Mineral wool

Mineral wool is molten rock — basalt, dolomite, or blast furnace slag — spun or blown into fine fibres and bound into a mat. Like every bulk insulation it works by trapping air in spaces too small for convection to get going, and its thermal conductivity is consequently within a factor of two of glass wool, polyurethane foam and expanded polystyrene.

What distinguishes it is what happens in a fire. It is rock, so it does not burn, does not contribute fuel, and does not produce smoke — and the high-temperature grades hold together well above 1000 °C. Every organic foam insulation burns, and the flame retardants added to slow that down have their own history.

After the Grenfell Tower fire in 2017, regulation in several countries restricted combustible insulation on tall buildings regardless of its thermal performance, and mineral wool is the material that answer points to.

Rock is melted at around 1500 °C and the stream is thrown onto spinning wheels, which fling droplets that draw out into fibres as they fly — the same principle as candy floss, at ten times the temperature. A binder is sprayed on and cured, and the mat is cut into batts, rolls or boards.

Slag wool uses blast furnace slag instead of quarried rock, which is a genuine use for a large industrial by-product.

Building insulation in walls, roofs and floors. Acoustic insulation and studio treatment, where the same fibrous structure absorbs sound. Fire stopping and protection of structural steel. Industrial and marine pipe and vessel insulation. And, in a completely different application, as a hydroponic growing medium.

Observed in nature first: fibres form naturally around Hawaiian volcanoes and are called Pele's hair. Industrial production dates from the 1870s in Wales and the United States, and the material became a mass building product after the Second World War as insulation standards rose.

Energy intensive to make — melting rock is not cheap — and it repays that many times over in a heated building, which is the clearest case of embodied energy being the wrong thing to optimise in isolation.

The health question has been examined thoroughly and the answer is reassuring in a specific way. Early concern arose by analogy with asbestos, since both are mineral fibres. The distinguishing property is biopersistence: asbestos fibres are durable and stay in lung tissue for decades, and modern mineral wool fibres dissolve in lung fluid within weeks. The International Agency for Research on Cancer moved glass and rock wool out of its possible-carcinogen category in 2001 on that basis. The fibres are still a mechanical irritant to skin and airways, and handling precautions are about that.