Mineral wool

Mineral wool is the collective term for two industrial insulation materials produced by melting minerals and drawing the glowing melt out into long, fine fibres. The two variants are stone wool (from basalt, diabase and similar volcanic rocks) and glass wool (from recycled glass mixed with quartz sand). In English they are called stone wool / rock wool and glass wool respectively; in the EU standards the abbreviation MW (Mineral Wool) covers both.

The key point — which is easily overlooked because it is counter-intuitive — is that the insulating effect itself does not come from the fibres. It comes from the air between the fibres.

Still, trapped air is one of the best insulators we have: λ for air at 20 °C is approximately 0,026 W/(m·K) [Engineering Toolbox; ASHRAE Handbook]. That is lower than λ for finished mineral wool (0,032–0,040). The job of the fibres is therefore not to conduct less heat — their job is to hold the air in place and break the three forms of heat transfer.

The three heat transfers that mineral wool must stop:

1. Conduction: Heat spreads as molecules collide with one another. In a solid this is efficient; in air it is very slow. Mineral wool consists of 80–95 vol-% air and only 5–20 vol-% mineral fibres — so conduction is almost entirely through the trapped air.

2. Convection (flow): Warm air rises, cold air sinks — giving a constant circulation that transports heat. If the air spaces are too large, a small air circulation can develop inside the insulation. This is why there is a lower limit for density: below about 30 kg/m³ the air spaces become so open that convection raises λ. The fibres' job is to "stop the wind" inside the material.

3. Radiation: Heat is transferred as infrared light without an intermediary. Between two warm surfaces, radiation passes freely through the air. The mineral fibres interrupt the radiation path — each fibre absorbs a small part of the radiation and re-emits it in all directions. The more fibres in the path of the radiation, the better it is broken. This is why higher density reduces the radiation contribution, and it explains why high-temperature products (where radiation dominates) are built with higher density than ordinary building products.

Mineral wool works because it holds still air and breaks radiation between the fibres. The fibres are not the insulation — they are the scaffolding that keeps the air insulating.

The production process looks, in broad terms, like this for both types:

1. Raw materials are melted at 1400–1500 °C in a large furnace. Stone wool melts basalt, diabase, limestone and coke (the latter both as fuel and chemical reduction). Glass wool typically melts 70% recycled glass + 30% new raw materials such as soda, dolomite and quartz sand [Rockwool.com and Saint-Gobain Isover sustainability reports 2023].

2. Fibre formation. The glowing melt is poured onto a rotating spinner disc (stone wool) or blown out through nozzles (glass wool). The centrifugal force flings the melt out into fine threads that solidify almost instantly in the surrounding air.

3. Binder is sprayed on. A thin film of typically phenol-formaldehyde resin (2–5 wt-% of the finished product) is added to glue the fibres together.

4. Curing in an oven at 200–250 °C. The resin polymerises and becomes solid. The mat acquires its final thickness and density.

5. Cutting and forming. For pipe sections the mat is moulded around a mandrel with the fibres oriented radially. For lamella mats, thick mat is cut into narrow strips that are tilted 90° and glued to aluminium foil. For wire-mesh mats, a galvanised steel mesh is sewn onto one side.

Stone wool and glass wool solve the same basic task — holding still air and breaking radiation — but they are made from different raw materials and have different practical properties.

Choice of principle — when do you choose which?

As a rule of thumb:

• Glass wool = "normal HVAC" up to 180 °C. Cheap, light, good enough. When economy counts.
• Stone wool = higher temperatures, fire requirements, sound requirements, mechanical strength. When the requirements are pushed.

For the specific product choice — Climpipe Section Alu2 vs. Rockwool 800, Industribatts vs. CLIMCOVER Slab — see sortiment/mineraluld-former. At the conceptual level the rule is: higher temperature, tougher requirements → stone wool.

The mineral fibres are pure minerals. They do not burn, melt late, and last forever. But a mat of loose fibres falls apart, so we add a binder — typically phenol-formaldehyde resin at 2–5 wt-% of the finished product [EN 14303 Annex; Rockwool and Isover datasheets].

Why is the binder necessary?

• It glues the fibres together so the mat holds its shape.
• It gives the pipe section the necessary stiffness so it does not collapse around the pipe.
• It maintains the density — without binder, the fibres would settle together under vibration.
• It provides hydrophobisation (silicone in the binder) that makes water bead off.

Why is the binder problematic?

• It is organic and can burn. It is the one "dirt" component in an otherwise pure mineral product.
• At temperatures above 250 °C the binder slowly burns off and releases small amounts of formaldehyde and ammonia. This is why new mineral-wool installations smell noticeably the first time the heat is turned on for an industrial pipe, and why test areas must be cordoned off during high-temperature insulation at power stations [arbejde-med-isoleringsmaterialer.pdf, BAR-BA 2009, p. 19].
• The binder is the reason the finished product is often classified A2-s1,d0 and not A1, even though the fibre itself is A1 (see the next section).

REACH and the formaldehyde trend:

Phenol-formaldehyde resin is regulated under REACH (the EU's chemicals legislation), and formaldehyde is classified as carcinogenic category 1B (H350) when inhaled in pure form [ECHA dossier 50-00-0]. This does not mean that finished mineral wool is carcinogenic — the binder is fully polymerised and locked into the matrix — but the pressure to find alternatives is real.

Two notable moves:

• Knauf Insulation: ECOSE Technology — a sugar-based bio-binder without formaldehyde emissions. Gives lighter brown mats and lower VOC emission.
• Rockwool: new generation of binders — several products with bio-based binder being introduced; Rockwool highlights "lower formaldehyde emission".
• Isover Saint-Gobain has equivalent low-emission binders in several product lines.

Thermal performance is essentially identical — it is the emission profile and indoor-climate quality that are improved.