Rock Wool

What is Rock Wool

Rock wool is a man-made vitreous fibre insulation produced by melting natural rock, predominantly volcanic basalt or diabase, together with steel slag and limestone in a cupola or electric furnace at roughly 1400 to 1500 degrees Celsius, then spinning the molten stream into fine fibres on rotating wheels. The fibres are sprayed with a small fraction of thermosetting binder and a water-repellent additive, collected on a conveyor as a wool mat, and cured in an oven before being cut into batts, rolls, boards, or pipe sections. The finished fibre diameter is typically in the 3 to 7 micrometre range, and binder content is usually only 2 to 5 percent by weight, which is what allows the product to remain non-combustible.

The material belongs to the broader category that EN 13162 calls factory-made mineral wool (MW). Mineral wool divides into two families that share a manufacturing concept but differ in raw material: glass wool, spun mostly from recycled container glass and sand, and stone wool, spun from rock and slag. Rock wool and stone wool are interchangeable names for the same product. The practical distinction engineers draw between the two families is service temperature and density: stone wool tolerates substantially higher temperatures and is available in higher densities, which makes it the default where fire resistance or mechanical loading governs.

The industrial history is older than most realise. Natural mineral fibre was first observed in Hawaii, where wind drew threads of glass from molten lava (the strands islanders called Pele's hair). Commercial slag wool production began in Wales and the United States in the 1870s, and the modern spun-stone-wool process that underlies today's industry was developed in the early twentieth century, with large-scale European production established by mid-century. The founding of the company now known as ROCKWOOL in Denmark in 1937 is a common reference point for the modern industry.

Functionally, rock wool is a multi-duty material rather than a single-purpose one. The same fibre mat simultaneously slows conductive heat transfer (because still air trapped between fibres conducts poorly), absorbs airborne sound (because the porous matrix dissipates acoustic energy as friction), and resists fire (because the mineral fibre does not burn and melts only above 1000 degrees Celsius). No competing insulation chemistry, neither expanded polystyrene, extruded polystyrene, polyurethane, nor phenolic foam, delivers all three at once: the organic foams offer lower thermal conductivity per millimetre but are combustible and have far lower service temperatures.

In application scale, rock wool reaches from the building envelope to heavy industry. In construction it insulates external walls, cavity walls, ventilated rainscreen facades, flat and pitched roofs, floors, and internal partitions, and it forms the core of fire-rated walls and doors. In industry it lags steam pipework, boilers, furnaces, ducts, and process vessels operating up to several hundred degrees Celsius, and high-density grades line ships' bulkheads to meet marine A-class fire requirements. Horticultural rock wool, a separate low-density grade, is even used as a soilless growing substrate, though that is outside the scope of this thermal-insulation guide.

Product Forms and Classification

Rock wool reaches the jobsite in several physical forms, and choosing the wrong form is a common procurement error: a flexible batt cannot carry a render facade, and a rigid board wastes money in an unloaded cavity. The form is largely a function of the binder content and the manufactured density, which together set how rigid, how strong, and how easily compressed the product is. The table below summarises the main forms, their typical density windows, and where each belongs.

Batts and slabs are the workhorse of building insulation. Semi-rigid batts in the 30 to 60 kg/m3 range are friction-fitted between studs, joists, or rafters; their slight springiness lets them grip the cavity without fasteners. They are sized to standard stud spacings and are the cheapest way to add thermal and acoustic value to an unloaded cavity. They cannot, however, take any compressive load, so they are never used under screeds or as the substrate for direct render.

Rigid boards carry the loads that batts cannot. By raising density to 80 to 200 kg/m3 the binder locks the fibres into a board that resists point and distributed loads, which is what a rendered external wall, a walkable flat roof, or a load-bearing floor demands. Continuous-insulation boards used outboard of the structural wall, products such as ROCKWOOL Comfortboard and Cavityrock, fall in this group, as do the high-density industrial slabs used in firewalls and marine bulkheads.

Pipe sections are pre-formed cylindrical shells, usually hinged with a longitudinal slit so they snap around the pipe, sized to standard nominal pipe diameters. Their density is tuned for the mechanical robustness needed on hot pipework plus low thermal conductivity at elevated temperature. Loose and granulated wool is blown or poured into irregular cavities, attics, and voids where a rigid product cannot reach. Lamella mats orient the fibres perpendicular to the facing so the mat bends easily around curved tanks and vessels while keeping a relatively high crush strength in the radial direction.

A second axis of classification is whether the product is faced. Many boards and pipe sections are supplied with an aluminium foil, glass-tissue, or reinforced-foil facing that acts as a vapour retarder, a radiant barrier, or a finish surface. The facing changes the product's fire classification and its water-vapour behaviour, so it must be specified deliberately rather than treated as cosmetic.

Density Grades and Performance

Density, expressed in kilograms per cubic metre, is the single most informative number on a rock wool datasheet because it correlates with compressive strength, fire-resistance duration, acoustic absorption, and, less obviously, with thermal conductivity. Two boards of the same thickness but different density behave very differently under a render load or a two-hour fire test. The table below maps the common density bands to the properties they unlock and to where they are specified.

