Does insulation in a stud wall actually reduce noise?

An empty stud wall is two rigid panels with an air gap, which is acoustically a very effective drum: the cavity resonates and couples the two faces together. Filling it with an open, fibrous, absorbent material breaks that resonance up. That is a genuine and worthwhile improvement, and it is also much smaller than the marketing around acoustic products suggests, because the studs still connect the two faces mechanically.

Step by step

1. Understand what insulation does acoustically.It absorbs sound energy within the cavity and damps the air resonance between the two board faces. It is not a barrier — sound passes through mineral wool easily. It is a damper.
2. Use an open fibrous material.Mineral wool — rock or glass — is the standard. Denser acoustic-grade batts perform somewhat better than lightweight loft roll, and the difference is real but not dramatic. Rigid foam boards are poor acoustically because they are closed-cell and stiff.
3. Fill the cavity, but do not compress it.Friction-fitted so it fills the depth without being squashed. Compressed wool loses much of its absorption, and gaps let sound straight past. Full and loose is the target.
4. Do not leave gaps at the edges or around services.Cut around cables and boxes neatly. An unfilled corner is a hole in the treatment.
5. Add mass at the same time.Two layers of 12.5mm board, or a single layer of dense acoustic board, on each face does more than the insulation. The two together do considerably more than either alone.
6. Deal with the mechanical connection if you want a real change.The studs conduct vibration directly from one face to the other. Resilient bars, a staggered stud arrangement, or two independent frames are what break that path, and they are the difference between a modest improvement and a big one.
7. Seal the perimeter.A continuous bead of acoustic sealant where the board meets the floor, ceiling and adjoining walls, and around every penetration. Air paths defeat everything else, and this step costs almost nothing.
8. Set expectations honestly.Filling an empty partition takes it from poor to reasonable — you will notice it. It will not make a bedroom wall behave like a party wall, and it will do nothing about footsteps.

Tips

• It is essentially impossible to add insulation to a boarded wall without taking one face off, which is why doing it at the framing stage is the moment that matters.
• If a wall is being built between two bedrooms, or between a bedroom and a bathroom, insulating it costs very little relative to the whole job and is one of the few decisions you cannot easily revisit.
• The same principle applies between floors: mineral wool between the joists improves both airborne and impact performance as part of a system.
• Acoustic mineral wool and thermal mineral wool are largely the same material at different densities. On an internal partition the thermal rating is irrelevant; the density and the fit are what matter.

Common mistakes

• Expecting insulation alone to solve a noise problem — It damps a cavity. It is not a barrier, and used alone the improvement is real but small.
• Using rigid foam board — Closed-cell and stiff, so it absorbs very little and can even couple the two faces together. Fibrous and open is what you want.
• Compressing the wool to fit — Compression reduces absorption significantly. Cut it slightly oversize and let it friction-fit without being squashed.
• Insulating but not sealing — A gap under the skirting or around a socket lets sound straight through, past everything you have just paid for.

Questions people ask