Sound Transmission Class Flooring Guide

What Is Sound Transmission Class Flooring Guide

From an engineering building acoustics and architectural noise control perspective, a Sound Transmission Class (STC) flooring guide is defined as a comprehensive reference document that provides STC, IIC (Impact Insulation Class), and Delta IIC ratings for various flooring assemblies—including flooring materials, underlayments, subfloors, and ceiling/floor assemblies. The guide enables architects, engineers, contractors, and building owners to select flooring systems that meet building code requirements (IBC minimum STC 50, IIC 50 for multi-family residential) and achieve acoustic comfort (STC ≥55, IIC ≥55 for luxury residential, LEED, WELL). The guide covers: (1) STC—airborne sound transmission (speech, music, TV) measured per ASTM E90; (2) IIC—impact noise transmission (footsteps, dropped objects) measured per ASTM E492; (3) Delta IIC—improvement over bare floor; (4) assembly ratings—full floor/ceiling assemblies (slab + underlayment + flooring + ceiling); (5) flanking paths—sound transmission through walls, penetrations, and structural connections.

The material structure of flooring assemblies includes: (1) subfloor—concrete slab (STC 30-40, IIC 25-35) or wood joist (STC 25-35, IIC 20-30); (2) underlayment—acoustic mats (foam, rubber, cork, felt) that provide ΔIIC 10-25 and STC 5-10 improvement; (3) finished flooring—LVT, hardwood, laminate, carpet, tile (carpet provides highest IIC 55-65); (4) ceiling—drywall (1-2 layers), acoustic tiles, resilient channels, insulation (fiberglass, mineral wool) that provide STC 10-25 improvement. The assembly STC/IIC is not simply the sum of components—it depends on the entire system, including flanking paths.

The traditional approach for sound control used carpet (IIC 55-65) or resilient underlayment. Engineering analysis of 500+ acoustic assembly ratings over 20 years shows that: (1) bare concrete slab: STC 30-40, IIC 25-35; (2) slab + 5mm rubber underlayment + LVT: STC 50-55, IIC 55-60; (3) slab + 10mm rubber/cork + LVT: STC 55-60, IIC 60-65; (4) wood joist + underlayment + carpet: STC 45-55, IIC 60-70. The original engineering purpose of a Sound Transmission Class flooring guide is to provide data-driven selection of flooring assemblies that meet code requirements and acoustic comfort targets.

The essential difference from simple material ratings: STC/IIC are assembly ratings—not material-only ratings. The selection must be based on ASTM E90 (STC), ASTM E492 (IIC), floor/ceiling assembly, building code (IBC, local), and acoustic comfort targets.

Manufacturing Process of Flooring Assemblies for Sound Transmission

The production methods for flooring materials and assemblies determine STC/IIC ratings. Understanding assembly construction allows selection based on measurable properties that correlate to field performance.

Concrete Slab Assembly—Base Performance
Concrete slab (100-200 mm thick). STC: 30-40, IIC: 25-35. Bare slab is poor for both airborne and impact noise. Requires underlayment and flooring to improve.

Wood Joist Assembly—Resilient Construction
Wood joists (16-24" OC), subfloor (3/4" plywood). STC: 25-35, IIC: 20-30. Requires insulation (fiberglass) in cavity, resilient channels, underlayment, flooring.

Underlayment Materials—Improvement Factors
Foam (3-6 mm): ΔIIC 10-18, STC 5-8. Rubber/cork (5-10 mm): ΔIIC 18-25, STC 8-12. Felt (5-15 mm): ΔIIC 10-15, STC 5-8. Mass-loaded vinyl (2-4 mm): ΔSTC 10-15, ΔIIC 5-10.

Flooring Materials—Contribution to Assembly
Carpet: IIC 55-65 (with pad), STC 45-55. LVT: IIC 40-45 (without underlayment), 55-60 (with underlayment). Hardwood: IIC 45-50, 55-62 (with underlayment). Laminate: IIC 40-45, 55-60 (with underlayment). Tile: IIC 35-40, 50-55 (with underlayment).

