The acoustic insulation inside a plasterboard partition cavity is one of the least visible and most consequential decisions in office fitout design. Once the wall is closed and painted, nobody can see what is inside it. But what is inside it determines whether the room feels private or exposed, whether conversations carry through to the next room, and whether the wall delivers the acoustic rating it was specified to achieve.
The choice of insulation product is not straightforward because different materials perform differently across frequency ranges, respond differently to compression, interact differently with moisture, and have different fire classifications. Specifying “insulation” on a drawing without naming a product, a density, and a thickness leaves the installer to make a choice that may not align with the acoustic intent of the design. And substituting one insulation type for another during construction, which happens frequently when the specified product is not available on the delivery date, can change the wall’s acoustic performance by enough to make a noticeable difference in the occupied space.
This article compares the main insulation types used inside plasterboard office partitions, explains what each one actually achieves acoustically, and identifies where the specification matters most and where it matters least.
Glass Wool: The Industry Standard
Glass wool is the most widely used insulation in commercial plasterboard partitions. It is manufactured from recycled glass and sand, spun into fine fibres, and formed into batts or rolls of varying densities and thicknesses. It is lightweight, easy to handle, widely available, and cost-effective. For the majority of office partition applications, glass wool provides adequate acoustic performance and represents good value.
Acoustically, glass wool works by absorbing airborne sound energy as it passes through the fibrous matrix. The fibres convert sound energy into heat through friction, reducing the amount of sound that reaches the opposite face of the wall. The absorption is most effective at mid to high frequencies, which is where most speech energy sits, making glass wool a practical choice for partitions intended to provide speech privacy.
Standard glass wool batts for acoustic applications are typically specified at densities of 11 to 14 kilograms per cubic metre, in thicknesses of 50mm to 75mm to match the stud cavity depth. At these densities, the batts are soft and compressible, which means they need to be installed carefully to avoid gaps at the edges and compression in the centre. A batt that is compressed to fit a slightly narrow cavity loses some of its acoustic effectiveness because the compressed material has less air space for sound absorption.
Rock Wool: Higher Density, Broader Frequency Performance
Rock wool, also known as stone wool or mineral wool, is manufactured from volcanic rock or blast furnace slag, melted and spun into fibres in a process similar to glass wool production. The resulting product is denser, stiffer, and heavier than glass wool, with typical densities of 40 to 80 kilograms per cubic metre for acoustic-grade products. This higher density gives rock wool several acoustic and practical advantages over glass wool in partition applications.
Acoustically, the higher density of rock wool provides better absorption at lower frequencies than glass wool of the same thickness. Low-frequency sound, such as bass from music, rumble from mechanical services, and the lower registers of male voices, is harder to absorb because the sound waves are longer and carry more energy. The denser fibre structure of rock wool is more effective at converting this low-frequency energy into heat, which means a partition insulated with rock wool will provide more consistent performance across the full frequency range.
The stiffness of rock wool batts also makes them easier to install without gaps or compression. The batts hold their shape during handling and friction-fit into the stud cavity without sagging or slumping. This practical advantage translates into more consistent acoustic performance because the insulation fills the cavity more uniformly, with fewer gaps at the edges and fewer areas of compression in the centre.
Polyester Insulation: The Non-Irritant Alternative
Polyester insulation is manufactured from recycled PET fibres and is used in some office fitouts as an alternative to glass wool and rock wool. Its primary advantage is that it does not contain any mineral fibres that can cause skin or respiratory irritation during handling. This makes it a preferred choice in fitouts where the insulation is being installed in an occupied or partially occupied space, or where the building owner has a preference for non-irritant products.
Acoustically, polyester insulation performs adequately at mid to high frequencies but is less effective than glass wool and significantly less effective than rock wool at lower frequencies. The fibres are softer and less dense, which means they absorb less energy per unit thickness. For partitions where the acoustic requirement is moderate, such as general office walls and non-confidential meeting rooms, polyester may provide sufficient performance. For rooms requiring high acoustic isolation, such as boardrooms or quiet meeting rooms, the lower performance at low frequencies makes polyester a poor substitute for rock wool.
