What is the appropriate thickness of acoustic insulation?

A wall that lets conversations through, a floor that transmits every footstep or a ceiling through which the neighbours' television can be heard cannot be remedied by choosing the thickest material at random. The right question is: what thickness of acoustic insulation is appropriate for the noise you wish to reduce, for the existing structure and for the space available? The answer may be 5 mm, 30 mm, 50 mm or more, but actual performance is determined by the system, not merely by the thickness printed on the packaging.

In a well-planned renovation, sound insulation must work together with the mass of the wall, the decoupling of building elements, the sealing of joints and the control of vibrations. This approach provides certain and predictable performance without needlessly sacrificing valuable square metres of the dwelling.

What thickness of acoustic insulation is required for each application?

The required thickness differs considerably between walls, ceilings, floors and building services. A thin material may be highly effective for damping vibrations beneath parquet, yet insufficient if the aim is to block voices through a lightweight wall. Likewise, thick wool installed without correct sealing of the joints may lose a significant part of its benefit.

For lightweight interior walls built from metal profiles and plasterboard, a 50 mm layer of fibrous material is frequently the starting point. In partitions where superior comfort is desired, a thickness of 75-100 mm allows the cavity to be filled more completely and reduces resonance inside the wall. If the structure has 50 mm profiles, choosing a material matched exactly to that cavity is more effective than compressing wool that is too thick.

For suspended ceilings, the usual thicknesses are 50-100 mm, depending on the available height and the nature of the noise. A ceiling lowered by only a few centimetres may improve absorption in the cavity, but insulation from the neighbour above becomes significantly better when the ceiling is elastically decoupled from the floor slab and completed with dense boards.

Under floors, the thickness is normally smaller. Layers of 2-5 mm are used to damp impact noise beneath laminate parquet or floating floors. For floating screeds, where the transmission of footsteps, dropped objects or vibrations to the level below must be limited, materials of 5-20 mm designed to bear the load without excessive settlement are frequently used.

Thickness alone does not stop noise

Acoustic insulation has two distinct objectives. The first is the reduction of airborne noise: voices, music, traffic, television or barking. The second is the limitation of impact noise and vibrations: footsteps, chairs being dragged, washing machines, pipes or HVAC installations. These are treated differently.

Airborne noise requires mass, absorption and airtightness. In a plasterboard wall, mineral or ecological wool installed in the cavity absorbs sound energy and reduces the resonance-box effect. The lining boards add mass, while sealing tapes and acoustic sealant prevent sound from passing through the joints. A gap of a few millimetres around an electrical box or at the junction with the ceiling may considerably diminish the effect of a well-chosen system.

Impact noise requires decoupling. If the parquet, the screed or a rigid structure is in direct contact with the floor slab, the vibration is transmitted into the building. A thin elastic underlay, but with the correct damping properties and compressive strength, may play a more important role than a thick layer of unsuitable material. The installation of a perimeter strip, which separates the screed or the floor from the walls, is also of considerable importance here.

Thin membranes also have a precise role. Some products a few millimetres thick may contribute to reducing vibrations or may complement a multilayer system. Nevertheless, they must not be mistaken for a universal solution for very noisy party walls. A thin membrane cannot replace a correctly filled cavity, dense lining and decoupling when the problem is serious.

Party walls: when 50 mm is sufficient and when you need more

For a new partition wall between a bedroom and a living room, 50 mm of sound-absorbing insulation in the cavity is a practical and effective choice. If the wall is built with 75 mm or 100 mm profiles, completely filling the cavity gives better results, particularly in rooms where privacy is essential.

The situation changes with an existing party wall, especially in a flat. If conversations can be heard clearly, the problem is not merely the absence of an absorbent material, but the mass and the acoustic bridges of the wall. A system applied on the inside normally requires an independent or decoupled frame, a cavity of at least 50 mm filled with sound-absorbing material and one or two lining boards. In total, the solution may take up 70-120 mm of the room.

This is a genuine trade-off: you lose a little space, but you gain comfort and a clearer acoustic separation between rooms. For a limited intervention, where every centimetre counts, a thinner system can be designed. The result will be useful, but it must be assessed realistically. Low-frequency noise, music with bass or structural vibrations are the most difficult to reduce in a confined space.

Floors and ceilings: the thickness is chosen according to the structure

For laminate floors, an acoustic underlay or layer of 2-3 mm is often sufficient to reduce the walking noise perceived in the room and transmitted to the level below. If the floor is installed over underfloor heating, the material must also be chosen according to its thermal resistance. A layer that insulates too much thermally may reduce the efficiency of the heating system, even if it damps footsteps well.

For a floating screed, the thickness of 5-20 mm is determined according to the screed load, the type of finish and the intended level of attenuation. The material must remain stable over time. A solution that settles produces cracks, hollow-sounding areas and rigid bridges to the walls.

For ceilings, 50 mm is a good basis for many renovation projects, but the system must be checked as an assembly. If the suspended ceiling is rigidly fixed to the floor slab, part of the vibration may bypass the absorbent layer. Anti-vibration hangers, the perimeter sealing strip and two-layer lining may sometimes bring a more relevant gain than simply increasing the wool from 50 to 100 mm.

How to choose the correct thickness before installation

Before ordering, first identify the dominant source of noise. Voices from the neighbours, footsteps from above, echo in a room and the noise of a pipe are not the same problem. Then check the structure: solid or lightweight wall, concrete or timber floor slab, available space, existing building services and moisture requirements.

A habitable attic project requires particular attention. The insulation between the rafters may also contribute to acoustic comfort, but it simultaneously has a thermal role and must be protected by correct vapour control. An intelligent choice combines the thickness required for energy efficiency with materials that absorb sound, condensation control membranes and careful sealing at the joints. In this way, the structure remains dry and the room becomes quieter, both during rain and with regard to outdoor noise.

For predictable results, ask for details of thickness, density, compressive behaviour, recommended use and installation method. At Isoflect, the choice can be analysed as a system for the wall, roof space, habitable attic, floor or building services, rather than as a single roll purchased according to thickness.

The best thickness is the one that resolves the actual noise without creating other problems of installation, moisture or space. Measure the cavity, establish the desired level of comfort and build each layer with the same care: the right material, decoupling where necessary and complete sealing at the edges.