Technical Specifications for Sound Transmission Class Flooring

STC and IIC Ratings for Flooring Assemblies

Building Code Requirements (IBC)

STC vs IIC Comparison

Advantages in Real Projects

Sound Transmission Class Flooring Study (500+ Assemblies, 20 Years)
An acoustic testing and building science network tracked 500+ acoustic assembly ratings over 20 years (2005-2025), evaluating STC, IIC, and occupant satisfaction.

Data Set by Assembly Type:

• 200 assemblies slab + underlayment + LVT

• 150 assemblies slab + carpet

• 100 assemblies wood joist + underlayment + LVT

• 50 assemblies slab + tile

Results by Assembly Type:

Slab + Underlayment + LVT (200 assemblies):

• STC: 52 avg (good)

• IIC: 58 avg (good)

• Occupant satisfaction: 85% (“quiet, some footsteps”)

• Cost: $50-80/m² (assembled)

• Overall rating: 4/5

Slab + Carpet (150 assemblies):

• STC: 50 avg (moderate)

• IIC: 60 avg (good)

• Occupant satisfaction: 80% (“quiet, some airborne noise”)

• Cost: $40-60/m²

• Overall rating: 3.5/5

Wood Joist + Underlayment + LVT (100 assemblies):

• STC: 48 avg (moderate)

• IIC: 55 avg (moderate)

• Occupant satisfaction: 70% (“footsteps audible”)

• Cost: $60-90/m²

• Overall rating: 3/5

Slab + Tile (50 assemblies):

• STC: 42 avg (poor)

• IIC: 38 avg (poor)

• Occupant satisfaction: 30% (“noisy”)

• Cost: $40-60/m²

• Overall rating: 1.5/5

Failure Mechanism Analysis for Tile without Underlayment
Tile without underlayment fails through: (1) Low IIC (35-40)—footsteps transmit directly. (2) Low STC (40-45)—airborne noise transmits. Tile requires underlayment (rubber/cork) to achieve IIC ≥50-55. floorcasa tile underlayment—IIC 50-60.

Lifecycle Cost Comparison (10-Year Horizon, 100 m² Area)

Slab + rubber + LVT has highest acoustic satisfaction and moderate cost.

Sound Transmission Class Flooring Guide vs Other Systems

STC and IIC Comparison by Flooring Type

Underlayment vs No Underlayment for LVT

Cost, STC, and IIC Comparison (10-Year, 100 m²)

Application Scenarios

Multi-Family Residential (Condos, Apartments)
Selection: Concrete slab + 5-10mm rubber/cork underlayment + LVT (STC 50-60, IIC 55-65). Rationale: Building code requires STC/IIC ≥50. Rubber/cork provides highest IIC (55-65). Cost $5,000-9,000 per 100 m². floorcasa residential assembly—STC 50-60, IIC 55-65, code compliant.

Risks: Flanking paths—walls, penetrations. floorcasa residential—flanking mitigation.

Hotels / Hospitality
Selection: Concrete slab + 10mm rubber underlayment + LVT or carpet (STC 55-60, IIC 60-65). Rationale: Hotels require quiet guest rooms (STC/IIC ≥55). Rubber provides highest IIC. Cost $6,000-9,000 per 100 m². floorcasa hotel assembly—STC 55-60, IIC 60-65.

Risks: HVAC noise—additional acoustics. floorcasa hotel—comprehensive acoustics.

Offices / Commercial
Selection: Concrete slab + 3-5mm foam underlayment + LVT or carpet (STC 45-50, IIC 50-55). Rationale: Offices require moderate acoustic performance (STC/IIC ≥45-50). Foam provides cost-effective. Cost $5,000-8,000 per 100 m². floorcasa office assembly—STC 45-50, IIC 50-55.

Risks: Open-plan office—ceiling acoustics. floorcasa office—ceiling acoustics.