The relationship between density and thermal conductivity is not monotonic, which trips up many first-time specifiers. At very low density the fibre matrix is too open and radiant and convective heat transfer through the air voids raises lambda. As density rises the air is increasingly subdivided and immobilised, lambda falls, and it reaches a broad minimum somewhere in the 40 to 100 kg/m3 region for most products. Beyond that, packing the fibres tighter increases solid conduction through the glass itself and lambda creeps up again. This is why the lowest declared lambda values often appear on mid-density boards rather than the densest ones.

Fire-resistance duration, by contrast, increases steadily with density. A two-hour or four-hour rated firewall, or a marine A60 bulkhead that must hold a 60-minute integrity and insulation rating, relies on a high-density slab because the denser fibre mat resists shrinkage and slumping for longer once the binder has burned off and the fibre approaches its sintering point. This is the structural reason firewall and marine grades sit at the top of the density range rather than being chosen for thermal performance.

Acoustic absorption also tracks density and thickness together. A porous mineral mat absorbs airborne sound by converting the acoustic particle velocity into friction within the fibre network. Mid-density semi-rigid slabs of roughly 45 to 90 kg/m3 are the classic acoustic-treatment density; properly thick assemblies reach a noise reduction coefficient (NRC) approaching or exceeding 1.0 across the speech band. Going far denser does not keep improving absorption, because once the airflow resistivity is high enough the surface begins to reflect rather than absorb.

The engineering takeaway is that density is a multi-objective lever, not a quality score. A higher number is not automatically better: it is better only for the objective that needs it. Specifying a 150 kg/m3 board in an unloaded loft wastes money and slightly worsens lambda, while specifying a 45 kg/m3 batt under a screed guarantees a callback. The discipline is to identify the governing duty, structural, fire, or acoustic, and let that set the minimum density, then confirm the thermal performance follows.

Thermal, Fire and Acoustic Properties

Rock wool is selected for the combination of three physical behaviours that almost no other single material delivers together: low thermal conductivity, non-combustibility, and high sound absorption. Understanding each behaviour, and its limits, is what separates correct specification from optimistic assumption.

Thermal conductivity (lambda). Heat moves through any insulant by three paths in parallel: solid conduction along the fibres, gas conduction and convection through the trapped air, and radiation across the voids. Rock wool minimises all three by holding a large volume of still air in a fine fibre network. For building products EN 13162 declares lambda at a 10 degrees Celsius mean temperature, and stone wool values fall between roughly 0.033 and 0.045 W/(m.K), with the bulk of facade and roof boards declared at 0.034 to 0.037 W/(m.K). The critical caveat for industrial use is that lambda is temperature dependent: it rises with mean temperature because gas conduction and radiation both grow, so a product reading 0.037 W/(m.K) at 10 degrees Celsius may read 0.07 to 0.09 W/(m.K) at a 300 degrees Celsius mean. High-temperature insulation must always be sized against the lambda-versus-temperature curve, not the headline figure.

Fire performance. This is rock wool's signature property. The fibre melts only above 1000 degrees Celsius, and with binder content of just a few percent the product carries the Euroclass A1 reaction-to-fire classification under EN 13501-1, the top non-combustible class, meaning it does not ignite, adds no fuel load, emits negligible smoke, and sheds no flaming droplets. It is essential to separate two distinct concepts: reaction to fire (A1, a material property) and fire resistance (an assembly property measured in minutes, such as EI 60 or marine A60). A1 is the prerequisite the material must satisfy; the assembly rating then depends on the board density, thickness, and fixing within a tested wall, floor, or bulkhead build-up.

Acoustic absorption. The same open porous structure that traps air for thermal purposes also dissipates sound energy as friction, making rock wool a strong broadband absorber. Mid-density mineral-wool slabs are the standard infill behind acoustic panels and inside partition cavities, and well-detailed assemblies reach an NRC at or above the 0.90 to 1.05 region across the principal speech frequencies. Acoustic performance is governed by airflow resistivity, density, and thickness together; in practice the thickness of the cavity and the air gap behind the absorber matter as much as the wool itself.

Water and moisture behaviour. Mineral fibre is inorganic and does not wick water, and most products are dosed with a hydrophobic additive, so short-term water absorption by partial immersion under EN 1609 is typically below 1 kg/m2. Rock wool is also highly vapour-open, with a water-vapour diffusion resistance factor (mu) of about 1, essentially the same as still air, which lets a wall assembly dry outward. It is not waterproof: prolonged saturation fills the voids with water, which conducts heat far better than air and collapses the insulation value, and standing water can leach binder. In wet, buried, or below-grade positions the wool must sit behind a membrane or rainscreen and any drained water must be free to escape. Other properties worth noting are chemical inertness (rock wool resists most acids and bases, and is non-corrosive to steel pipework when chloride content is controlled per ASTM C795), biological inertness (it does not feed mould or attract vermin), and dimensional stability under heat.

