Acoustic underlayment is the resilient layer between the subfloor and the finished floor that decouples the two and reduces structure-borne and airborne sound transmission. You never see it after the floor goes down. The owner feels its absence every time someone walks across the unit above, and the GC feels it the day field testing fails the IIC threshold and the certificate of occupancy gets held up.

This is the long version of the question we get asked weekly. What it does, the four materials competing for the spec, how to read ASTM E492 and E90 ratings without getting fooled by the assembly the test was run in, when code or HOA or DSA or OSHPD actually requires it, and how to choose between them when a real project is on the line. Written for architects, specifiers, builders, facility managers, and the owner who has to sign off on the warranty.

Typical specifier context: Substitution requests are common on hospitality renovations, multi-family residential projects, and institutional buildouts when a specified underlayment becomes unavailable or value engineering proposes an alternate. Design teams typically request manufacturer test reports for candidate assemblies that bracket the original acoustic target. Refer to published test reports for assembly-specific STC, IIC, and Delta IIC values when evaluating substitutions against the project’s contracted acoustic performance.

What acoustic underlayment does

Three functions, all related:

  • Reduces impact noise (footsteps, dropped objects, rolling chairs) from transmitting through the floor assembly into the space below.
  • Reduces airborne noise (voices, music) transmitted through the floor and ceiling assembly.
  • Decouples the finished floor from the structural subfloor, reducing vibration transmission.

Acoustic underlayment is most commonly used in multi-family residential, hospitality (hotels), schools and education (K-12 through colleges and universities), military and government facilities, healthcare environments, professional sports organizations, fitness centers, chain fitness clubs, private athletic clubs, and commercial buildings where impact and airborne noise control matters for occupant comfort and code compliance.

The four main types of acoustic underlayment

Recycled rubber

Widely specified for acoustic performance across the broadest range of floor coverings. Recycled rubber underlayment is widely recognized for its long-term resiliency and can often outlast the finished floor covering installed above it. Made from post-consumer tires in the case of recycled rubber. The premium option for schools, colleges and universities, military facilities, healthcare environments, professional sports organizations, fitness centers, chain fitness clubs, private athletic clubs, multi-family residential, hospitality, and commercial applications.

Cork

Natural, renewable material. Good acoustic performance in residential applications. Compresses more readily than rubber under heavy point loads. Common in single-family residential and lighter-duty applications. Reference the cork manufacturer’s published service-life and compression data for project-specific expectations.

Foam (polyethylene or polyurethane)

Lowest cost, lightest weight, and shorter service life than denser underlayment materials. Acoustic performance varies widely by density. Common in budget residential applications and under laminate flooring. Confirm service-life and acoustic data with the foam manufacturer.

Felt and fiber

Recycled-fiber underlayment offers moderate acoustic performance. Used in some hardwood and engineered wood applications. Less common in current multi-family specifications than rubber.

How acoustic ratings work

STC (Sound Transmission Class)

Measures airborne sound transmission through a floor and ceiling assembly, per ASTM E90. Higher number means better isolation. STC 50 is the IBC minimum between dwelling units, laboratory-tested. Quality multi-family and hospitality specifications routinely require higher numbers. Confirm the specific target against your project requirements.

IIC (Impact Insulation Class)

Measures impact sound transmission (footsteps, dropped objects), per ASTM E492. Higher is better. IIC 50 is the IBC minimum, laboratory-tested. HOAs and luxury projects routinely specify above the code floor. Confirm the specific target against your project requirements.

Delta IIC

Measures the improvement an underlayment provides over a reference floor assembly, per ASTM E2179. A Delta IIC of 20 means the underlayment improves the IIC of a reference bare concrete assembly by 20 points. Helpful for comparing underlayments to each other.

Field vs laboratory ratings

Laboratory ratings (STC, IIC) are measured in controlled conditions. Field ratings (FSTC, FIIC) measure actual installed performance and typically read below laboratory ratings. Some jurisdictions and projects require field testing. Refer to published test reports for assembly-specific values.

When is acoustic underlayment required?

By code

IBC Section 1207 sets the floor at STC 50 / IIC 50 between dwelling units, laboratory-tested. Multi-family and hospitality projects with hard-finished floors rarely meet this without underlayment in the assembly.

By HOA

Condo boards and apartment HOAs often require specific IIC ratings above code minimum on renovations that change the floor covering. Recycled rubber underlayment in the typical 5mm to 8mm range is a common retrofit specification. Confirm the HOA’s required rating and verify against published test reports.

By project type

Schools tracked through ANSI S12.60 for classroom acoustics. California DSA-reviewed school projects, HCAI (formerly OSHPD)-reviewed healthcare projects, and military housing built to UFC standards each carry their own acoustic obligations. Luxury hotels and Class-A condos routinely specify above code minimum regardless of jurisdiction. Confirm the specific target against your project’s requirements.

