Rock wool is made from basalt melted at high temperature and spun into fibre, with a binder and a dust-repellent oil added. The standard board for partition fill is 600 × 1200 mm in 50 mm thickness, at 80 or 100 kg/m3. The open structure gives high absorption across the mid and low-mid range, and the density helps in a shallow cavity where the material has to do its work in a short distance.
Glass wool is made from quartz sand, limestone and dolomite melted with soda ash and borax, then blown or spun into fine fibres that interlace into a porous mat. Boards are 600 × 1200 mm in 30 or 50 mm, at 48 or 64 kg/m3. It is lighter and, per unit weight, a very effective absorber at mid and high frequency.
The practical difference in a partition is at the low end. A denser, heavier fill resists the low-frequency movement of the cavity better, which is why rock wool at 80–100 kg/m3 is the usual choice behind a 122 mm acoustic wall. Glass wool is the sensible choice where the wall is thinner and the target is speech privacy rather than music isolation.
Two installation rules apply to both. The fill must fill the cavity — a compressed batt or a gap at the top is an acoustic hole — and it must not bridge between the two faces, because a compressed bridge transmits vibration. Where there is an air layer behind the wool, as in a retrofit, absorption at low frequency improves as the air layer gets thicker, and layering different densities increases it again.