Selecting an insulation product for acoustic purposes involves different criteria than selecting for thermal performance. A material with an excellent R-value per inch may have a lower Noise Reduction Coefficient than a denser product at the same thickness. This overview covers the most commonly specified acoustic insulation products in Canadian residential construction, with particular attention to how their physical properties affect sound absorption and transmission loss.
What to Measure: NRC vs. STC Contribution
Two metrics describe insulation performance in acoustic applications:
- Noise Reduction Coefficient (NRC): A single-number average of a material's sound absorption at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. Relevant when insulation is exposed (inside a room as a finish material), or when calculating cavity absorption in recording studios and listening rooms. A higher NRC means more of the incident sound energy is absorbed rather than reflected.
- Contribution to assembly STC: When insulation is inside a wall or floor cavity, its role is to damp standing waves and add transmission loss. Tests show that filling a wall cavity with acoustic-grade mineral wool typically adds 5–8 STC points to the assembly compared to leaving it empty — regardless of the material's NRC when used as a room absorber.
For most interior partition projects, the relevant question is not the NRC of the insulation alone, but the published STC of the complete assembly it is part of.
Mineral Wool (Rockwool / Stone Wool)
Mineral wool, manufactured from blast furnace slag or volcanic rock (basalt), is widely regarded as the best general-purpose acoustic insulation for wall and ceiling cavities. Its density — typically 32–80 kg/m³ for acoustic-grade batts — is significantly higher than standard fiberglass batts at comparable thicknesses, which translates directly to better sound damping in the mid-frequency range.
Key Properties
- Density: 32–80 kg/m³ (acoustic batts); compare with fiberglass at 8–12 kg/m³ for standard thermal batts.
- NRC: 0.90–1.05 at 50mm thickness (varies by density and frequency).
- Fire resistance: Non-combustible; melting point above 1000°C. Meets CAN/ULC-S102 requirements without additional treatment.
- Moisture resistance: Hydrophobic surface treatment; will not absorb moisture or support mould growth.
- R-value: Approximately R-3.7 to R-4.2 per inch — comparable to fiberglass for thermal purposes.
Acoustic-Grade vs. Thermal-Grade Products
Not all mineral wool products sold in Canada are optimised for acoustics. Thermal-grade mineral wool used in exterior wall and attic applications is often manufactured at lower densities (12–24 kg/m³) to reduce cost. For interior acoustic partitions, specify products marketed as "acoustic batts" — Rockwool Safe'n'Sound, Roxul AFB, and similar products from CGC Lafarge are the most widely stocked in Canada. These range from 32–48 kg/m³ and are designed to fit standard 89mm (2×4) and 140mm (2×6) stud bays.
Installed Cost in Canada (2025 Reference)
Mineral wool acoustic batts (e.g., Rockwool Safe'n'Sound 89mm): $0.95–$1.30/sq ft for materials; $1.80–$2.60/sq ft installed including labour for a basic wall installation. Prices vary by region and contractor.
Fiberglass Batts
Standard fiberglass batts — R-12, R-14, R-20 — are the most common wall insulation in Canadian residential construction. When installed in an interior partition, they do provide some acoustic benefit: an empty 89mm stud wall typically tests at STC 33–36, while the same wall with fiberglass batts reaches STC 38–42. However, the improvement is measurably lower than with mineral wool at the same thickness, primarily because fiberglass batts at standard thermal densities (8–12 kg/m³) have less resistive flow and absorb less mid-frequency energy.
High-density fiberglass products (32 kg/m³, such as Johns Manville 1000 Series or Owen Corning QuietZone) close the performance gap significantly and are used where fiberglass is preferred for installation flexibility or cost. These products typically add 6–7 STC points to a wall assembly compared to 5–8 for acoustic mineral wool of equivalent thickness — a modest difference in most residential applications.
The density of the insulation matters more than the material type. A dense fiberglass batt at 32 kg/m³ performs comparably to acoustic mineral wool in many assemblies. A standard R-12 batt at 10 kg/m³ is a thermal product that provides only incidental acoustic benefit.
