Building thermal insulation · Stone wool
ROCKWOOL Thermalrock S
- Thermal conductivity (λ)
- 0.034–0.036 W/m·K
- Sound absorption (NRC)
- 0.90–1.05
How do insulation and acoustic materials behave in fire — foams vs mineral wool?
In short
Seen from a fire point of view, insulation falls into two families. Mineral wools (stone wool, glass wool) are inorganic fibres that are hardly fuel at all, so many products reach a "non-combustible" level. Foams and plastic fibres (rubber foam, PE foam, acoustic foam, polyester fibre) are organic materials that can burn. Flame retardants make them harder to ignite or slower to spread flame in a stated test, but do not turn them into non-combustible materials. The key point is that each test standard measures something different: a pass in one is not a pass in another. Always ask which standard, at what thickness, and on the product with which facing.
Two families — how do they behave in a fire?
| Mineral wool (stone wool, glass wool) | Foams and plastic fibres | |
|---|---|---|
| --- | --- | --- |
| Material | Inorganic fibres from molten rock or glass, with a small amount of organic binder | Organic polymers (synthetic rubber, PE, polyurethane, polyester) |
| Is it fuel? | The fibres are not fuel; many products reach a non-combustible level | Can be fuel; the level depends on formulation and flame retardants |
| At very high temperature | Stone wool keeps its shape to very high temperatures; glass wool softens at a lower temperature than stone wool | Softens, melts, shrinks or chars; some types drip burning material and give off a lot of smoke |
| What can change the result | Facings such as foil, paper, fabric, and adhesives | Thickness, density, facings and covering materials |
The table describes the tendency of each family, not the result of any particular product, and it does not mean foams cannot be used. Many combustible materials are used correctly in buildings when they pass the tests the specification calls for, are installed in suitable locations and are covered as designed.
What flame retardants can and cannot do
A flame retardant makes a material harder to ignite from a small heat source, slower to spread flame, or self-extinguishing once the flame is removed — under the conditions of that test. In a large fire, organic materials can still burn and release heat and smoke. "Does not spread flame" or "fire retardant" on a label should therefore always be followed by the name of the standard and the result.
What does each test measure?
| Standard | Designed for | What is measured | Result reported |
|---|---|---|---|
| --- | --- | --- | --- |
| EN 13501-1 | Construction products (Europe) | A classification built from several tests: non-combustibility, heat of combustion, a room-corner burning item (SBI) and small-flame ignitability | Class A1 to F, with smoke (s) and flaming-droplet (d) sub-classes |
| BS 476 Part 6 | Wall and ceiling linings (UK) | Rate of heat release in a small combustion chamber | Fire propagation index (I and sub-indices) |
| BS 476 Part 7 | Wall and ceiling linings (UK) | How far flame spreads across the surface | Class 1 to Class 4 |
| ASTM E84 | Wall and ceiling surface materials (USA) | Surface flame spread and smoke density in a 10-minute tunnel test, compared with reference materials | Flame Spread Index and Smoke Developed Index |
| UL 94 | Plastic parts in devices and appliances | Small specimens exposed to a small burner flame; afterflame time and flaming drips | HB, V-2, V-1, V-0, 5V, and foam classes (HF) |
| California TB 117-2013 | Materials in upholstered furniture | Resistance to smouldering from a lit cigarette, not an open flame | Pass / fail for cover fabric, barrier and filling materials |
"Class 0" is not a test, and does not mean non-combustible
Class 0 is a level defined in building regulations from two test results together: Class 1 to BS 476 Part 7 plus a Part 6 index below set limits (or a material that is non-combustible throughout). A combustible material with a low-spread surface can therefore be Class 0. It describes the surface of the material; it does not say the core is not fuel.
Why a pass in one test is not a pass in another
- Different ignition sources — a smouldering cigarette, a small burner flame and a tunnel or room-corner fire are heat exposures of very different severity
- Different specimen size and orientation — a small vertical plastic bar does not tell you how a large sheet fixed to a ceiling behaves
- Different quantities measured — some tests look at spread over a surface, some at heat released, some only at whether a cigarette ignites the material
- Different scope — UL 94 was designed for plastic parts in equipment and TB 117-2013 for upholstered furniture; neither was designed to classify wall or ceiling linings of buildings
- Results are tied to thickness and facing — the same product at another thickness or with another facing may give a different result. For example, the AERO-ROOF document reports UL 94 V-0 at 13 mm and Class 0 to BS 476 Parts 6 and 7 at 10 mm: two different tests at two different thicknesses
ASTM E84 itself states that it compares surface burning behaviour, that a flame spread index alone does not classify a material as noncombustible, and that materials which melt, drip or fall out of position during the test may give low indices that are not directly comparable with materials that stay in place.