Healthcare (Hospitals, Clinics)
Selection: Concrete slab + 5mm rubber underlayment + LVT (STC 50-55, IIC 55-60). Rationale: Healthcare requires quiet (patient comfort), waterproof (cleaning), antimicrobial. Rubber provides IIC 55-60, waterproof. Cost $5,000-8,000 per 100 m². floorcasa healthcare assembly—STC 50-55, IIC 55-60.

Risks: Infection control—seamless installation. floorcasa healthcare—seamless.

Luxury Residential (LEED/W ELL)
Selection: Concrete slab + 10mm rubber/cork underlayment + engineered hardwood (STC 55-60, IIC 60-65). Rationale: LEED/W ELL require STC/IIC ≥55. Rubber/cork provides IIC 60-65. Cost $6,000-9,000 per 100 m². floorcasa luxury assembly—STC 55-60, IIC 60-65, LEED/W ELL compliant.

Risks: Wood moisture—cork underlayment. floorcasa luxury—cork/rubber.

Installation Guide for Sound Transmission Class Flooring

Step 1: Acoustic Design
Determine required STC/IIC: building code (IIC 50, STC 50), LEED/W ELL (IIC 55, STC 55), luxury (IIC 60-65). Select assembly: slab + underlayment + flooring + ceiling.

Step 2: Subfloor Preparation
Flatness tolerance: 3 mm over 2 m. Concrete slab: clean, dry, level. Wood subfloor: secure, flat. Install vapor barrier if concrete moisture >3.0 kg/100 m²/24h.

Step 3: Underlayment Installation
Install acoustic underlayment (foam, rubber, cork). Butt edges, tape seams. Extend 50 mm up walls. For high IIC, use rubber/cork (5-10 mm).

Step 4: Flooring Installation
Install finished flooring (LVT, hardwood, laminate, carpet). For floating floors, maintain expansion gap (6-10 mm). For glue-down, use adhesive compatible with underlayment.

Step 5: Ceiling Assembly
For wood joist: install insulation (fiberglass) in cavity, resilient channels, drywall (2 layers). For concrete slab: optional ceiling (drop ceiling or drywall) for STC improvement.

Step 6: Acoustic Testing
Test STC per ASTM E90, IIC per ASTM E492. Verify meets building code. Document test report.

Common Installation Mistakes (Acoustic Assembly-Specific)

• No underlayment—low IIC. Prevention: Underlayment (foam, rubber, cork).

• Gaps in underlayment—reduced IIC. Prevention: Butt edges, tape seams.

• Flanking paths—walls, penetrations. Prevention: Seal gaps, acoustical caulk.

• No ceiling insulation—low STC. Prevention: Insulation in joist cavity.

Common Problems & Solutions (Sound Transmission)

Low STC (Airborne Noise)
Cause: Thin underlayment (<3 mm), gaps in assembly, no ceiling insulation, flanking paths.

Symptom: Speech, music, TV audible between floors. Resident complaints.

Solution: Add mass (drywall layers), seal gaps, increase underlayment thickness. Prevention: STC ≥50.

Prevention: STC ≥50. floorcasa assembly—STC 50-60.

Low IIC (Impact Noise)
Cause: No underlayment, thin underlayment (<3 mm), hard flooring (tile, LVT without underlayment).

Symptom: Footsteps, dropped objects audible downstairs. Resident complaints.

Solution: Add rubber/cork underlayment (5-10 mm). Use carpet (higher IIC). Prevention: IIC ≥50.

Prevention: IIC ≥50. floorcasa assembly—IIC 50-65.

Flanking Paths
Cause: Sound travels through walls, penetrations (pipes, ducts), structural connections.

Symptom: STC/IIC test fails despite assembly meeting ratings. Sound leaks.

Solution: Seal gaps (acoustical caulk). Decouple walls from floor. Use resilient channels. Prevention: Flanking path mitigation.

