Sound insulation · Stone wool
ROCKWOOL Safe 'n' Silent Pro
- Thickness
- 25–100 mm
- Density
- 40–100 kg/m³
- Sound absorption (NRC)
- 0.90–1.00
Why does insulation in the wall cavity improve sound insulation?
In short
A drywall or double-leaf wall has an air cavity in the middle. Sound that gets through the first board bounces around the cavity and drives the board on the other side. Glass wool or stone wool in the cavity absorbs part of that energy, so less sound reaches the far side. It works fully only when the wall system is right — the two faces are not rigidly connected, there is enough mass, and there are no leaks. That is why an STC or Rw belongs to the whole tested wall, not to the insulation.
What happens inside the cavity?
A lightweight wall has boards on both sides (e.g. gypsum board or fibre-cement board), framing, and an air cavity. Sound passes through it in two main ways:
- Through the air in the cavity — the first board vibrates, the trapped air acts like a spring that pushes on the second board, and sound can also bounce back and forth inside the cavity
- Through the framing — if both boards are fixed rigidly to the same studs, vibration travels straight through the studs to the other side
Cavity insulation reduces the first path. The second has to be dealt with by the framing design: double studs, staggered studs, or resilient channels.
How much does insulation help?
Research by the National Research Council Canada (NRC), which tested more than 250 gypsum board walls, found that:
- When the two faces are rigidly connected, cavity insulation does very little
- When the two faces are properly isolated, adding insulation to the cavity raised the STC by about 10 points in the walls tested; the exact figure varies with the construction
- The type of insulation had a relatively minor effect on STC compared with isolation, board mass and cavity depth
- Insulation covering the whole face of the cavity worked better than partly filling it with the same amount of material
The SCG Cylence ZoundBlock installation guide shows a wall of one layer of 8 mm SCG SmartBoard on each side of C64/U66 steel framing (82 mm overall) at STC 41 without insulation and STC 48 with ZoundBlock S050 in the cavity. These figures belong to that wall only; change the boards, framing or cavity and the result changes. The guide does not say whether they are laboratory test results or calculated values.
Why is it a system result?
A wall's sound insulation depends on several things at once:
- Board mass — number of layers and weight per square metre
- Decoupling — how rigidly the two faces are connected
- Cavity depth — deeper cavities usually help more
- Cavity insulation — filling the cavity properly
- Leaks and flanking — outlets, joints, the space above the ceiling
Insulation is only one of these five. If another is weak — an opening, or sound going round via the ceiling — adding insulation helps only a little. See Sound transmission and flanking.
Insulation has no STC of its own
An STC or Rw shown next to an insulation product is the rating of a wall system described in the document. To claim that figure on a project, build the wall to those details. More in What are STC and Rw.
Choosing cavity insulation
- Choose a thickness that suits the cavity, so it fills the whole face without being packed too tightly
- Check whether the manufacturer has wall-system documents using that product, and whether the details match your wall
- Consider related points too, such as fire behaviour (see the product's documents) and ease of installation
- Both stone wool and glass wool are used in wall cavities; see Stone wool vs glass wool
Before installing, read What to know before insulating a drywall partition.
See solutions: Noise from next door / neighbours · Machine noise
Sources
- National Research Council Canada — Construction Technology Update No. 1, Control of Sound Transmission through Gypsum Board Walls (Warnock & Quirt, 1997)
- SCG — SCG Cylence ZoundBlock installation guide (STC table for example walls)
- ROCKWOOL (Thailand) — Safe 'n' Silent Pro data sheet
- ASTM E90 / ASTM E413 and ISO 10140-2 / ISO 717-1 — testing and rating the sound insulation of walls
Related products
Categories and example products for this topic
- ROCKWOOL Safe 'n' Silent Pro — Stone wool for wall cavities, NRC 0.90–1.00 at 50 mm (absorption, not an STC rating)
- SCG Cylence ZoundBlock — 32 kg/m³ glass wool in a moisture-barrier wrap, 50 or 100 mm to suit the framing
- ISO NOISE Standard — Glass wool batt 0.60 × 1.20 m, 50 mm, in a clear PE bag - fits the stud cavity in the bag
- ISO NOISE Premium — 100% polyester batt, 65 mm - an option for projects that prefer not to use glass wool
Sound insulation · Glass wool
SCG Cylence ZoundBlock
- Thickness
- 50–100 mm
- Density
- 32 kg/m³
- Thermal conductivity (λ)
- ≤ 0.033 W/m·K
