Rock wool (stone wool) is one of the most effective thermal insulants for buildings and industrial plants, but its real-world performance comes down to two numbers: thermal conductivity (λ, lambda) and R-value. Understanding what these figures mean is the key to specifying the right thickness, meeting energy codes, and comparing products fairly. Whether you are an architect designing a façade, a contractor insulating a roof, or a buyer comparing factory quotes, the figures below will help you make a confident decision. This guide explains how rock wool thermal conductivity and R-value are defined and measured, and what you can expect from Taishi Rock Wool boards — produced with a declared λ of ≤0.040 W/m·K and tested to GB/T 25975-2018, with EN 13162 and ASTM C612 reports available on request.
What Is Thermal Conductivity (λ) in Rock Wool?
Thermal conductivity — written as λ (lambda) and measured in watts per metre-kelvin (W/m·K) — describes how easily heat flows through a material. The lower the number, the better the insulation. Still air has a λ of roughly 0.025 W/m·K, and mineral wool gets close to that ideal because its fine fibres trap millions of tiny, still air pockets that resist heat transfer.
Taishi Rock Wool boards carry a declared thermal conductivity of ≤0.040 W/m·K, tested to GB/T 25975-2018 and reported in line with EN 13162 and ASTM C612. A value this low means less heat escapes in winter and less heat intrudes in summer — allowing a thinner, lighter build-up than many competing insulants for the same performance.
If you are new to the product, start with our guide on what rock wool board is and how it is made.
How Rock Wool Conductivity Is Measured and Declared
Thermal conductivity is measured in a guarded hot-plate or heat-flow-meter apparatus under controlled conditions, typically at a mean temperature of 10°C or 25°C. The result is the declared value (λD), which a manufacturer publishes on the product’s technical datasheet and CE marking.
For Taishi Rock Wool, that declared value is ≤0.040 W/m·K, verified to the Chinese national standard GB/T 25975-2018 and reported in line with European EN 13162 and American ASTM C612. When an engineer designs a building, they may apply a slightly higher design λ to account for joints, fixings and ageing — which is why choosing a product with a genuinely low declared λ leaves headroom for code compliance.
Rock Wool R-Value: Thickness Sets the Resistance
R-value is thermal resistance — how strongly a given thickness of insulation resists heat flow. It is simply thickness divided by conductivity:
R = d ÷ λ, where R is in m²·K/W, d is thickness in metres, and λ is in W/m·K.
Because λ stays essentially constant for a given product, R-value scales almost linearly with thickness: double the board thickness and you roughly double the R-value. Here is what that looks like for Taishi Rock Wool at λ = 0.040 W/m·K:
| Thickness | Metric R-value (m²·K/W) | Imperial R-value (ft²·°F·h/Btu) |
|---|---|---|
| 30 mm | 0.75 | 4.3 |
| 50 mm | 1.25 | 7.1 |
| 100 mm | 2.50 | 14.2 |
| 150 mm | 3.75 | 21.3 |
| 200 mm | 5.00 | 28.4 |
To put that in context, a 100 mm board at λ = 0.040 W/m·K gives an R-value of 2.5 m²·K/W — equivalent to a U-value of roughly 0.40 W/m²·K for the insulation layer alone. Thicker boards, or doubling layers, bring that figure down proportionally to meet stricter energy targets.
Taishi boards are supplied in the standard 1200×600 mm format in thicknesses from 30 mm up to 200 mm, so a single product line covers everything from thin internal liners to thick roof and external-wall build-ups.
Density: Strength and Sound Without Sacrificing λ
Density (kg/m³) affects compressive strength, sound absorption and handling — but a well-engineered rock wool holds a low λ across its density range. Taishi’s external-wall series spans 50–120 kg/m³, giving specifiers the compressive performance and fire rating required for ETICS/EIFS façade systems while keeping thermal conductivity at or below 0.040 W/m·K.
Higher-density boards also deliver better airborne sound insulation and dimensional stability, which is why the 50–120 kg/m³ external-wall series is a common choice for rainscreen and ventilated façade applications where both acoustic and thermal comfort matter.
Browse the full rock wool board range to match density and thickness to your application.
Why a Low λ Matters: Energy, Compliance and Fire Safety
A lower thermal conductivity delivers three benefits at once:
- Lower energy bills — a thinner wall can hit the same U-value, or the same thickness performs better.
- Easier code compliance — declared λ and R-value let you calculate U-values against local energy regulations with confidence.
- Fire safety without compromise — rock wool is rated A1 non-combustible, so the same layer that insulates also slows fire spread and adds no fuel load.
These numbers only mean something if the product is consistent batch after batch. Taishi Rock Wool is a China Famous Trademark produced across 7 bases with a combined 800,000-tonne annual capacity, using the 1450°C basalt-melt pendulum method to form fibres — the same process behind the EN 13162 and ASTM C612 test reports we make available on request.
Whether you need a 50 mm acoustic liner or a 200 mm external-wall board, we can quote the right density and thickness for your project — and ship worldwide through our export partner, Geyuan Insulation (Shandong) Co., Ltd. Contact us for a factory-direct quote with a 24-hour reply.
Related reading
- What Is Rock Wool Board? Manufacturing, Properties and Applications
- Rock Wool for Cold Storage and Sandwich Panels: Fire-Safe Insulation Guide
- Rock Wool vs Fiberglass Insulation: Complete Comparison
Buyer questions we get most
What is the R-value of rock wool?
R-value is thickness divided by thermal conductivity. At a typical λ of 0.040 W/(m·K), a 100 mm product gives R ≈ 2.5 m²·K/W.
Does a higher density give better insulation?
No. Density mainly governs compressive strength, fire performance and acoustic mass. Insulation value comes from thickness and λ, so paying for density does not lower a heating or cooling bill.
Why do two suppliers quote different λ values?
Density, fibre orientation and the test standard all move the number. Always ask for the tested λ and the standard it was tested to, then convert to R-value yourself.
About this guide
Who maintains it
This guide is maintained by the Taishi Rock Wool export team in Shandong, China — factory-direct rock wool and glass wool from seven production bases with a combined capacity of 800,000 tonnes a year. Technical questions go straight to the factory rather than to a trading desk: WhatsApp +86 175 6050 1246 or email cliu32626@gmail.com.
Standards and sources referenced
- GB/T 25975-2018 — Chinese national standard for rock wool and its products; our production and test reports follow it.
- EN 13162 — European product standard for mineral wool, including how thermal conductivity (λ) is declared.
- EN 13501-1 — fire classification of construction products; class A1 means non-combustible.
- ISO 8301 / ASTM C518 — steady-state thermal conductivity test methods behind any quoted λ figure.
- ASHRAE Handbook—Fundamentals — definitions of thermal conductivity and R-value used here.
- EURIMA — European Mineral Wool Manufacturers Association, technical guidance on mineral wool performance.
- Insulation Institute (NAIMA) — industry technical resources on insulation performance and specification.
Last updated: September 23, 2026