What Is Acoustic Underlayment? (STC Specs)
Acoustic underlayment is a resilient layer beneath a finished floor that can reduce impact noise when it is part of a tested floor–ceiling assembly. It does not have a stand-alone STC rating that proves room-to-room performance. Check the assembly’s laboratory STC and IIC results separately, then confirm they meet your local code and project requirements.
A floor can feel solid and still pass too much sound to the room below. Footsteps, dropped objects, and voices also travel in different ways, so one product number cannot describe every noise problem. The useful question is not simply “Which underlayment is best?” It is “Does this complete floor–ceiling assembly meet the right sound targets?”
I treat the test report as the starting point, not the product label. A small change to the floor finish, fasteners, ceiling, or perimeter detail can make the installed construction different from the one tested.
Planning: Code Targets and Assembly Test Data
Acoustic underlayment is one layer in a larger floor–ceiling system. Before choosing it, identify the required airborne and impact ratings, the building-code edition in force, and the tested assembly that matches the job. Product claims alone cannot establish that a floor meets code.
The 2021 International Building Code (IBC) sets a model-code baseline for applicable separations between dwelling units, suites, or dwelling areas and public or service areas. Under Sections 1206.2–1206.3, that baseline is STC 50 and IIC 50 when tested in a laboratory, or 45 for each when field-tested. Confirm the locally adopted code, amendments, applicability, and exceptions with the authority having jurisdiction (AHJ).
STC is a classification of how well an assembly limits airborne sound, such as speech. IIC classifies resistance to impact sound, such as footfalls. They measure different performance, so a floor may meet one target and miss the other.
ASTM E90 measures airborne sound transmission in a laboratory; ASTM E413 classifies the result as STC. ASTM E492 measures impact sound transmission in a laboratory; ASTM E989 classifies the result as IIC. ASTM E2179 measures how a floor covering reduces impact sound on a concrete floor. That reduction is not the IIC of a complete floor–ceiling assembly.
| Information shown | What it tells you | What it does not prove |
|---|---|---|
| Assembly STC from E90/E413 | Laboratory airborne performance of the tested construction | Impact performance or performance of a different assembly |
| Assembly IIC from E492/E989 | Laboratory impact performance of the tested construction | Airborne performance or a guaranteed field result |
| E2179 impact-sound reduction | Change in impact sound for a floor covering over a concrete floor | The assembly’s IIC |
| Field test result | Performance measured in a building using the applicable field method | A laboratory rating on the same basis |
There is no code-wide underlayment thickness, perimeter gap, or universal STC increase. Request the full report and check the exact underlayment, thickness, substrate, flooring, ceiling, attachment, and edge details. Next step: write down the required lab or field targets before comparing products.
Access and Clearances: Substrate, Elevation, and Perimeter Isolation
A tested assembly only applies when its parts and installation details match the report. Survey the existing floor, ceiling, and room edges before ordering materials. This helps reveal height conflicts, moisture issues, and rigid connections that can carry sound around the resilient layer.
Check whether the substrate is a concrete slab or a framed floor, and note its condition and flatness. Identify the finish floor, ceiling type, joist or slab construction, penetrations, and connections to walls. Also note door undercuts, thresholds, stairs, cabinets, appliances, and accessible routes that may be affected by the added floor height.
Follow the assembly’s specified substrate preparation and moisture limits. Do not assume that a product tested on concrete will perform the same way over wood framing, or that the same underlayment works under tile, wood, or resilient flooring. The report and manufacturer’s instructions should name the permitted use.
Perimeter isolation means keeping the resilient layer from making an unapproved rigid connection to walls, trim, or other parts of the structure. Use the edge detail stated in the tested assembly. A generic gap dimension is not a substitute for that detail, and trim must not press the floor into a rigid bridge.
Flanking transmission is sound that travels by a path around the main floor assembly. Continuous walls, partitions, pipes, ducts, and rigidly connected trim can all provide such paths. A high-rated floor assembly cannot by itself guarantee the room-to-room result if these paths are left unresolved. Next step: mark edges and penetrations on a plan, then confirm their details with the assembly documentation.
Execution: Underlayment and Finish-Floor Installation
Installation should follow the exact sequence and attachment method in the tested assembly. Prepare the substrate, place the underlayment, install the finish floor, and preserve the specified isolation at edges and penetrations. A substitute product or fastening method may change the system’s performance.
A practical sequence is:
- Prepare: Remove debris and address substrate conditions required by the report and product instructions. Check moisture where the assembly requires it.
- Place: Install the specified underlayment in the stated direction and layout. Keep seams, overlaps, and edge clearances as detailed.
- Finish: Install the approved floor covering using only the stated adhesive or attachment method.
