IIC and STC Ratings Guide: Condo and Apartment Flooring Permit Compliance

• STC (Sound Transmission Class): Lab-Measured Resistance to Airborne Sound Through a Wall, Floor, Door, or Window Assembly
• IIC (Impact Insulation Class): Lab-Measured Resistance to Impact Sound Through a Floor-Ceiling Assembly
• ASTC (Apparent STC): The Field-Measured Equivalent of STC — Includes Flanking and Real Construction Conditions
• AIIC (Apparent IIC): The Field-Measured Equivalent of IIC
• Field Penalty: ASTC and AIIC Typically Test 3 to 5 Points Lower Than the Lab STC and IIC of the Same Assembly

STC and IIC are the two ratings that govern almost every multi-family and hotel acoustic specification in the United States. STC handles airborne sound — voices, music, television, HVAC noise, anything that travels through the air. IIC handles impact sound — footfall, dropped objects, furniture moving, anything that strikes the floor structure directly.

Both ratings come from controlled laboratory tests. Real construction in real buildings always performs a few points lower because lab tests isolate the assembly from flanking paths (the routes sound takes around the wall through the structure, doors, penetrations, and shared cavities). When designers spec STC 50 on a wall schedule, they should be reading that as a target of ASTC 45 to 47 in the field.

The deeper background on the difference between airborne and impact noise lives in our airborne vs structure-borne noise guide.

The STC rating you should target depends on which building element you are specifying and what use class the space falls under. Walls between dwelling units have a different floor than interior partitions inside the same unit. Exterior windows facing a busy street have a different floor than interior doors. The chart below maps the working windows for the most common elements.

The door and window rows are where most code-compliant walls fail in the field. A wall rated at STC 50 with a door rated at STC 24 in the same partition delivers an effective STC closer to 28 because the door becomes the dominant flanking path. Door and window STC has to be specified in conjunction with the surrounding wall STC, not independently.

IIC rating depends on the floor assembly type and the finished floor material. Concrete floor assemblies generally outperform wood-frame assemblies on impact insulation. Carpet over pad nearly always tests higher than hardwood, LVT, or tile because soft finished floors absorb impact at the source.

The underlayment column is where most condo flooring permit applications get won or lost. A bare concrete slab alone almost never meets the IIC 50 code minimum. Adding a rated acoustic underlayment under any finished floor brings the assembly into the compliant range. The product line that handles this lives at the Floor Blokker acoustic underlayment page.

• IBC Section 1207.1: STC 50 (Lab) for Walls Between Dwelling Units
• IBC Section 1207.2: IIC 50 (Lab) for Floor-Ceiling Assemblies Between Dwelling Units
• Field Equivalent (Approximation): ASTC 45 and AIIC 45 in the Built Assembly
• Hotel Brand Standard: STC 55 to 60 — Marriott, Hilton, Hyatt, IHG All Spec Above Code
• Luxury Condo Standard: STC 55 to 60+ Driven by Buyer Expectation, Not Code

The International Building Code Section 1207 establishes the floor for residential acoustic separation in the United States. Almost every state and municipality adopts IBC 1207 either by reference or with minor amendments. The Florida Building Code adopts IBC 1207 directly for multi-family construction. Most jurisdictions enforce STC 50 lab-tested as the minimum for walls between dwelling units and IIC 50 for floor-ceiling assemblies between dwelling units.

Hotels and luxury condos almost always exceed code by 5 to 10 STC points because the brand standard or buyer expectation is higher than the legal minimum. A code-compliant STC 50 wall is fine for a budget apartment but reads as cheap construction in a luxury condo where buyers expect to hear effectively nothing from neighboring units. The deeper background on building codes for acoustics lives in our building codes acoustics guide.