Polyester insulation is also more expensive per square metre than glass wool and rock wool, which means the cost saving from avoiding mineral fibres comes with a performance compromise and a price premium. For fitouts where acoustic performance is a priority, the trade-off is rarely justified.
Why Density Matters More Than Thickness
Tenants and designers sometimes assume that thicker insulation means better acoustic performance. A 100mm batt must be better than a 50mm batt, the reasoning goes. But the relationship between thickness and performance is not linear, and in many cases the density of the insulation is a more important variable than its thickness.
A 50mm batt of rock wool at 60 kilograms per cubic metre will outperform a 75mm batt of glass wool at 11 kilograms per cubic metre across most of the frequency range. The denser product absorbs more energy per unit of thickness because the fibre structure is tighter and the air pockets within the material are smaller. Increasing the thickness of a low-density product adds some performance, but the gains diminish quickly beyond 50 to 75mm because the additional thickness is filled with the same sparse fibre structure that was already losing effectiveness.
This is why the specification should name both the density and the thickness, not just the thickness. A drawing that specifies “75mm acoustic insulation” without naming the product or the density leaves the installer free to use the cheapest available product, which may be a lightweight glass wool batt that does not deliver the performance the designer intended.
How Insulation Choice Interacts With Framing Type
The interaction between insulation choice and framing type is where the acoustic specification becomes a system decision rather than a component decision. In a single-stud partition, the insulation in the cavity absorbs some of the airborne sound crossing from one face to the other, but the structural bridge through the studs is the dominant transmission path. Upgrading the insulation from glass wool to rock wool in a single-stud wall will improve performance, but the improvement is limited because the stud bridge is still transmitting vibration directly between the two faces.
In a double-stud partition, where the structural bridge is eliminated, the cavity insulation becomes the primary acoustic mechanism. The sound must cross the air gap between the two stud rows, and the insulation absorbs a significant portion of the energy during that crossing. In this configuration, the choice between glass wool and rock wool makes a larger difference because the insulation is doing more of the acoustic work. Upgrading from glass wool to rock wool in a double-stud partition can improve the overall wall performance by 2 to 4 decibels, which is a meaningful improvement in a system that is already performing at a high level.
Fire Classification and Compliance Implications
Insulation products used in fire-rated plasterboard partitions must comply with the fire classification requirements of the tested wall system. Glass wool and rock wool are both non-combustible materials that achieve Group 1 fire classification, making them suitable for use in fire-rated construction without restriction. Polyester insulation, depending on the product, may achieve Group 2 or Group 3 classification, which may limit its use in fire-rated partitions depending on the tested system requirements.
The fire classification matters because a plasterboard partition that is specified as fire-rated must use exactly the insulation product that was part of the tested system. Substituting a different insulation type, even one that has a similar acoustic profile, invalidates the fire rating of the wall unless the substitution is supported by a separate test report or an engineering assessment. This is a compliance requirement that is frequently overlooked during construction, particularly when the specified product is not available and a substitute is used without checking the fire rating implications.
Making the Right Specification for Each Room
The insulation specification should be matched to the acoustic requirement of each room, which means different rooms in the same office may appropriately use different insulation products. A general open-plan boundary wall may perform adequately with standard glass wool. A meeting room requiring moderate privacy may benefit from higher-density glass wool or standard rock wool. A boardroom or executive office requiring confidential-level privacy should use high-density rock wool in combination with double-stud framing and two layers of plasterboard on each face.
This room-by-room approach is more cost-effective than applying the highest specification uniformly, and it avoids the opposite problem of applying the lowest specification uniformly and discovering after occupation that key rooms do not perform as needed. The classification of rooms by acoustic requirement should happen during design, and the insulation specification should follow directly from that classification.
If you are specifying plasterboard partitions and want to make sure the insulation choice matches the performance you need, we can help. We build partition systems to tested acoustic standards across complete fitouts and standalone partition projects.