Rock Wool vs Glass Wool

A property-by-property comparison of two non-combustible mineral wool insulations, so you can match the material to what your specification actually demands.

Which should you choose?

Choose rock wool when fire integrity, high-temperature service, load-bearing strength or water resistance leads the specification: it is non-combustible with a melting point above 1,000°C, spans 40–220 kg/m³ densities with corresponding compressive strength, and is hydrophobic-treated to a water-repellency rate above 98%. Choose glass wool when the priority is low-cost, lightweight thermal insulation in a dry, non-loaded, moderate-temperature cavity, where its lighter weight and marginally lower thermal conductivity per kilogram help. Both are non-combustible mineral wools classified Euroclass A1 or A2 under EN 13501-1; the practical difference is temperature endurance, density and durability, not chemistry.

Six-property comparison

Rock wool figures are Taishi® board values from our datasheets and CE documentation. Glass wool figures describe the product as a class, using typical published ranges rather than one tested sample.

Property	Rock wool (Taishi®)	Glass wool (as a class)
Reaction to fire	Non-combustible; Euroclass A1 (EN 13501-1) declared on the CE certificate to EN 13162	Non-combustible; typically Euroclass A1 or A2
Max service temperature	Melting point above 1,000°C; continuous service to about 650°C on high-density grades	Softens at a lower temperature, typically around 600°C
Thermal conductivity (λ)	About 0.038–0.050 W/(m·K), depending on grade and density	About 0.030–0.040 W/(m·K) as a class
Density & compressive strength	40–220 kg/m³; compressive resistance declared per grade, up to 200 kPa (EN 826) on high-density boards	Lighter, lower density; limited compressive resistance
Acoustic absorption	Strong; NRC >0.45, with better low-frequency absorption at high density	Effective, particularly at mid-to-high frequency
Water resistance & durability	Hydrophobic >98–99%; dimensional stability ≤1.0%	Can be treated water-repellent, but more compressible and moisture-sensitive over time

Reaction to fire: both non-combustible, separated by temperature

Both rock wool and glass wool are non-combustible mineral wools. Taishi® rock wool is declared Euroclass A1 for reaction to fire under EN 13501-1, as stated on our CE certificate to EN 13162 (Intertek NB 2649); it does not ignite, adds no fuel to a fire and releases no significant smoke. Glass wool, as a class, is also inorganic and generally reaches Euroclass A1 or A2-s1,d0 under the same standard. On the reaction-to-fire class alone the two are broadly comparable. The decisive fire distinction is not the class but how each behaves as temperature rises, which the fibre's melting and softening point sets — the next property.

Temperature endurance: rock wool holds integrity far hotter

This is where the two materials genuinely diverge. Taishi® rock wool is spun from basalt-type rock and has a melting point above 1,000°C — above 1,150°C on the fireproof black grade — with a maximum continuous service temperature up to about 650°C on high-density boards. That lets a single fire barrier or industrial jacket keep its shape and insulating function through a prolonged, high-temperature fire. Glass wool, as a class, is spun from recycled glass and sand and softens at a markedly lower temperature, typically around 600°C; above that its fibre structure begins to slump and lose integrity. For passive fire protection, firestops and high-temperature process insulation, the higher melting point is the reason rock wool is specified.

Thermal conductivity: glass wool marginally lower per kilogram

For everyday thermal insulation the two are close. Taishi® rock wool declares thermal conductivity limits of 0.038 to 0.050 W/(m·K) across the range — ≤0.038–0.040 W/(m·K) on standard building boards, rising to ≤0.050 W/(m·K) on the densest marine grades — to EN 13162 and GB/T test methods. Glass wool, as a class, commonly reports a marginally lower value of around 0.030 to 0.040 W/(m·K) at low density, which is one reason it is popular in lightweight, cost-led building envelopes. The gap is small and narrows once temperature, moisture or load enter the picture, because rock wool holds its stated value across a wider service range where glass wool is compressed or heated.

Density and strength: rock wool carries load, glass wool does not

Taishi® rock wool is manufactured across a broad density band, from about 40 to 220 kg/m³, and that density buys mechanical strength: compressive resistance is declared per grade — at least 40 kPa on standard wall and roof boards, at least 80 kPa on the densest roof board, and up to 200 kPa (EN 826) on the highest-density marine grade — with point loads of ≥200 to ≥700 N on roof boards. This is what lets rock wool sit under a loaded roof deck, behind a rendered façade, or as a structural sandwich-panel core. Glass wool, as a class, is a lighter, lower-density product usually supplied as soft rolls or semi-rigid batts; it offers limited compressive resistance and is not normally specified where the insulation itself must carry load or resist foot traffic.

Acoustics: both absorb sound, rock wool leads at low frequency

Both materials are open, porous fibre and absorb airborne sound well. Taishi® multipurpose rock wool board reaches a noise reduction coefficient above 0.45, and its higher-density grades improve absorption at low frequencies and add mass to a partition, which raises airborne sound insulation. Glass wool, as a class, is also an effective absorber and performs particularly well at mid-to-high frequencies thanks to its light, open structure. Its lower density generally makes it less effective than high-density rock wool at low frequencies and in dense partition or floating-floor build-ups. Where the target is a heavy, low-frequency-rated partition or a machine enclosure, density favours rock wool; for light mid-to-high lining, glass wool is often sufficient.

Water and durability: density and treatment favour rock wool

Taishi® rock wool is hydrophobic-treated to a water-repellency rate above 98 to 99% by series, with low moisture absorption and dimensional stability within 1.0%. It is inorganic, rot-proof, does not feed mould, and holds its thermal performance for decades in humid, coastal or marine service — the same board range carries CCS Works Approval for shipbuilding. Glass wool, as a class, can also be supplied water-repellent-treated, but being a lighter product it is generally more compressible and more sensitive to moisture over a long service life, particularly where it is not kept dry. Both are inorganic and non-combustible; the durability gap is one of density and water handling, not of chemistry.

Choosing for your application

Match the material to the property that leads your specification. These are general guidelines; send your project details and we will recommend a specific series.

Rock wool is usually the better fit for

•High-rise façades, EIFS/ETICS render systems and curtain-wall fire stops
•Flat and metal roof decks that must carry load (FM Approvals Class 4470, Hardroof 180)
•Industrial and high-temperature insulation — process pipe, vessels and mechanical rooms up to about 650°C
•Marine and offshore bulkheads and structures (CCS Works Approval)
•Acoustic partitions, machine enclosures and floating floors where low-frequency performance matters
•Humid, coastal or wash-down environments that need water repellency

Glass wool is often sufficient for

•Lightweight, dry, non-loaded building cavities and stud partitions
•Cost-led thermal insulation where temperature and moisture stay moderate
•Mid-to-high frequency acoustic lining in light framing

Not sure which grade fits your project?

Send your application, target temperature, thickness and quantity. We will match a Taishi® rock wool series and share datasheets and certificates.

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