Mineral wool — all forms

Mineral wool is the collective term for two industrial insulation materials made by melting minerals and drawing the 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 insulating effect itself comes not 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 approx. 0,026 W/(m·K). The fibres' job is to hold the air in place and break up convection currents and thermal radiation. That is why a mineral wool mat is 80–95 % air measured by volume, while only 5–20 % is the mineral fibres themselves.

The production process looks roughly like this (both types):

1. Raw materials are melted at 1400–1500 °C in a large furnace. Stone wool melts basalt, diabase and coke (as fuel and chemical reduction). Glass wool typically melts 70 % recycled glass + 30 % new raw materials such as soda, dolomite and quartz sand.
2. Fibre formation. The glowing melt is poured onto a rotating spinning disc (stone wool) or blown out through nozzles (glass wool). Centrifugal force flings the melt out into fine threads that solidify almost instantly in the air.
3. Binder is sprayed on. A thin film of phenol-formaldehyde resin (2–5 % by weight of the finished product) is added. The binder glues the fibres together so they hold their shape as a mat or a moulded piece.
4. Curing in an oven at 200–250 °C. The resin polymerises and becomes firm. The mat takes on its final thickness and density.
5. Cutting and shaping. For pipe sections the mat is moulded around a mandrel so it takes a cylindrical form with radial fibres. For lamella mats a thick mat is cut into narrow strips ("lamellae") that are tipped 90° and glued onto alu foil. For wire-mesh mats a galvanised steel mesh is stitched onto one side.

The binder is the key to understanding many of mineral wool's properties. It is the one "dirty" component in an otherwise pure mineral product. At high temperatures (>250 °C) the binder burns off slowly and releases small amounts of formaldehyde and ammonia — which is why new mineral wool installations smell noticeably the first time the heat is turned on on an industrial pipe, and why test areas must be cordoned off during high-temperature insulation work at power plants [arbejde-med-isoleringsmaterialer.pdf, BAR-BA 2009].

New generations of binders are moving away from phenol-formaldehyde. Knauf Insulation has ECOSE Technology (sugar-based), and Rockwool is experimenting with bio-based resins. Both give lighter mats and lower VOC emission, but the same thermal properties.

Stone wool and glass wool solve the same basic task, but there are differences that have practical bearing on which material you choose.

When is each chosen?

• Glass wool: HVAC in buildings — heating pipes, cold water, ventilation ducts, domestic water. Everything up to 180 °C. When economy matters and the temperature is normal. Isover's CLIMPIPE Section Alu2 is probably the most widely used pipe-section product in Danish office and residential construction.
• Stone wool: Higher temperatures (steam, hot process pipes), fire-engineering requirements (penetrations, section separations), acoustic requirements (residential construction between floors), and wherever mechanical strength is required (industry, large ducts, tanks). Rockwool 800 is the stone wool counterpart to CLIMPIPE.
• Hybrid choice: For hot-water pipes at 60–80 °C both glass and stone wool can be used. Many selections are governed by stock holding and price at the insulator's supplier, not by technical requirements.

An example of the two materials meeting: Isover's U Protect Pipe Section Alu2 is a glass wool product line, but when the temperature requires it, Isover also has stone wool versions of the same pipe section — together they cover a broad temperature range from 180 °C up to 660 °C [isover-technical-insulation.com].

λ (lambda, thermal conductivity coefficient) is measured in W/(m·K) and states how many watts are conducted through 1 m² of the material when there is 1 m thickness and a 1 K (=1 °C) temperature difference. Lower λ = better insulation.

Mineral wool has a base value of approx. λ = 0,037 W/(m·K) at 10 °C — this is the value the compendium Grundlæggende teknisk Isolering (Preben Kristensen, AMU Syd, 2025) cites as typical for "mineral wool such as stone wool and glass wool" [Grundlæggende isolering revideret 220825.docx].

But λ is not constant. It rises with temperature, because both the heat conduction in the trapped air and the thermal radiation between the fibres increase:

This shows that λ at 200 °C is approximately doubled compared with 10 °C. The consequence is very important:

When you dimension insulation on a hot pipe, λ must not be read off at 10 °C — it must be taken at the actual operating temperature (or the mean temperature between the medium and the surface).

In practice it is the supplier's calculation program (Isover's IsoDim, Rockwool's Rockassist, Paroc Calculus) that does this correctly. If you calculate by hand or in a simple formula, look it up in the product's datasheet table and use λ at roughly the mean temperature between the medium temperature and the expected surface temperature (typically 30–50 °C above room temperature).

The difference can be decisive: a 60 °C pipe insulated with a 40 mm pipe section loses 25–30 % more heat than a 20 °C pipe with the same thickness, simply because λ is higher at 60 °C.