Sound insulation in timber construction: why the concrete approach does not work

Three measures that do not have the expected effect on timber

A thin underlay does not achieve its stated value on timber. An elastic underlay that delivers an 18 dB impact sound improvement on a stiff concrete floor often achieves no more than 8 to 10 dB on a timber joist floor. The joist floor flexes along with it, so the underlay decouples less effectively. Acoustic product values on the packaging are usually measured on concrete and cannot simply be used as design values on timber.

A lining wall on the timber frame is counterproductive when it is fixed to the structure. In concrete construction a lining wall is placed on free-standing battening, decoupled from the structure. In timber frame construction the battening is often fixed directly to the timber studs. Timber transmits vibrations very efficiently, so the lining wall no longer forms a separate acoustic layer and sometimes even acts as an amplifier. Measured effect: an improvement of 3 dB instead of the intended 12 to 15 dB.

A ceiling suspended from the joists does not solve the problem either. A suspended ceiling that hangs from the timber joist floor moves with the floor above it and passes the vibrations on instead of damping them. Every suspension point forms a sound bridge.

What does work in timber construction

Adding mass is the most effective measure, because it compensates for the difference in mass compared with concrete. A dry-laid screed of cement-bonded fibreboard, 2 to 3 layers and 40 to 60 kg per square metre, on a resilient intermediate layer delivers an 8 to 14 dB impact sound improvement on a timber joist floor. In historic joist floors a sand bed of 80 to 120 kg per square metre between the joists can double the mass and improve the sound insulation by 6 to 10 dB, provided the joist floor can carry this weight. Besides sand, loam or rammed earth is also used, with comparable acoustic benefits and additional thermal and moisture-regulating properties.

Decoupling the floor from the load-bearing structure is the second route. In CLT floors, resilient bearings such as elastomers or Sylomer strips work well at the contact points between floor and wall. The floor then rests on the bearing instead of directly on the wall. Effect: a 4 to 8 dB reduction in the flanking transmission. This has to be designed in from the start, not added later during construction.

Making the ceiling independently supported is the measure that delivers the most per euro. A ceiling on its own battening, decoupled from the joist floor above it, with mineral wool in the cavity and without a single suspension point to the joists, gives an 8 to 15 dB impact sound improvement. The biggest gain is at low frequencies, where timber construction is most vulnerable. This is usually the measure with the highest return per euro invested.

Finally, the flanking transmission has to be limited. In timber frame construction sound travels through the structure via the wall-floor junctions. Decoupling at the separating construction, with elastic strips and interrupted battening, limits this path. Without that measure even the best floor will not achieve its calculated value, because the sound travels around it.