Ask any property manager at a stacked-flat building what kills tenant retention and the answer is almost never paint colour. It is the upstairs unit. A pair of work boots on hardwood at 6 AM, a kid running drills in the hallway, a rolling office chair on an LVT floor. Acoustic complaints between floors are the dominant liveability issue in multi-family housing, dormitories, military barracks, hotels, and patient wards. The fix happens at construction or renovation, not after the lease is signed.

What follows is the practical framework we use when an architect, GC, or facility team needs a real answer on how to get a floor assembly that will not generate noise complaints, organized by what works structurally and where the products land.

Typical application context: Conversions of existing multi-story buildings into multi-family residential or hospitality use are common settings for acoustic underlayment specification. Concrete slabs and wood-frame assemblies that meet code on paper sometimes underperform once finished floors are installed and field testing is conducted. Specifying an acoustical underlayment between the structural floor and the finish floor is the most common mid-project intervention to bring field-tested performance into the contracted range.

Two types of sound transmission

Impact noise (structure-borne)

Footsteps, dropped objects, dragged furniture, rolling chairs. Impact noise travels through the floor assembly as vibration and radiates into the space below as sound. Measured by IIC (Impact Insulation Class). Higher number is better.

Airborne noise

Voices, music, TV, conversation. Airborne sound travels through the air, hits the floor, and transfers through the assembly. Measured by STC (Sound Transmission Class). Higher is better.

Impact noise is harder to control. Most acoustic interventions for floors focus on reducing impact noise because that is what residents notice and complain about.

Code requirements (general framework)

IBC Section 1207 sets the floor: STC 50 and IIC 50 between dwelling units, laboratory-tested per ASTM E90 and E492 respectively. Field-tested numbers (FSTC and FIIC) typically come in below laboratory ratings, which is why developers, HOAs, and hospitality brands often specify above the IBC floor. Acoustic performance depends on the complete floor assembly; assembly-specific STC, IIC, and Delta IIC values should be confirmed against published test reports for each candidate product.

California is stricter than most jurisdictions. Title 24 references both IIC and STC obligations, and CALGreen adds optional but increasingly common voluntary measures. School projects routed through DSA reference ANSI S12.60 for classroom acoustics. Healthcare projects often default to OSHPD-reviewed assemblies with documented acoustic performance. Confirm the requirement that actually applies to your project before specifying.

Five methods to reduce noise between floors

1. Acoustic underlayment

The most common and cost-effective intervention. A layer of acoustic underlayment (rubber, cork, or specific foams) installed between the subfloor and the finished floor covering decouples the floor surface from the structure below, reducing impact noise transfer.

Recycled rubber underlayment in the typical 5mm to 12mm range is widely specified for impact-noise reduction across the broadest range of finish floors (hardwood, tile, LVT, engineered wood). Acoustic performance depends on the complete floor assembly. Refer to published test reports for assembly-specific STC, IIC, and Delta IIC values.

2. Floating floor assemblies

Decoupling the finished floor from the structural floor reduces both impact and airborne noise. A floating floor sits on a resilient layer (acoustic underlayment) without being mechanically fastened to the subfloor below.

3. Suspended ceilings

On the receiving (lower) side of the assembly, a suspended ceiling with resilient channels or sound isolation clips decouples the ceiling drywall from the joists above. This significantly reduces both airborne and impact noise reaching the unit below.

4. Mass added to the assembly

More mass in the floor and ceiling reduces sound transmission. Concrete topping (gypcrete is common at 1.5" to 2"), additional layers of drywall, and dense-fiber insulation in the joist cavity all add mass.

5. Sealing flanking paths

Sound moves through gaps. Penetrations at ductwork, plumbing, and electrical boxes are common flanking paths. Acoustic sealant at the floor-wall junction and around penetrations preserves the rated performance of the assembly.

Typical assemblies and what they achieve

Acoustic performance depends on the full assembly, not any single layer. These are general reference assemblies. Verify with manufacturer-tested ratings for your specific products.

Wood-frame floor with acoustic underlayment

Joists, plywood subfloor, acoustic underlayment, finished floor, resilient ceiling channels with two layers of drywall, fibrous insulation in cavity. Acoustic performance depends on the complete floor assembly. Refer to published test reports for assembly-specific values.

Concrete slab with acoustic underlayment

Concrete slab, acoustic underlayment, finished floor. Slab thickness and underlayment specification both influence performance. Refer to published test reports for assembly-specific values.

Concrete slab with gypsum concrete topping and underlayment

Concrete slab, gypcrete topping, acoustic underlayment, finished floor. The combination commonly used in multi-family where higher acoustic performance is required. Refer to published test reports for assembly-specific values.

Choosing acoustic underlayment for noise reduction

Specify based on these factors:

  1. Required IIC and STC ratings for your jurisdiction and project.
  2. The full floor assembly (subfloor type, concrete topping, finished floor).
  3. Service life expected for the building (decades for multi-family ownership).
  4. Sustainability and LEED targets — see our sustainability documentation.
  5. Compatibility with the floor covering above.

Recycled rubber underlayment scores well on all five for schools, colleges and universities, military facilities, healthcare environments, professional sports organizations, fitness centers, chain fitness clubs, private athletic clubs, multi-family, hospitality, and commercial applications.

Common mistakes that kill acoustic performance

  • Skipping the acoustic sealant at floor-wall junctions. Sound flanks around the assembly.
  • Mechanically fastening the finished floor through the underlayment to the subfloor. Defeats the decoupling.
  • Using a thinner underlayment than the tested assembly. Acoustic ratings do not scale linearly with thickness.
  • Substituting a different underlayment product without verifying it achieves the same tested rating in the same assembly.
  • Ignoring flanking paths through ducts, plumbing, and electrical boxes.

FAQs

What IIC rating do I need for a multi-family condo?

Code minimum is generally IIC 50, laboratory-tested. HOAs and project specifications often require higher numbers. Acoustic performance depends on the complete floor assembly. Confirm your specific HOA bylaws and local code requirements, then verify candidate assemblies against published test reports.

Carpet over standard pad can meet code-minimum impact ratings in some assemblies but does not match the performance of a hard floor over a quality acoustic underlayment. For high-IIC targets or hard finishes, acoustic underlayment is the standard intervention.

Retrofit installations require removing the existing finished floor and installing the underlayment under a new finished floor. Adding underlayment to an existing assembly without disturbing the finish floor is not generally feasible.

Yes. Resilient channels, sound isolation clips, and a suspended ceiling with additional drywall layers on the receiving side of the assembly significantly improve both IIC and STC. Often used in combination with floor-side acoustic underlayment for maximum performance.

Field testing (FIIC and FSTC) measures real installed performance and typically reads below laboratory ratings. Some jurisdictions require field testing as part of certificate of occupancy. Confirm field-testing requirements before construction starts.

Specify QuietSound for impact noise reduction

QuietSound is the recycled-rubber acoustical underlayment we manufacture for projects exactly like this. Tested assemblies on file for hardwood, LVT, tile, and engineered wood. Schools, military, healthcare, fitness centers, multi-family, hospitality, commercial. If you want the IIC and STC reports for a specific assembly that matches your project, send the build-up and we will get them across. usrubber.com/quietsound-acoustical-underlayment/ or (833) 877-8223. For gym and sports floors above occupied space, see Survivor SportFloor.

Standards referenced: IBC Section 1207; ASTM E90; ASTM E492; ANSI/ASA S12.60 (schools); California Title 24 and CBC; UFC 3-450-01 (military). Confirm against current published versions before relying on specific provisions.

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.