Cellulose Insulation
Cellulose — finely shredded newsprint treated with boric acid for fire and pest resistance — is used primarily as blown-in insulation in attics and retrofit wall cavities. Its acoustic properties are often overlooked, but dense-pack cellulose (55–70 kg/m³ settled density) is one of the most effective acoustic insulation options for retrofit applications where batts cannot be easily installed.
Dense-pack cellulose is injected into closed wall cavities through 50mm holes drilled between studs, then packed to a density that prevents settling. The material's high density and irregular fibre structure give it excellent absorption across the frequency range. Studies by the National Research Council of Canada have shown that dense-pack cellulose in 89mm stud walls can produce assembly STC values comparable to mineral wool acoustic batts at similar installed costs.
Limitations
- Moisture: Cellulose is hygroscopic. In high-humidity applications or where vapour management is critical, mineral wool is generally preferred.
- Settling: Improperly installed blown cellulose can settle by 15–20% over time, leaving an empty zone at the top of the cavity that reduces acoustic performance.
- Fire rating: Cellulose requires a fire-rated covering (typically drywall) on both sides; it is not independently non-combustible like mineral wool.
Acoustic-Grade Drywall
Standard 12.7mm drywall contributes to STC primarily through mass. Acoustic-grade drywall products — sold as QuietRock (USG), SoundBreak (CertainTeed), and equivalent products from CGC Lafarge — achieve higher STC contributions through two mechanisms: greater mass (typically two layers of gypsum laminated together, or a viscoelastic polymer layer sandwiched between gypsum sheets) and constrained-layer damping.
Constrained-layer damping (CLD) works by sandwiching a viscoelastic material between two rigid layers. When the assembly flexes under sound pressure, the polymer layer shears and converts mechanical energy to heat. The result is substantially higher STC per unit of wall thickness compared to standard drywall.
Performance Reference
| Drywall Product | Thickness | Mass (kg/m²) | Approximate STC (single layer, on studs) |
|---|---|---|---|
| Standard gypsum board | 12.7mm | 9.8 | 33–36 |
| Standard gypsum board | 15.9mm | 12.2 | 35–38 |
| QuietRock 510 | 12.7mm | 11.7 | 45–52 |
| QuietRock 525 | 15.9mm | 13.8 | 52–60 |
| SoundBreak XP | 15.9mm | 13.5 | 50–56 |
These are single-layer values on a simple stud wall. When combined with insulation and a second drywall layer or used on a decoupled assembly, the STC values are substantially higher.
Cost Consideration
Acoustic drywall costs $3.50–$6.50/sq ft for materials in Canada (2025), compared to $0.40–$0.65/sq ft for standard 12.7mm board. The cost is justified where wall thickness cannot be increased and the target STC would otherwise require double drywall plus resilient channels. For most residential renovations, two layers of standard 12.7mm drywall with acoustic sealant at the perimeter delivers comparable performance at lower material cost.
Spray Polyurethane Foam (SPF) — Not an Acoustic Product
Open-cell spray polyurethane foam is sometimes marketed with acoustic claims, but its use as a primary acoustic insulation in interior partitions is limited. Open-cell SPF has an NRC of approximately 0.70–0.80, and when used to fill a wall cavity it does reduce sound transmission compared to an empty cavity. However, its density (8–12 kg/m³) is similar to standard fiberglass batts, and it provides no specific advantage over batts for interior acoustic applications. Closed-cell SPF has even lower NRC values and is not suitable for acoustic cavity fill.
Product Comparison Summary
| Product | Density (kg/m³) | NRC | Fire Rating | Best Application |
|---|---|---|---|---|
| Mineral wool acoustic batt | 32–48 | 0.90–1.05 | Non-combustible | New walls, ceilings, party walls |
| Dense-pack cellulose | 55–70 | 0.85–0.95 | Class 1 (with covering) | Retrofit into existing cavities |
| High-density fiberglass batt | 28–36 | 0.80–0.95 | Non-combustible | Interior partitions; lower cost |
| Standard fiberglass batt | 8–12 | 0.55–0.75 | Non-combustible | Thermal; limited acoustic benefit |
| Acoustic-grade drywall (CLD) | N/A (wall panel) | N/A | Type X (fire-rated) | Where wall thickness is constrained |
Sources: Rockwool North America, NRC Canada Acoustics Resources, NRC Building Materials Technical Series (IRC).