• Jurisdiction: City of Hallandale Beach, Broward County, Florida
• Requirement: Condo Floor Assemblies Must Meet or Exceed STC 50 and IIC 50 (Lab-Tested)
• Code Basis: Florida Building Code Section 1207 Adopted at the Municipal Level
• Permit Trigger: Any Flooring Replacement in a Condo Unit That Removes the Original Carpeting
• Required Documentation: Manufacturer Spec Sheets Showing STC 50 and IIC 50 for the Proposed Floor Assembly

Hallandale Beach is one of several South Florida coastal municipalities that enforce condo flooring acoustic requirements at the city permit level. The driver is straightforward — when an owner removes existing carpet (which carries its own incidental impact insulation) and replaces it with a hard surface like LVT, hardwood, or tile, the floor-to-ceiling acoustic performance can drop by 15 to 25 IIC points overnight. The unit below files a complaint and the building’s HOA ends up adjudicating.

The permit office requires the owner or contractor to document that the new floor assembly will meet or exceed STC 50 and IIC 50. The documentation is usually a manufacturer spec sheet (or an acoustic test report) showing the proposed floor type plus the underlayment together hit the target rating on a representative subfloor.

Floor Blokker acoustic underlayment is rated to deliver STC 50 and IIC 50 across a range of LVT, engineered hardwood, and tile finishes on both 6-inch concrete and wood-truss subfloors. The spec sheets include lab-tested values that meet Hallandale Beach permit requirements. For sister-jurisdiction projects that ran the same compliance path on a multi-family vertical, see the LVT soundproofing underlayment case study.

The right STC and IIC target for a condo depends on the building class, the buyer expectation, and the local code. Code-minimum apartments and entry-level condos hit STC 50 / IIC 50 lab-tested. Luxury condos run higher because buyers compare units against quiet single-family experience.

The middle two rows are where most condo developers land. Code-minimum is legal but generates complaint volume. Ultra-luxury adds cost that most buildings cannot recover. Mid-market and luxury (STC 52 to 60) is the right band for almost any condo project that wants to keep complaints below the threshold where the HOA gets involved.

Field findings from a Central Florida apartment complex acoustic review (2017) make the lab-versus-field gap concrete. An architecture firm asked for an audit on standard wall details that had been spec’d at STC 50+ but were generating noise complaints in the built units. Figures 1 and 2 below show the wall schedule details for the two assembly types tested. The wall STC reference assemblies appear in Figures 8, 9, and 10 at the bottom of this page.

WF-9 Tenant-to-Tenant Double-Stud Wall

• Stud Configuration: Two Sets of 2×4 Studs With a Shared 10-Inch Top Plate
• Spec’d Lab STC: 55
• Built STC (With Top Plate): 51
• Field ASTC: 50 (Above Florida Building Code Minimum)
• Flanking at Wall Base: 3 to 5 dB

The shared top plate diminished the wall’s lab STC of 55 to a built STC of 51 because it allowed direct propagation through the top plate. Adding a thin bead of acoustic sealant along the wall base would have lifted the field STC by several more points. The WF-9 ASTC test data is shown in Figure 3, and the top-plate effect on the assembly’s STC is illustrated in Figures 8 and 9 at the bottom of this page.

WF-4 Breezeway Corridor-to-Unit Wall (Kitchen)

• Decoupling: Resilient Channel
• Compromise: Wood Blocking Added to Support Kitchen Cabinets
• Wall Schedule STC: 56 (No Acoustic Test Number Referenced)
• Tested STC With Blocking: 50
• Door STC: 24 (Dominant Flanking Path)
• Effective STC at Door Zone: ~28

The blocking screws penetrated through the drywall into the studs, defeating the resilient channel decoupling. The kitchen door was the dominant flanking path: an STC 24 door in an STC 50 wall delivered effective STC closer to 28 in the door zone. The WF-4 Kitchen ASTC test data is shown in Figure 4, and the calculated STC for this assembly with blocking is shown in Figure 10 at the bottom of this page.

