Sound insulation is the most common technical doubt families raise about a timber house. The doubt comes from experience: in a house built from a lightweight timber frame, or in an old timber house, the neighbours or the floor above are often audible. The experience is real, but the conclusion is not, because solid timber and a lightweight frame are two different things acoustically.

In an old timber house the floor is usually thin and has no resilient layer, so footsteps carry straight down, and in a lightweight frame the walls have little mass while the cavities carry sound onwards. A modern solid timber house works in a third way, and the eight points below decide how quiet it turns out: mass, impact noise, the build-up of floors and internal walls, junctions, services noise and the moment these decisions have to be made.

1. Mass is the basis

The simplest rule for damping airborne noise is mass: the heavier the structure, the less sound passes through it. This is where solid timber and a lightweight frame differ most. Cross-laminated timber, or CLT, is a dense timber panel whose mass per square metre is several times that of a frame of thin studs and boards.

That does not make a CLT wall quiet automatically, but it gives a strong starting point on which the remaining layers are built. The properties of the material are described in our article on cross-laminated timber, and how solid timber differs from other ways of building in our comparison of a prefab, modular or CLT house.

2. Airborne and impact noise are two problems

The two are constantly confused, but the solutions differ. Airborne noise is speech, music and television travelling through the air, and mass and airtightness work against it. Impact noise travels through the structure: footsteps, dropped objects, dragged chairs, toys on the floor.

Mass alone does not help against impact noise, because the sound moves through the structure rather than the air. What helps is breaking that path with a resilient layer that stops impact energy carrying onwards. This is where it is decided whether footsteps upstairs are heard below.

3. The floor build-up decides most

If one thing has to be right, it is the floor. The typical good solution is a floating floor: on the CLT deck a resilient layer, on that a load-spreading layer or screed, and only then the floor covering. The resilient layer breaks the sound bridge and keeps the floor separated from the structure; this is the most effective single sound insulation measure against impact noise.

Often a mass layer is added as well, sand or gravel for instance. From below, a decoupled ceiling on resilient hangers or independent battens helps. When the build-up is worked out in the design the result is good; when it is not, no floor covering will fix it.

4. Internal walls and bedroom noise

For internal walls the aim is different: noise between rooms. Three things work together here. First, mass: a heavier wall gives better sound insulation. Second, layers: two boards damp better than one. Third, decoupling: when the two faces of a wall are not rigidly connected, sound does not pass straight across.

In practice this is decided at layout stage: a bedroom is better not placed next to the living room or the technical room where that can be avoided. The relationship between layout and noise is described in our article on the house floor plan.

5. Junctions, holes and doors

The weakest point is usually not the wall but a hole in it. A socket placed back to back on both sides of a wall carries sound straight through, and an unsealed penetration does the same. The biggest hole is the door: an ordinary internal door damps very little, so a bedroom door can be the weakest link in the whole wall.

The remedies are simple and cheap when they are in the design: sockets are offset, penetrations sealed, and the door given a heavier leaf and a sealed threshold. Done later they are awkward, although a door is one of the few parts still replaceable in a finished house.

6. Services noise: ventilation, heating and pipes

The most common source of noise in a new house is not the neighbour but the house’s own services. A ventilation unit fixed rigidly to a wall carries vibration into the structure. Rigidly fixed pipework carries the sound of running water. The indoor unit of a heat pump behind a bedroom wall is audible.

All three are sound insulation decisions solved in the design: the position of the unit in the technical room, resilient fixings, silencers on the ducts and insulation of the pipework. The ventilation solutions are described in our article on ventilation in a detached house.

7. External noise and windows

When a house stands beside a road, the noise comes from outside. Here the weakest link is the window rather than the wall, because a CLT wall with its insulation and facade damps external noise well. For windows the sound insulation is decided by the glazing build-up and the seals; using panes of different thickness in the unit helps against noise.

The second factor is the ventilation air intake: if fresh air is drawn from the road side, noise comes with it, so its position is decided together with the position of the house. Assessing a plot and its noise level is covered in our article on buying a plot for a house.

8. When sound insulation is decided

Everything above is in the structure, not in the finishes. The floor build-up, the layers of internal walls, the positions of sockets, the fixing of pipework and the silencers on ducts belong to the structural and services design. Once the concrete is poured and the walls closed, those changes are expensive or impossible.

The practical conclusion is the same as for most technical decisions in a factory-built house: decide before production. The order of decisions is described in our article on house construction stages.

What to ask the designer

Four questions give clarity. First: what is the floor build-up, and does it include a floating floor on a resilient layer? Second: what are the layers of the internal walls beside the bedrooms? Third: how are the services fixed, and do the ventilation ducts have silencers? Fourth: have the positions of sockets and penetrations been thought through for noise?

When the answers are specific, the sound insulation has been designed; when the answer is “CLT is solid, there are no problems with it”, that is an assumption rather than a solution. It is also worth asking a builder’s earlier clients how their houses sound in daily use, and our article on builder reviews and references explains how to reach them.

Summary

Mass gives a CLT house a better starting point than a lightweight frame, but the quiet is decided by the floor build-up, the layers of internal walls, the sealing of junctions, the fixing of the services and the windows. If you would like to know which sound insulation solution suits your house, write to us through the contact page and we will go through the layout with you.