Everything above concerns the reaction to fire of a material, not the fire resistance of a wall, floor or opening, which is the result of a complete tested system. See reaction to fire, non-combustible and fire resistance.
Examples from stone-wool manufacturer documents
- General-purpose stone-wool slab: ROCKWOOL Thermalrock S is stated as non-combustible, class A1 to EN 13501-1, excluding the foil facing — an example of why the facing has to be considered separately
- Stone-wool slab for fire-protection systems: the ROCKWOOL Conlit H110 document states that ROCKWOOL stone wool is non-combustible with a melting point of approximately 1,000 °C; it is used as part of passive fire-protection systems
- Stone-wool slab for the curtain-wall / floor-edge gap: ROCKWOOL Rocksafe Plus is stated as non-combustible to BS 476 Part 4 with a melting point of at least 1,000 °C to DIN 4102-17. The fire-resistance result to BS 476 Part 20 belongs to the tested installation, not to the slab alone
See also stone wool in passive fire protection.
Questions to ask a seller
- Which standard was it tested to, and which edition?
- What class or index was obtained — not just "passed" or "does not spread flame"?
- At what thickness and density was it tested, and does that match the product you are buying?
- Was it tested with the facing or adhesive that will actually be used?
- Who did the test, and is there a report or certificate to see?
- Is the result for a material or for a tested system?
- Which standard does the project specification or the law require, and does the result match it?
If a product's documents do not state a fire test result, treat it as no data yet. Do not infer one from the material type or from a similar-looking product.
Work with fire requirements
Linings, ceilings and insulation in escape routes, plant rooms, kitchens, tall buildings and public buildings must follow building-control law and the project specification, and should be decided by the designer or a fire-safety engineer. This article explains principles; it is not an assurance that any material may be used for any job.
Sources
- Scottish Government — Building standards technical handbook, Annex 2.E Reaction to fire (EN 13501-1 classes and the tests behind each)
- ASTM International — ASTM E84 Standard Test Method for Surface Burning Characteristics of Building Materials (scope and limitations)
- Singapore Civil Defence Force — Fire Code, Clause 3.13 Classification of spread of flame (definition of Class 0; BS 476 Parts 6 and 7)
- UL Solutions — Combustion (Fire) Tests for Plastics (UL 94 classes and methods)
- Interpower — The UL 94 Standard for Safety (UL 94 scope: plastic parts in devices)
- Legal Information Institute, Cornell Law School — 16 CFR 1640.1 (TB 117-2013 as the smoulder-resistance standard for upholstered furniture materials)
- California CEQAnet — Technical Bulletin 117-2013 (scope)
- Czech Technical University — Compartment and façade large scale tests: behaviour comparison of different insulating materials in case of fire (abstract)
- PCC Group — Is PUR foam flammable? (typical Euroclass of PUR foam and the effect of coverings)
- ROCKWOOL Asia — Frequently Asked Questions (stone wool and fire classification)
- GetBestInsulations — verified product data for ROCKWOOL Thermalrock S, Conlit H110, Rocksafe Plus and AERO-ROOF (based on the manufacturer documents listed on each product page)
Related products
Categories and example products for this topic
- ROCKWOOL Thermalrock S — General-purpose stone-wool slab; non-combustible, A1 to EN 13501-1 (excluding foil)
- ROCKWOOL Conlit H110 — Specially formulated 110 kg/m³ stone wool used as part of passive fire-protection systems
- ROCKWOOL Rocksafe Plus — Curtain-wall safing slab; non-combustible (BS 476 Part 4); fire result is system-based
Fire protection · Stone wool
ROCKWOOL Conlit H110
- Thickness
- 25–100 mm
- Density
- 110 kg/m³
- Thermal conductivity (λ)
- 0.039 W/m·K
Fire protection · Stone wool
ROCKWOOL Rocksafe Plus
- Thickness
- 135 mm
- Density
- 80 kg/m³
- Thermal conductivity (λ)
- 0.035 W/m·K