Not all mineral wool is the same. Standard thermal insulation — the kind typically used to fill a stud wall cavity in a domestic build — has a density of around 15–25kg/m³. At this density, it provides good thermal resistance and some limited acoustic benefit. But for meaningful acoustic performance — the kind required for Part E compliance in a separating wall or floor, or for achieving Rw 45dB or above in a commercial partition — it is largely ineffective. The right grade of acoustic mineral wool starts at around 45kg/m³, and high-performance acoustic applications call for 60–100kg/m³.
The reason density matters is that acoustic absorption and transmission loss in a cavity are driven by the flow resistivity of the fill material — its resistance to air movement through its structure. Low-density materials have low flow resistivity: sound energy passes through the cavity with limited attenuation. High-density acoustic mineral wool has much higher flow resistivity, and significantly reduces the sound energy that passes through the cavity, contributing meaningfully to the overall assembly performance.
How acoustic mineral wool works
In a stud wall, floor, or ceiling assembly, the acoustic fill in the cavity contributes to the assembly's airborne sound performance through two mechanisms. First, it absorbs sound energy that crosses the cavity — reducing the energy available to drive the receiving surface. Second, it damps the resonance of the cavity itself: an empty cavity resonates at frequencies governed by its dimensions and can amplify sound transmission at those frequencies; fill suppresses this.
The practical outcome is that replacing standard thermal fill with acoustic-grade mineral wool in an otherwise identical assembly typically delivers an additional 4–8dB Rw improvement. In assemblies designed to meet Part E, this is the difference between specifying the right amount of additional mass and decoupling, and having to add more to compensate.
Acoustic mineral wool is used in combination with acoustic plasterboard and resilient clips or channels in high-performance wall and ceiling assemblies. In floor systems, it is used alongside acoustic membranes and acoustic floor screed.
Typical use cases
- Separating stud walls in new builds and conversions — acoustic mineral wool fill is a standard component in Part E compliant timber-frame and metal-stud separating walls; standard thermal fill does not achieve the same performance
- Floor voids in timber joist construction — acoustic mineral wool between joists contributes to airborne sound performance in separating floors; the combination of mass layers, resilient clips, and acoustic fill is the standard approach in Part E floor assemblies
- Ceiling voids — in ceiling builds where a resilient clip or channel system is used, acoustic fill in the void above the ceiling board adds absorption and suppresses cavity resonance
- Commercial acoustic partitions — walls designed for speech privacy (Rw 45dB and above) require acoustic-grade fill; standard thermal insulation in the same partition typically underperforms by 5–8dB Rw
- Upgrading existing walls and ceilings — where access is being made for rewiring, replumbing, or replastering, acoustic mineral wool can be added to improve performance at low additional cost
Technical notes
Acoustic mineral wool density grades are typically described by kg/m³: 45kg/m³ for entry-level acoustic applications, 60kg/m³ for standard acoustic partitions, and 80–100kg/m³ for demanding applications. Published performance data (Rw, DnT,w + Ctr for assemblies containing acoustic mineral wool fill) is available on request for specification purposes. For Part E compliance, the complete assembly — including fill grade — must be assessed against the required minimum performance values. BS 8233 provides guidance targets for non-residential spaces. We can advise on the correct fill grade and density for your assembly and performance target.
What's the difference between acoustic mineral wool and thermal insulation?
Thermal insulation (standard mineral wool, glass wool, or foam insulation products) is designed to resist heat transfer. Low density (15–25kg/m³) is sufficient for this purpose — the thermal resistance (R-value) of mineral wool depends primarily on its thickness and fibre structure, not its density.
Acoustic mineral wool is a specific product category manufactured to a higher density (45–100kg/m³) to achieve the flow resistivity required for meaningful acoustic cavity performance. It is physically heavier and denser than thermal insulation of the same thickness. Some products carry both thermal and acoustic performance data — but a product sold as thermal insulation, even if it has a published acoustic figure, will typically underperform acoustic-specific grades in the same cavity at the same thickness.
The practical rule: if the wall, floor, or ceiling assembly is required to meet an acoustic performance standard — Part E, BS 8233, or a specified Rw value — specify acoustic mineral wool by grade (kg/m³), not by brand or generic "acoustic insulation" description. If you are unsure which grade is right for your project, describe your assembly and we'll advise on the correct specification.