By choice

Single-family homeowners spec acoustic underlayment in basements, second-floor bedrooms, and home theatres when noise control matters even though no code or HOA requires it.

Choosing the right acoustic underlayment

Five questions to answer before specifying:

  1. What IIC and STC ratings does the project require? (Code, HOA, spec)
  2. What is the finished floor covering? (Hardwood, tile, LVT, engineered wood, carpet)
  3. What is the subfloor? (Concrete slab, plywood, gypcrete topping)
  4. How long does the building owner expect the floor system to last?
  5. Are there LEED, EPD, HPD, or other sustainability targets? (See our sustainability documentation.)

Answer those five questions, then match the underlayment to the assembly. For most multi-family, hospitality, and commercial projects, recycled rubber underlayment is the strongest match.

Installation considerations

  • Subfloor must be flat (typically within 1/8" over 10 ft) and dry. Confirm moisture levels meet the floor covering manufacturer’s specifications.
  • Acoustic sealant at floor-wall junctions prevents flanking and preserves the rated assembly performance.
  • Mechanical fastening through the underlayment to the subfloor defeats the decoupling. Floating-floor installations or specific adhesives are the standard approaches.
  • Penetrations (plumbing, electrical, ducts) need acoustic sealant treatment to prevent flanking noise paths.
  • Field testing after installation verifies actual performance against the laboratory ratings.

Common acoustic underlayment mistakes

  • Substituting a lower-cost product without verifying it achieves the same tested rating.
  • Using a thinner underlayment than the assembly was tested with.
  • Skipping acoustic sealant at perimeter and penetrations.
  • Mechanically fastening the floor through the underlayment.
  • Assuming a single rated assembly applies to all floor coverings; the same underlayment performs differently under hardwood, tile, and LVT.

FAQs

Do I need acoustic underlayment under hardwood floors?

In multi-family buildings, almost always. Hardwood by itself transmits impact noise readily. Code, HOA, and project specifications generally require acoustic underlayment under hardwood in shared-wall and stacked-unit configurations.

LVT installations need underlayment compatible with the LVT manufacturer’s warranty and the floor system. Recycled rubber underlayment from manufacturers who have published LVT-specific tested assemblies is the standard for commercial and multi-family LVT installations.

Yes, with specific acoustic underlayment products designed and tested for use under tile and stone. Confirm the assembly’s documented IIC and STC ratings and the underlayment’s compatibility with the mortar system.

5mm to 12mm is the common range for multi-family and hospitality projects. Specifications vary by acoustic target, finish floor, and code requirement. Acoustic performance depends on the complete floor assembly. Refer to published test reports for assembly-specific values rather than selecting thickness in isolation.

Recycled rubber acoustic underlayment with published EPD, HPD, and FloorScore certification contributes to LEED v4 and v4.1 credits in Materials and Resources and Indoor Environmental Quality categories. Confirm specific credit contribution with the manufacturer’s LEED documentation.

Service life varies by material, density, and application. Recycled rubber underlayment is widely recognized for its long-term resiliency and can often outlast the finished floor covering installed above it. Cork and foam tend to perform shorter than recycled rubber under typical multi-family and commercial loads. For multi-decade installations, recycled rubber is the standard specification. Confirm service-life expectations against each manufacturer’s published technical data.

Specify QuietSound for proven acoustic performance

QuietSound is the recycled-rubber acoustical underlayment we manufacture in Colton, California. Engineered for assemblies in K-12 schools, colleges and universities, military housing, healthcare facilities, professional sports organizations, chain fitness clubs, private athletic clubs, fitness centers, multi-family residential, hotels, and commercial buildings. Tested assemblies on file for hardwood, LVT, tile, and engineered wood. May contribute toward LEED credits; third-party EPD and HPD on record; FloorScore certified. When you need the test reports to back up a specification, usrubber.com/quietsound-acoustical-underlayment/ has the current set, or call (833) 877-8223. Related reading in our Media Room: rubber vs cork vs foam comparison, reducing noise between floors, and gym flooring thickness guides.

Standards referenced: ASTM E90; ASTM E492; ASTM E2179; ASTM E1007; ASTM E336; IBC Section 1207; ANSI/ASA S12.60. Confirm against current published versions.

Disclaimer: This article is for general informational purposes only. Product specifications, acoustic ratings (STC, IIC, Delta IIC), LEED credit eligibility, building code requirements, and pricing change over time and vary by project. U.S. Rubber Recycling, Inc. does not provide architectural, engineering, structural, or legal advice. Confirm performance specifications and code compliance with a licensed acoustical consultant, architect, structural engineer, building official, and the most recent product technical data sheets and third-party test reports before specifying a product or making a purchase decision. Article current as of the publication date shown above.