Acoustic Engineering · US Multifamily · Technical Guide
IIC & STC
Explained
for SPC
The two acoustic numbers that determine whether your BTR community passes code inspection — and the installation detail that voids both of them regardless of product specification.
IIC — Impact Insulation Class
STC — Sound Transmission Class
△IIC — Product Contribution
FIIC — Field Test
IIC 50US Code Minimum
△IIC 225mm + 1.5mm IXPE
−4–8 dBGap bridge penalty
Quick Reference — US Multifamily
IIC 50IBC § 1207 minimum
STC 50Airborne minimum
IIC 55+Luxury / HOA target
IIC 60+Healthcare / WELL v2
Definitions
Two numbers.
One critical distinction.
IIC
Impact Insulation Class
IIC measures a floor/ceiling assembly’s resistance to impact sound — footfall, dropped objects, furniture dragging, children running. Tested by a standardized tapping machine per ASTM E492. Higher IIC = better impact isolation.
This is the number most BTR specifications reference. It describes the entire assembly — structure, flooring, ceiling — not just the floor product alone. See our US BTR flooring guide for full specification tables.
This is the number most BTR specifications reference. It describes the entire assembly — structure, flooring, ceiling — not just the floor product alone. See our US BTR flooring guide for full specification tables.
US Code Minimum (IBC § 1207)
IIC 50 — standard multifamily. IIC 55–60 for luxury / HOA specifications.
STC
Sound Transmission Class
STC measures resistance to airborne sound — voices, TV, music, HVAC noise — transmitting through the floor/ceiling assembly per ASTM E90. Higher STC means less airborne sound passes between floors.
STC is less flooring-specific than IIC. It depends primarily on the mass and construction of the structural slab or joist assembly. SPC flooring’s contribution to STC is measurable but modest — typically +2 to +4 dB above bare slab. View Ecoflors SPC product range →
STC is less flooring-specific than IIC. It depends primarily on the mass and construction of the structural slab or joist assembly. SPC flooring’s contribution to STC is measurable but modest — typically +2 to +4 dB above bare slab. View Ecoflors SPC product range →
US Code Minimum (IBC § 1207)
STC 50 — IBC standard multifamily. Luxury typically specifies STC 55+.
Performance Scale — Residential & Commercial Reference
Scale normalized to 80 = maximum reference. Values based on published ASTM E492 / E90 test data ranges.
△IIC — The Product Number
The number on your
test report is not the IIC.
Concrete Podium Assembly — Cross Section
≥10mm expansion gap — all perimeters — NON-NEGOTIABLE
Air cavity
△IIC = The product’s contribution above bare slab
Your SPC product test report shows △IIC — the improvement the flooring assembly provides above the reference slab. A △IIC of 22 dB does not mean your floor achieves IIC 22. It means your floor improves whatever slab you install it over by 22 dB.
If your structural slab tests at IIC 28 (typical 150mm reinforced concrete), and your SPC assembly provides △IIC 22, your system IIC is approximately 50 — the US code minimum.
If your structural slab tests at IIC 28 (typical 150mm reinforced concrete), and your SPC assembly provides △IIC 22, your system IIC is approximately 50 — the US code minimum.
IXPE thickness drives the delta
The pre-attached IXPE underlayment is the primary variable in SPC’s △IIC contribution:
1.0mm IXPE: △IIC 16–19 dB — wood-frame garden-style BTR
1.5mm IXPE: △IIC 20–24 dB — concrete podium mid-rise
2.0mm IXPE: △IIC 24–28 dB — luxury high-rise specification
See 6mm SPC specifications →
1.0mm IXPE: △IIC 16–19 dB — wood-frame garden-style BTR
1.5mm IXPE: △IIC 20–24 dB — concrete podium mid-rise
2.0mm IXPE: △IIC 24–28 dB — luxury high-rise specification
See 6mm SPC specifications →
⚠ FIIC vs Lab IIC — Field vs Laboratory
Laboratory IIC (tested in controlled conditions) is always higher than Field IIC (FIIC, tested in actual buildings). The difference is typically 3–8 dB due to flanking transmission through walls, pipes, and structural connections. When specifying for code compliance, confirm whether the code requires lab IIC or field IIC — and specify accordingly.
