higher-value mix (premium fire-rated stone wool, faced/board products). Reported segment margins are healthy for building products: Owens Corning's insulation EBITDA margin runs around 23% [12]; Rockwool's Insulation segment EBITDA margin was 21.3% in 2025 [13].
• Demand cyclicality. Volume tracks construction — new residential (housing starts), commercial/industrial building, and repair-and-remodel — so results are cyclical and interest-rate sensitive, layered on top of a structural code-driven growth trend (Section 6). Owens Corning notes North American residential-insulation demand typically follows housing starts with about a three-month lag, with peak construction and remodeling activity in the second and third calendar quarters [12].

6. What drives demand

• New construction. Housing starts and commercial/industrial building set the baseline; the business is rate- and cycle-sensitive [9].
• Repair and remodel. Reinsulation and renovation add a steadier, less cyclical layer of demand.
• Building-energy codes. Each tightening of the International Energy Conservation Code (IECC) and state codes such as California's Title 24 raises required R-values, meaning more insulation per building — a durable volume tailwind independent of the number of buildings. Model energy codes are updated nationally but adopted state by state or locally, producing an uneven rather than uniform regulatory tailwind [26][27].
• Decarbonization and retrofit. DOE says opaque envelopes affect 28% of building energy use, equal to 11% of U.S. primary energy use, and that nearly 93% of today's residential buildings and 60% of commercial buildings are expected still to exist in 2050 — creating a long retrofit runway independent of new starts [28]. Net-zero building goals and renovation-wave incentives push insulation as the cheapest operational-carbon cut.
• Fire safety. Codes favoring noncombustible materials in high-rise, cladding, and assembly occupancies steer specifiers toward stone wool, which does not burn — a demand driver sharpened globally after high-profile cladding fires [9].
• Industrial and OEM. Pipe and equipment insulation for refineries, power plants, and HVAC, plus appliance and automotive acoustic/thermal parts, are steady non-construction end markets [12].
• Data centers. A fast-growing outlet needing thermal, acoustic, and fire protection at once. Rockwool reported that technical-insulation and OEM growth was supported by data-center projects and tougher fire regulations [13][29].
• Horticulture. Stone wool cut into cubes and slabs (Rockwool's Grodan brand) is a leading hydroponic growing medium for greenhouse vegetables and cannabis — small today, but a structurally growing controlled-environment-agriculture niche [30].

Capacity investment. Producers are responding to demand: Rockwool broke ground on a $175 million Washington plant expected to begin commercial production in 2028 [25]; Knauf's McGregor, TX facility was designed for 500 million square feet of annual output [21]; Johns Manville plans a new Georgia blowing-wool line for 2027 [22]. Rockwool stated that North American stone-wool demand is expected to exceed capacity until 2028, requiring supplemental imports from Europe [13]. These investments indicate real demand but could reduce regional scarcity pricing once commissioned.

7. Regulation

• Worker/fiber safety and cancer classification. Airborne mineral wool fibers cause mechanical skin, eye, and respiratory irritation, so handling requires PPE. On cancer risk, the International Agency for Research on Cancer reclassified glass, rock, and slag wool to Group 3 — "not classifiable as to carcinogenicity to humans" in 2001–2002, based on worker studies showing no clear link to respiratory disease [31]. In the U.S., the National Toxicology Program removed bio-soluble building-insulation glass wool from its Report on Carcinogens, and California's Prop 65 modified its listing so ordinary bio-soluble insulation fibers no longer carry a cancer warning [32]. Modern fibers are engineered to be bio-soluble (they dissolve in lung fluid); in the EU this is certified via "Note Q" and the EUCEB mark [33].
• Binder chemistry. Traditional binders are phenol-formaldehyde resins; formaldehyde is a regulated hazardous air pollutant, and makers have moved heavily to low- and no-formaldehyde and bio-based binders, often with GREENGUARD indoor-air certification [33].
• Air permitting and emissions. Melt furnaces emit particulates, SO₂, formaldehyde, and VOCs, so plants need Clean Air Act permits. EPA's 2015 rulemaking universe comprised eight major-source rock/slag-wool facilities and 30 wool-fiberglass facilities (10 major sources and 20 area sources) [34]. Compliance may require fabric filters, scrubbers, wet electrostatic precipitators, thermal controls, binder reformulation, monitoring, and permitting. Permitting and local opposition are a real siting hurdle — Rockwool's Ranson, WV plant drew years of community and air-emissions controversy before opening [35].
• Product codes and labeling. Products must meet ASTM fire and noncombustibility tests (e.g., E84, E136) and the FTC R-value Rule for insulation labeling; green-building programs (LEED, embodied-carbon "Buy Clean" rules) increasingly reward recycled content and Environmental Product Declarations — favorable to slag wool and high-recycled-cullet fiberglass [26].

