Mineral wool insulation

Glass wool or stone wool, and how do you choose?

Mineral wool insulation comes in two families spun from different melts. Glass wool is light and springy, supplied mostly as rolls and soft batts; stone wool has a higher density, is stiffer and holds its shape even in large boards. Thermally they sit close together: glass wool has a conductivity of roughly 0.032 to 0.040 W/(m·K), stone wool in facade and roof boards 0.035 to 0.045 W/(m·K). The decision is therefore not about conductivity at all but about mechanics, and about where the material sits in the build-up.

Flexible batts belong wherever a frame holds them, between rafters, in timber-frame walls, in partitions and in suspended ceilings. The batt is cut a few millimetres oversize, pressed between the members and fills the cavity without gaps. Stone wool goes where stiffness and compressive strength are needed, and its stability under load is what makes it the only sensible choice under a screed or behind a render.

Why does the A1 fire class decide so many specifications?

Glass wool and stone wool are both reaction to fire class A1 to EN 13501-1, that is, non-combustible and contributing nothing to a fire. An external insulation system built with expanded polystyrene is normally classified as a system in class B-s1,d0, which Slovak fire regulation restricts on taller buildings; the governing standard is STN 92 0201 and the fire authority signs off the specific project. Once a fire report rules out combustible insulation, mineral wool is the only established answer at ordinary cost.

On a ventilated facade the case is clearer still. The cavity behind the cladding is a continuous vertical duct that behaves like a chimney in a fire, so insulation in a ventilated build-up is almost always specified as stone wool in Slovakia. The same reasoning applies to fire-separating elements and their penetrations, where non-combustibility is a condition rather than an advantage.

How does mineral wool behave when it gets damp?

Mineral wool is vapour-open and hygroscopic: it passes vapour with almost no resistance, and it can take up airborne moisture and release it again. That is its strength in build-ups meant to dry out, and it is also its greatest weakness, because the same open structure cannot tolerate bulk water.

A soaked batt slumps, loses thickness and with it most of its thermal resistance, and it never recovers its original shape once dried. It therefore has to be protected from water both during construction and in the finished build-up: an uncovered roof or a leaking penetration can ruin an entire bay overnight, and the damage is invisible once the lining goes on. In closed build-ups a vapour barrier or vapour retarder with an appropriate resistance belongs on the warm side, with the resistance falling from inside to outside across the build-up.

How does mineral wool compare with the other common insulants?

Read across that table and the pattern is clear. There is no best insulant, only a best answer to a specific constraint: fire class, moisture exposure, acoustic performance, summer behaviour or price. A specification that names one product for every position in the building is almost always a specification that has not been thought about.

What does mineral wool do for acoustics?

Wool is a porous, open-structured material, so it damps sound even where it is not the main mass of the element. In a double-boarded plasterboard partition, filling the cavity with wool markedly improves airborne sound insulation, because it suppresses the resonance of the air trapped between the linings. In suspended ceilings and pitched roofs it does the same job, and it is one of the cheapest acoustic improvements available in a light structure.

Impact sound between dwellings works differently. There the result is decided by the resilient layer under the screed and by its rigorous separation from every wall and pipe. A high-density mineral wool board made for that position is the standard material, but it only performs if the screed nowhere touches the masonry and the perimeter strip stays continuous until the floor covering is laid.

What should be checked before mineral wool is specified?

Three things, and none of them is the price per cubic metre. First, the declared conductivity for the specific product and thickness, because the range within the family is wide enough to change a U-value calculation. Second, the mechanical property that the position demands, whether that is compressive strength, tensile strength perpendicular to the face, or stability under a heated screed. Third, the protection the material will get from water between delivery and completion, since that is where most of the performance is actually lost.