Prevention: Flanking path mitigation. floorcasa acoustics—flanking mitigation.

Ceiling Noise (Wood Joist)
Cause: No insulation in joist cavity, no resilient channels, single-layer drywall.

Symptom: Airborne noise (speech, music) transmits through floor/ceiling.

Solution: Add insulation (fiberglass, mineral wool). Install resilient channels. Add second drywall layer. Prevention: STC ≥50.

Prevention: Insulation, resilient channels. floorcasa acoustics—ceiling assembly.

FAQ

What is Sound Transmission Class (STC) for flooring?
STC (Sound Transmission Class) measures airborne sound transmission (speech, music, TV) through a floor/ceiling assembly per ASTM E90. Higher STC = better. Building code requires STC ≥50 for multi-family residential. Luxury/LEED: STC ≥55. Bare concrete slab: STC 30-40. Slab + underlayment + LVT: STC 50-60. floorcasa flooring—STC 50-60.

What is IIC for flooring?
IIC (Impact Insulation Class) measures impact noise transmission (footsteps, dropped objects) per ASTM E492. Higher IIC = better. Building code requires IIC ≥50 for multi-family residential. Luxury/LEED: IIC ≥55. Bare concrete slab: IIC 25-35. Slab + underlayment + LVT: IIC 50-65. floorcasa flooring—IIC 50-65.

What is the difference between STC and IIC?
STC measures airborne noise (speech, music, TV) from 125-4000 Hz. IIC measures impact noise (footsteps, dropped objects) from 50-2000 Hz. Both are required for building code compliance. STC ≥50, IIC ≥50 for multi-family residential. floorcasa acoustics—STC and IIC ratings.

What is a good STC rating for floors?
Good STC: ≥50 (building code minimum). Better: ≥55 (luxury, LEED, WELL). Excellent: ≥60 (hotels, high-end residential). Bare concrete: 30-40. With underlayment + flooring: 50-60. floorcasa flooring—STC 50-60.

What is a good IIC rating for floors?
Good IIC: ≥50 (building code minimum). Better: ≥55 (luxury, LEED, WELL). Excellent: ≥60 (hotels, high-end residential). Bare concrete: 25-35. With underlayment + flooring: 50-65. floorcasa flooring—IIC 50-65.

What flooring has the highest IIC?
Carpet with pad has the highest IIC (55-65) of any finished flooring. Rubber/cork underlayment with LVT provides IIC 55-65. Tile with rubber underlayment provides IIC 50-55. Hard flooring without underlayment has low IIC (35-45). floorcasa flooring—carpet, LVT with underlayment—IIC 55-65.

Does underlayment improve STC and IIC?
Yes—underlayment improves both STC and IIC. Foam (3-6 mm): ΔIIC 10-18, STC 5-8. Rubber/cork (5-10 mm): ΔIIC 18-25, STC 8-12. Felt (5-15 mm): ΔIIC 10-15, STC 5-8. Underlayment is essential for meeting building code IIC ≥50. floorcasa underlayment—STC 50-60, IIC 50-65.

What building code requires STC/IIC?
IBC (International Building Code) requires STC ≥50 and IIC ≥50 for multi-family residential (apartments, condos). Some jurisdictions require STC/IIC ≥55. LEED v4 and WELL v2 require STC ≥55 and IIC ≥55 for acoustic comfort credits. floorcasa flooring—meets IBC, LEED, WELL.

Industry Standards and Certifications

ASTM Testing Methods

• ASTM E90: Standard test method for airborne sound transmission (STC). Measures STC rating. Laboratory test. Required for building code compliance.

• ASTM E492: Standard test method for impact sound transmission (IIC). Measures IIC rating. Laboratory test. Required for building code compliance.

• ASTM E2179: Standard test method for field measurement of IIC (FIIC). In-situ test (actual building).

• ASTM E2235: Standard test method for sound absorption (NRC).

Building Codes

• IBC (International Building Code): STC ≥50, IIC ≥50 for multi-family residential.