8. Competitive dynamics and consolidation

The industry is concentrated and hard to enter: CR4 of 61% and an HHI of 1,106 [1], protected by the capital cost of furnaces, freight-based regional moats, brand/spec relationships, code approvals, and environmental permitting. The competitive battleground has two fronts:

1. Glass wool vs. stone wool. Fiberglass is cheaper and lighter and dominates U.S. residential; stone wool wins on fire performance, density, and moisture resistance and is gaining share in commercial and fire-critical work [9]. Several leaders (Owens Corning, Johns Manville, CertainTeed) make both.
2. Mineral wool vs. foam and cellulose. Spray foam and rigid foam boards compete on R-value-per-inch and air-sealing; cellulose competes strongly in loose-fill retrofit work; mineral wool counters on fire safety and, increasingly, embodied carbon. Material selection depends on cavity geometry, thickness, air sealing, moisture, fire requirements, installed cost, and contractor familiarity [9][36].

Consolidation has been steady: Owens Corning bought Thermafiber (stone wool, 2013) and Paroc (2018); Knauf acquired USG (2019), adding ceilings; foreign leaders keep adding U.S. capacity (Rockwool's plants, Knauf's McGregor facility) rather than importing [12][19][21][24][25]. Expect continued capacity build-out by well-capitalized incumbents rather than new independent entrants.

9. Risks

• Cyclicality and rates. A housing/construction downturn or higher-for-longer interest rates hits volumes and utilization directly [9].
• Energy-cost volatility. Gas, coke, and power price spikes — and future carbon costs — compress margins on an energy-intensive process [1][4]. A furnace outage or poorly timed rebuild can be as damaging as commodity inflation because fixed costs continue while saleable output stops.
• Overcapacity. New lines are large, lumpy additions; if several land into soft demand, utilization and pricing suffer.
• Substitution. Foam plastics, continuous-insulation assemblies, and cellulose can take share; binder chemistry faces embodied-carbon and formaldehyde scrutiny [9][33].
• Permitting and community opposition. Air-emissions permitting and local resistance can delay or block plants (Ranson, WV) [35].
• Labor. Manufacturers need skilled operators and maintenance technicians, while builders need installers. Shortages downstream can delay insulation demand even after a housing start has occurred. Installation quality also matters to realized thermal performance, so material can lose share if contractor availability or ease of handling is unfavorable [7].
• Supply-chain and input risk. Owens Corning identifies chemicals, resins, transportation, sole- or limited-source materials, tariffs, and supply-chain interruption as margin risks [12].
• Legacy liability overhang. Both Owens Corning and Johns Manville passed through asbestos-driven bankruptcies decades ago. That liability stems from asbestos, a different product — modern mineral wool is not a listed carcinogen [31][32] — but the history still shapes corporate reserves, insurance, and public perception, and mislabeling risk persists.
• Customer concentration. Big-box retailers and building-products distributors hold buying power over residential volumes.

10. How to invest and the outlook

Public-market routes.

• Owens Corning (NYSE: OC) — the largest U.S. insulation maker and the most liquid way to own the theme, though insulation is only about a third of the company (roofing and doors are the rest); pays a dividend and buys back stock [12].
• Rockwool A/S (Copenhagen: ROCK-B; OTC: RKWBF/RKWCF) — the closest listed pure-play on stone wool and its fire-safety/energy-code tailwind, accessed via a foreign listing or OTC [13].
• Saint-Gobain (OTC: CODYY) — diversified exposure through CertainTeed [15][16].
• Armstrong World Industries (NYSE: AWI) — an adjacent play on mineral-fiber ceilings and non-residential construction [17].
• Berkshire Hathaway (NYSE: BRK.B) — owns Johns Manville, but the exposure is immaterial to the whole [16][18].
• TopBuild (NYSE: BLD) and Installed Building Products (NYSE: IBP) — downstream distribution and installation exposure rather than manufacturing; they track insulation demand without furnace/energy risk [7][23].

