IIC, STC and ΔIIC: Flooring Acoustic Ratings Explained
Acoustic ratings answer different questions. A useful specification names the metric, test method, floor-ceiling assembly and project requirement together.
Three metrics, three procurement questions
| Metric | What it describes | Method and comparison basis |
|---|---|---|
| IIC: Impact Insulation Class | Impact sound transmission through the tested floor-ceiling assembly. | Laboratory measurement under ASTM E492; rating under ASTM E989. Compare the full assembly. |
| STC: Sound Transmission Class | Airborne sound insulation of the tested separating assembly. | Laboratory measurement under ASTM E90; rating under ASTM E413. |
| ΔIIC: change in IIC | The improvement attributable to the tested covering under a specified concrete-floor procedure. | ASTM E2179 with the applicable rating procedure. Keep the reference floor, covering and test conditions visible. |
Read every layer in the report
Flooring and underlay
Record SKU, thickness, attached or separate pad, adhesive/floating method and perimeter details.
Concrete or timber
Record the structural floor and dimensions. A concrete test result does not establish the result over a lightweight joist floor.
Ceiling and cavity
Record ceiling suspension, cavity and insulation. These can contribute materially to the result.
A test assembly is a system, not a stack of independently additive ratings. Do not add a product’s ΔIIC to a published IIC from a different floor and present the sum as a tested result.
Use the adopted code and the correct field rating
As a specific model-code example, 2021 IBC Section 1206 specifies STC 50 for the covered separating assemblies when tested under ASTM E90, with an NNIC 45 field route under ASTM E336. For the covered floor-ceiling assemblies, it specifies IIC 50 under ASTM E492 or NISR 45 under ASTM E1007. Read the scope and local adoption before using those values on a project.
The project may use another edition, local amendments or a stricter owner requirement. Field metrics are not simply laboratory IIC or STC with a standard deduction. There is no universal “subtract 5–10 points” conversion.
A pad option is not an assembly guarantee
IXPE or EVA backing can be part of the proposed flooring system. Its contribution must be assessed with the tested product and assembly. Pad thickness, expansion factor or material name does not establish the required acoustic rating.
ABA / RSVP refers here to a layered construction with vinyl layers above and below a rigid SPC core. It is not an abbreviation for “Acoustic-Backing-Attached”. An attached pad, if specified, is a separate part of that build.
What to send with an acoustic enquiry
- Project requirementSend the exact metric, threshold, method, building location and whether laboratory or field evidence is required.
- Existing or proposed assemblyInclude structure, ceiling, underlay, flooring and installation details.
- Evidence reviewSubmit the complete applicable report to the project acoustic consultant or specifier. Record changes to the build before ordering.
Questions from buyers and specifiers
Can the same plank have different IIC results?
Yes. Structural floor, ceiling, underlay and installation affect the tested assembly. Quote the result with those conditions.
Is ΔIIC independent of every substrate?
No. It is obtained under the concrete-floor procedure and its specified conditions. It is useful for appropriate comparisons, not a universal result on any substrate.
Can IIC alone demonstrate airborne sound compliance?
No. Impact and airborne sound are different assessment questions. Use the metric and method specified for each.
Does an extra underlay always help?
Do not assume so. It can change support and joint behaviour as well as acoustics. Use only the underlay arrangement permitted for the selected flooring.
Related buyer resources
Sources and further reading
Send the build and destination
Tell us the construction, wear layer, size, quantity per colour, destination and required documents. This gives the quotation a clear product and delivery basis.