- Protect isolation: Do not add screws, nails, adhesive, or trim details that rigidly bridge the resilient layer unless the tested design calls for them.
- Record: Keep product labels, installation notes, and the report for the installed assembly.
The same caution applies to penetrations and transitions. Do not invent a sealant or gap detail: use the method specified for that assembly and coordinate it with the flooring and wall trades. If a detail is unclear, resolve it before the finish floor hides the work. Next step: compare each installed layer and edge condition with the report before covering it.
Verification: STC/IIC Evidence and Field Testing
Verification means checking both the paperwork and the finished construction. Laboratory ratings describe a controlled test assembly; field results describe a building and may reflect flanking paths and site conditions. Use the test basis required by the code, contract, or AHJ, and do not compare unlike ratings as if they were identical.
For airborne field testing, ASTM E336 is commonly used; for impact field testing, ASTM E1007 is commonly used. Field measurements are not interchangeable with laboratory tests under E90 and E492. Confirm the applicable field classification method and acceptance criteria for the project.
| Project stage | Check | Record or decision |
|---|---|---|
| Planning | Code edition, separation type, required ratings | Applicable lab or field STC and IIC targets |
| Product review | Full assembly report and test methods | Whether construction details match the project |
| Before flooring | Substrate, height, moisture, edges, penetrations | Corrections needed before concealment |
| Closeout | Installed materials, attachments, perimeter details | Documentation and any required field test |
A product sheet that says “STC rated” is not enough to establish the floor–ceiling assembly’s STC. Ask what was tested, under which method, and whether the claimed value belongs to the whole assembly or only a component. Next step: arrange field testing if required, and give the tester the assembly details and acceptance criteria.
Real-World Installation Scenarios
These examples are illustrative planning cases, not measured test results. They show how the same underlayment question changes with the building and the noise path. In each case, the correct comparison is between the project and a documented assembly, not between isolated product claims.
Concrete apartment floor: A homeowner wants to reduce footsteps heard below. First confirm whether the floor covering and underlayment match an applicable E2179 report or a complete assembly tested for IIC. If code compliance depends on assembly IIC, an impact-reduction figure alone does not answer that question.
Framed floor between units: A builder is selecting a finish over wood framing. The matching report must include the relevant floor, underlayment, framing, ceiling, and attachment details. A slab test or a different ceiling configuration is not a reliable substitute.
Renovation with a low doorway: The selected assembly adds height and may affect the door, threshold, or accessible route. Check those clearances before purchase, then retain the tested edge isolation. A field fix that rigidly connects the finish floor to the wall may compromise the intended design.
Across all three, the key decision is whether the proposed installation reproduces a tested assembly and satisfies the applicable target. Next step: resolve differences from the report with the design professional, manufacturer, or AHJ before installation.
Conclusion and FAQ
Acoustic underlayment can help control impact sound, but it is not a stand-alone STC solution. Specify the complete assembly, check STC and IIC separately, follow the tested details, and verify the code edition and test basis for your location. Keep the report and installation record with the project documents.
Does acoustic underlayment have an STC rating?
STC classifies airborne sound performance of an assembly, not an underlayment by itself. Check what the product’s stated test actually covers.
What is the difference between STC and IIC?
STC measures airborne sound isolation, such as speech. IIC measures impact sound transmission, such as footsteps. One does not replace the other.
What STC and IIC does the 2021 IBC require?
For applicable separations, its model-code baseline is 50 in laboratory testing or 45 in field testing for both STC and IIC. Check local adoption and exceptions.
Does an E2179 result equal an IIC rating?
No. ASTM E2179 reports impact-sound reduction for a floor covering over a concrete floor. It is not the IIC of a complete floor–ceiling assembly.
Can I compare laboratory STC with field STC directly?
Do not treat them as identical. Laboratory and field methods test different conditions. Use the basis and acceptance criteria required for the project.
Will thicker underlayment always improve sound control?
No universal thickness or performance increase applies. Use the thickness and product specified in the applicable tested assembly.
Can screws or trim reduce acoustic performance?
They can create rigid connections that bridge the resilient layer. Follow the tested attachment and perimeter details rather than adding unapproved fasteners or trim contact.
Why can a well-rated floor still transmit noise?
Sound may travel through flanking paths such as walls, partitions, pipes, ducts, or rigid trim. Underlayment alone cannot guarantee the room-to-room result.
What documents should I request before buying?
Request the full laboratory report, including test methods and the exact floor, underlayment, substrate, ceiling, attachment, and edge details. A short product claim may not provide that information.
When is field testing needed?
Use field testing when required by the adopted code, project contract, or AHJ. Airborne and impact field tests commonly use ASTM E336 and E1007, respectively.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