Private-market routes. Direct plant, JV, or greenfield investment (the model Rockwool and Knauf use); the private ownership of Knauf/USG and Johns Manville; the horticulture growing-media niche (Grodan-style stone wool substrate for controlled-environment agriculture) [30]; and the distribution/contractor layer that installs the product. Private-market exposure is more accessible in regional installation, specialty distribution, mechanical-insulation fabrication, fire-stopping, and energy-retrofit contractors than in greenfield manufacturing.

Near-term drivers to watch (forward-looking). Housing starts and mortgage rates; the pace of state IECC and Title 24 code adoption (a volume ratchet); energy prices and furnace-electrification progress; data-center construction; and the tightening of fire-safety codes. The judgment call: cyclical construction risk sits on top of genuinely durable structural tailwinds — codes, decarbonization retrofits, and fire safety — that should lift insulation content per building over time, with stone wool the likeliest share-gainer in fire-critical and commercial work (Rockwool expects North American stone-wool demand to exceed capacity until 2028 [13]). Energy cost, permitting, and new capacity timing are the swing factors that decide whether those tailwinds convert into margin.

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Sources

1. U.S. Census Bureau. 2022 Economic Census — Concentration & Comparative Statistics, NAICS 327993 (Mineral Wool Manufacturing) (receipts, firm count, CR4/CR8/CR50, HHI). 2022. https://data.census.gov/
2. U.S. Environmental Protection Agency. AP-42: Compilation of Air Pollutant Emissions Factors, Chapter 11.18 — Mineral Wool Manufacturing. 2020. https://www.epa.gov/sites/default/files/2020-10/documents/c11s18.pdf
3. U.S. Environmental Protection Agency. AP-42: Compilation of Air Pollutant Emissions Factors, Chapter 11.13 — Glass Fiber Manufacturing. 2020. https://www.epa.gov/sites/default/files/2020-10/documents/c11s13.pdf
4. U.S. Energy Information Administration. 2022 Manufacturing Energy Consumption Survey (MECS), Table 6.1. https://www.eia.gov/consumption/manufacturing/data/2022/pdf/Table6_1.pdf
5. U.S. Census Bureau / NAICS. NAICS 327993 — Mineral Wool Manufacturing: definition, inclusions, exclusions. 2022. https://www.census.gov/naics/
6. U.S. Census Bureau. County Business Patterns, NAICS 327993 (establishments, employment, annual payroll). 2023. https://www.census.gov/programs-surveys/cbp.html
7. TopBuild Corp. Form 10-K for fiscal year ended December 31, 2025. https://www.sec.gov/Archives/edgar/data/1633931/000110465926020481/bld-20251231x10k.htm
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17. Armstrong World Industries. SEC 10-K / results — net sales ~$1.6B; mineral-fiber ceilings ~74% of sales. 2024. https://www.tradingview.com/news/tradingview:c7528c844f255:0-armstrong-world-industries-inc-sec-10-k-report/
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30. Grodan (ROCKWOOL). Stone wool growing media for hydroponics / controlled-environment agriculture. https://www.grodan.com/global/solutions/stonewool-growing-media/
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32. California OEHHA. Modification of the Listing of Glass Wool Fibers under Proposition 65 (bio-soluble insulation fibers de-listed); NTP Report on Carcinogens. 2011. https://oehha.ca.gov/proposition-65/crnr/modification-listing-glasswool-fibers-airborne-particles-respirable-size-glass-wool-fibers-inhalable
33. EURIMA / EUCEB. Bio-solubility, "Note Q" exoneration, and formaldehyde-free binder trends. https://www.eurima.org/regulatory-compliance
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35. GreenBuildingAdvisor. New Rockwool Plant Divides a West Virginia Community — air permitting and community opposition. https://www.greenbuildingadvisor.com/article/news-new-rockwool-plant-divides-a-west-virginia-community
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