Getting a partition to 47 dB without doubling the stud

Rw 47 dB is the usual target for a partition between an apartment and a common corridor. A standard single-layer CW 75 partition with wool gets close, and then people start adding layers. Usually that is the expensive way round.

The three levers

Sound through a partition is governed by mass, by decoupling of the two faces, and by leaks. Adding a second board addresses only the first, and it adds cost, weight and thickness.

Mass

A second layer of 12.5 mm board buys roughly 3–4 dB. A single layer of a denser board buys much of the same for less build-up: Knauf Diamant is around 1000 kg/m³ against about 680 for standard board, worth roughly 3 dB on its own while staying a single-layer wall.

Decoupling

Staggering the studs so no single stud touches both faces is the biggest single improvement available, and it costs one extra track. A staggered CW 75 partition with wool will beat a straight double-boarded one, at less thickness.

Sealing

This is the one that decides whether the wall performs as calculated. An unsealed 2 mm gap at the head track can cost more than the second board gained. Perimeter sealant, sealed service penetrations and no back-to-back sockets in the same bay are free or nearly free, and they are the difference between a wall that tests at 47 and one that tests at 41 with exactly the same materials.

A build that gets there

• CW 75 studs at 600 mm, staggered on a 100 mm track
• 50 mm Rockwool Rockmin Plus in the cavity — the cavity wants damping, not filling
• One layer of 12.5 mm board each face, Diamant where the figure is tight
• Perimeter sealed on both faces before jointing
• Sockets offset by at least one bay

That is about 42 PLN net per square metre in materials at base tier, which we price out line by line on the home page. The second board would add roughly 15 and about 3 dB — worth it only after the sealing and the staggering are already done.