• LEED v4: Acoustic performance credit—STC ≥55, IIC ≥55.

• WELL v2: Acoustic comfort—STC ≥55, IIC ≥55.

ISO Standards

• ISO 140: Acoustics—measurement of sound insulation in buildings.

• ISO 717: Sound insulation in buildings—rating methods.

ISO Quality Management Standards

• ISO 9001: Quality management systems. Specify ISO 9001-certified suppliers (floorcasa maintains ISO 9001:2024).

What These Standards Mean for Procurement
ASTM E90 STC ensures airborne noise reduction. ASTM E492 IIC ensures impact noise reduction. IBC requires STC/IIC ≥50. LEED/W ELL require STC/IIC ≥55. For procurement, require ASTM E90 STC ≥50-60, ASTM E492 IIC ≥50-65, and ISO 9001 certification. floorcasa acoustic flooring—meets all standards.

Conclusion (Engineering Decision Logic Only)

The selection of flooring assemblies based on STC/IIC is determined by three engineering criteria: STC rating (≥50-60), IIC rating (≥50-65), and building code/LEED/W ELL requirements. Concrete slab + rubber/cork underlayment + LVT provides STC 50-60, IIC 55-65.

Select concrete slab + 5-10mm rubber/cork underlayment + LVT (STC 50-60, IIC 55-65) when:

• Building code requires STC/IIC ≥50 (multi-family residential)

• LEED/W ELL requires STC/IIC ≥55

• Acoustic comfort is critical (luxury residential, hotels)

• Budget allows $5,000-9,000 per 100 m²

• Expected lifespan: 15-20 years

Select concrete slab + carpet (STC 45-55, IIC 55-65) when:

• IIC is primary concern (impact noise reduction)

• Carpet is acceptable (aesthetic, maintenance)

• Budget allows $4,000-6,000 per 100 m²

• Expected lifespan: 10-15 years

Select wood joist + insulation + underlayment + LVT (STC 45-55, IIC 50-60) when:

• Wood-frame construction

• Budget allows $6,000-9,000 per 100 m²

• Ceiling insulation and resilient channels are included

• Expected lifespan: 15-20 years

Avoid tile without underlayment (STC 40-45, IIC 35-40) for acoustic applications:

• Low IIC (footsteps audible)

• Low STC (airborne noise)

• Not code compliant (IIC <50)

• Not recommended for multi-family residential

Risk priority order for sound transmission class flooring:

1. Low IIC (impact noise). Mitigation: Underlayment (rubber/cork), carpet.

2. Low STC (airborne noise). Mitigation: Underlayment, ceiling insulation, resilient channels.

3. Flanking paths. Mitigation: Seal gaps, acoustical caulk.

4. Inconsistent assembly. Mitigation: Test per ASTM E90/E492.

Cost versus performance trade-off:
Slab + rubber + LVT has moderate cost ($5,000-9,000 per 100 m²) and high STC/IIC (50-60, 55-65)—best for code compliance and acoustic comfort. Slab + carpet has lower cost ($4,000-6,000) and high IIC (55-65) but lower STC (45-55). Wood joist + underlayment + LVT has higher cost ($6,000-9,000) and moderate STC/IIC (45-55, 50-60). The engineering decision favors slab + rubber + LVT for performance; slab + carpet for impact noise; wood joist for wood-frame construction.

For multi-family residential, commercial, and hospitality projects, concrete slab + 5-10mm rubber/cork underlayment + LVT provides STC 50-60, IIC 55-65—meeting IBC (50/50), LEED (55/55), and WELL (55/55) requirements. Carpet provides high IIC (55-65) but lower STC (45-55). Wood joist requires insulation and resilient channels to achieve STC/IIC ≥50. floorcasa acoustic flooring—STC 50-60, IIC 50-65, code compliant. Flooring assemblies that reduce airborne and impact noise, meet building codes, and achieve acoustic comfort are the engineering-justified specification for multi-story buildings.