⚠ The expansion gap rule that voids performance
When perimeter expansion gaps are bridged — by rigid sealant, compressed skirting, or sloppy framing — the floating assembly mechanically locks to the wall. The resilient layer can no longer deflect freely under impact. Real-world IIC drops 4–8 dB. This is the most common acoustic failure mode on BTR projects where the product specification was correct.
System IIC Calculator
Calculate your
system IIC.
Adjust your assembly parameters
Structural Slab / Assembly Type
SPC Total Thickness
5
mm
IXPE Underlayment Thickness
1.5
mm
Separate Acoustic Mat
Perimeter Expansion Gap
Estimated System IIC
50
ASTM E492 reference assembly estimate
Structural slab baselineIIC 28
SPC rigid core contribution+8 dB
IXPE underlayment+14 dB
Acoustic mat+0 dB
Gap penalty+0 dB
System IIC (estimated)IIC 50
Estimates based on published ASTM E492 laboratory data ranges. Field IIC (FIIC) will typically be 3–8 dB lower due to flanking paths. Not a substitute for project-specific acoustic testing. Request △IIC test reports →
Failure Modes
Six reasons acoustic
specs fail in the field.
01
Expansion gap bridged
Perimeter gaps filled with rigid sealant, grout, or compressed by skirting boards. The floating assembly can no longer deflect freely under impact.
IIC Penalty
−4 to −8 dB
02
Wrong IXPE for slab type
1.0mm IXPE specified for concrete podium instead of wood-frame. Insufficient resilience to absorb impact energy transmitted through rigid slab.
IIC Penalty
−4 to −6 dB
03
Low core density compression
Budget SPC at 1.60–1.75 g/cm³ compresses permanently under load, reducing the effective thickness of the resilient layer over time. Ecoflors SPC: 1.95–2.05 g/cm³
IIC Penalty over time
−2 to −4 dB
04
Flanking transmission
Sound bypasses the floor/ceiling assembly through walls, pipes, structural connections. Lab IIC does not account for flanking. Field IIC is always lower.
Lab vs Field Gap
−3 to −8 dB
05
Missing mid-room T-bar
Rooms wider than 8m without a mid-room T-bar transition create unrestricted thermal expansion runs. Combined with gap failure, acoustic performance degrades significantly.
Combined with gap failure
Up to −10 dB
06
Underlayment seams unsealed
When using a separate acoustic mat, unsealed seams between mat panels create flanking paths. Pre-attached IXPE eliminates this variable entirely.
IIC Penalty
−2 to −5 dB
Specification Reference
SPC acoustic spec
by project type.
| Project Type | Structure | SPC Spec | IXPE | Target IIC | Certification |
|---|---|---|---|---|---|
| Garden-Style BTR | Wood frame, 3–4 storey | 5mm / 0.5mm wear | 1.0mm pre-attached | IIC 50–52 | FloorScore + CARB 2 |
| Mid-Rise BTR | Concrete podium, 5–12F | 5–6mm / 0.5mm wear | 1.5mm pre-attached | IIC 52–55 | FloorScore + GREENGUARD Gold |
| Luxury High-Rise | Concrete, 12+ storey | 6mm / 0.5mm wear | 2.0mm + acoustic mat | IIC 55–60 | GREENGUARD Gold + WELL v2 |
| Hotel Guest Rooms | Concrete, varies | 6mm / 0.7mm wear | 1.5mm pre-attached | IIC 55+ | FloorScore + GREENGUARD Gold |
| Hotel Corridors | Concrete | 6mm / 0.7mm wear EN 685 Cl.43 | 1.5mm pre-attached | IIC 50+ | EN 685 Class 43 on CE DoP |
| Commercial Office | Concrete | 5mm / 0.5mm wear | 1.0–1.5mm | IIC 50 | FloorScore + LEED v4 EQ Cr.2 |
| Healthcare / NHS | Concrete | 3mm LVT Dryback / 0.7mm | Separate acoustic mat required | IIC 60+ | GREENGUARD Gold + ASTM F1700 Cl.IV |
Acoustic-Tested SPC · US Projects
Acoustic-Tested SPC
for US Projects
IXPE pre-attached at the factory. △IIC test reports available per assembly. FloorScore · CARB 2 · GREENGUARD Gold. FOB quotes with full documentation.