WF-4 Breezeway Corridor-to-Unit Wall (Closet)

• Wall Type: Same as WF-4 Kitchen
• Wood Blocking: Less (Lighter Shelves)
• Field ASTC: 43 (Slightly Below Florida Building Code)
• Implication: Even Reduced Blocking Affects Resilient Channel Performance

Same wall type as the kitchen test, with less wood blocking, but the field result still came in below code. Demonstrated that even small mechanical bridges through a resilient channel system meaningfully drop the assembly below the Florida Building Code minimum. The WF-4 Closet ASTC test data is shown in Figure 5.

Test 4: Kitchen Flooring (FC-1) — AIIC 43

• Finished Floor: 20mm Wear-Layer LVT
• Substrate: 3/4″ Gypcrete, AccuQuiet D18 Underlayment, 3/4″ OSB
• Truss + Ceiling: 20″ Wood Floor Truss, R-13 Batt, Resiliently Mounted 5/8″ Type X Drywall
• Field AIIC: 43 (Below Florida Building Code Minimum)
• Issue 1: Underlayment Only Under LVT Section (Flanking Around Perimeter)
• Issue 2: Floor Perimeter Isolation Strips Only in Kitchen (Additional Flanking)

The Gypcrete moves footfall energy laterally around the underlayment into the OSB subfloor, bypassing the acoustic isolation. The AccuQuiet D18 underlayment spec (Figure 6b) also raised a documentation concern: the construction and referenced assemblies sheet listed no Acoustic Test number for the floor system, and the manufacturer’s literature claimed 6 to 12 IIC points of improvement without providing a third-party test report. As a rule, IIC and STC ratings on floor schedules should have an accompanying lab test reference available in the design package. AIIC test data for this floor is shown in Figure 6a.

Test 5: Living Room Flooring (FC-1) — AIIC 58

• Floor Assembly: Same as Test 4
• Finished Floor: Carpet and Pad (Instead of LVT)
• Field AIIC: 58 (Well Above Florida Building Code)
• IIC Swing vs LVT: ~15 Points
• Implication: Carpet-to-Hard-Surface Replacement Is the Largest Acoustic Decision in a Condo

This is the single largest acoustic difference any finished-floor decision can make and the reason condo flooring permits in jurisdictions like Hallandale Beach trigger documentation requirements specifically when carpet is being replaced with a hard surface. AIIC test data is shown in Figure 7.

• Shared Top Plate or Bottom Plate: Direct Path Around Decoupled Studs (Drops STC 3 to 5 Points)
• Wood Blocking Through Resilient Channel: Fasteners Bridge the Decoupling, Defeating the Channel Benefit
• Standard Solid Core Doors in High-STC Walls: STC 24 Door in STC 50 Wall Delivers Effective STC ~28 in the Door Zone
• Penetrations Without Acoustic Sealant: Outlets, Conduit, HVAC Boots — All Flanking Paths
• Missing Bead at Wall Base: Floor-to-Wall Joint Leaks Without Acoustic Caulk
• Underlayment Adhered Down When Floating Was Spec’d: Loses 3 to 5 IIC Points

The pattern across these failure modes is the same: the lab STC of an isolated assembly is the upper bound. Anything that creates a coupling path, a flanking path, or a leak between source and receiver pulls the field result down toward that path’s transmission characteristic. A wall is only as good as its weakest acoustic element.

For deeper coverage on how vibration and structure-borne paths move through real building assemblies, see our how vibration travels through buildings guide.

STC and IIC compliance is a design problem, a construction problem, and a documentation problem. Spec the lab values 5 points above the field target. Construct the assembly without bridging the decoupling layers. Document the resulting performance with manufacturer spec sheets or field tests when the jurisdiction (like Hallandale Beach) requires it. Skipping any of those three steps produces complaints, permit denials, or HOA disputes — usually all three. Contact Us for an STC and IIC review on a specific project or permit application.

Wall STC reference assemblies referenced throughout the field findings. These figures will be replaced with cleaner combined versions in a follow-up update.