Understanding Fire Resistance Terminology in Mineral Wool

When specifying building materials, understanding fire resistance terminology is the difference between a safe structure and a potential hazard. In the world of mineral wool (stone wool), you will frequently encounter two major classification systems: Class A and Class 1.

1. Class A (ASTM E84 / NFPA 255)

In North America, fire performance is primarily measured by the ASTM E84 "Tunnel Test." This test evaluates how a material contributes to fire growth across its surface.

• Flame Spread Index (FSI): Measures how far and fast flames travel.

• Smoke Developed Index (SDI): Measures the amount of smoke emitted.

• The Rating: To earn a Class A (or Class I) rating, the material must have:

• FSI: 0–25

• SDI: 0–450

• Mineral Wool Performance: Most unfaced mineral wool achieves an FSI of 0 and an SDI of 0, making it the safest possible Class A material.

2. Class 1 (BS 476 / International Variations)

The "Class 1" designation is often derived from the British Standard BS 476 Part 7. It measures the lateral spread of flame across the surface of a product.

• The Rating: Class 1 is the highest rating in this specific test, indicating very low flame spread.

• European Equivalent: Under the Euroclass system (EN 13501-1), the equivalent of a Class 1/Class A rating is A1 or A2.

• A1 (Non-combustible): No contribution to fire. Mineral wool typically sits in this category.

• A2 (Limited-combustible): Negligible contribution to fire.

3. Key Differences: Why the Label Matters

4. Why Mineral Wool Wins in Fire Ratings

• Melting Point: It can withstand temperatures exceeding 1,150°C (2,100°F).

• No Toxic Gasses: It doesn't release harmful gases associated with burning foam insulation.

• Passive Fire Protection: Acts as a fire barrier, slowing heat transfer between rooms.