Standards and Key Specifications

Rock wool is one of the most heavily standardised building products, which is good news for buyers because almost every relevant property is declared on a certified datasheet. The two anchor product standards are EN 13162 in Europe and ASTM C612 in North America, supported by reaction-to-fire (EN 13501-1), corrosion (ASTM C795), and terminology (ISO 9229) standards. The table below decodes the parameters that actually drive a selection decision and the standards that define them.

EN 13162 is the European harmonised standard for factory-made mineral wool thermal insulation in buildings. It mandates CE marking, a declared thermal conductivity, and a string of class codes covering dimensional tolerances, dimensional stability under temperature and humidity, compressive stress, tensile strength, point load, and water absorption. Products outside its scope are those with declared thermal resistance below 0.25 m2.K/W or declared lambda above 0.060 W/(m.K) at 10 degrees Celsius. When buying for a European envelope, the Declaration of Performance (DoP) tied to EN 13162 is the document of record.

ASTM C612 covers mineral fiber block and board thermal insulation and is the standard quoted on most North American industrial jobs. Its defining feature is classification by maximum use temperature: Type IA and IB to 232 degrees Celsius (450 degrees Fahrenheit), Type II to 454 degrees Celsius (850 degrees Fahrenheit), Type III to 538 degrees Celsius (1000 degrees Fahrenheit), Type IVA and IVB to 649 degrees Celsius (1200 degrees Fahrenheit), and Type V to 982 degrees Celsius (1800 degrees Fahrenheit). Specifying the correct C612 type is the single most important step for high-temperature pipe, boiler, and vessel insulation, because using a low-temperature type on hot equipment causes premature binder burnout, shrinkage, and gapping.

Reaction to fire is governed by EN 13501-1 in Europe, where stone wool earns the top A1 class, and by ASTM E84 (the surface burning characteristics test) in North America, where qualifying products report a flame spread index and smoke developed index at or near zero. These are material classifications and are distinct from the assembly fire-resistance ratings established by furnace tests such as EN 1364 or ASTM E119.

Corrosion and chemistry matter when wool contacts metal. ASTM C795 specifies the thermal insulation for use over austenitic stainless steel, limiting leachable chloride and requiring a pass on the C692 stress-corrosion test and the C871 chemical analysis, which protects against chloride-induced stress-corrosion cracking on stainless pipework. For any insulation in contact with stainless process lines, a C795-compliant grade should be specified rather than a generic board.

Selection Decision Factors

To convert the preceding chapters into a specified product, work the decision sequence below in order. Most selection failures come not from a single wrong number but from deciding density or form before the governing duty is identified. These eight steps double as a fixed RFQ template.

1. Identify the governing duty: decide first whether thermal, fire-resistance, acoustic, or structural load dominates. That single decision sets the minimum density band before any thermal sizing begins. A firewall is a fire decision; a render facade is a structural one; a partition is acoustic.
2. Choose the product form: batt or slab for unloaded cavities, rigid board for render and traffic loads, pipe section for cylindrical pipework, loose fill for irregular voids, lamella mat for curved vessels. The form follows the duty, not personal habit.
3. Set the density grade: 30 to 60 kg/m3 for unloaded thermal and acoustic work, 90 to 140 kg/m3 for render facades and pipe shells, 140 to 200 kg/m3 for floors, firewalls, and marine bulkheads. Confirm the compressive stress at 10 percent deformation against the service load.
4. Size thickness for R-value or surface temperature: use R equals thickness divided by lambda, reading lambda at the actual mean operating temperature. For envelopes, sum layer resistances to hit the target U-value and deduct 10 to 30 percent for thermal bridging at studs and brackets.
5. Specify the standard and class: EN 13162 with the relevant class codes for European building work, or the correct ASTM C612 type (I to V) for North American and high-temperature industrial work, matched to the maximum service temperature with margin.
6. Confirm the fire requirement: Euroclass A1 reaction to fire is intrinsic to stone wool, but verify the assembly fire-resistance rating (EI 60, EI 120, A60) is achieved by the specific density, thickness, and fixing in a tested build-up.
7. Address moisture and facing: select a hydrophobic-treated grade for damp exposure, add a foil or tissue facing where a vapour retarder or radiant barrier is needed, and ensure drained water can escape. For stainless contact, require an ASTM C795-compliant low-chloride grade.
8. Total cost of ownership (TCO): compare delivered board price plus fixing labour, accessories (fasteners, render carrier, facing), and the cost of any thickness increase needed to offset thermal bridging. The cheapest board per cubic metre is rarely the cheapest installed wall.

One dimension that buyers routinely overlook is manufacturer serviceability and documentation: a current Declaration of Performance or ASTM-referenced datasheet, third-party fire-test reports for the exact assembly, Environmental Product Declarations where green-building credits apply, and reliable regional stock for the specified thickness. The mainstream stone wool suppliers, ROCKWOOL (Comfortbatt, Comfortboard, Cavityrock, and the ProRox SL 940 and SL 960 industrial boards), Paroc, Knauf Insulation, Saint-Gobain Isover, and Owens Corning Thermafiber, all publish full technical documentation and hold tested-assembly libraries, which is what makes them defensible choices on audited projects. Always pull the current datasheet for the exact thickness and product family, because declared lambda, density, and class codes shift between families within a single brand.