Impact Insulation Class (IIC): Ratings, Delta IIC, and Field Testing

Impact Insulation Class, or IIC, is a single-number rating that measures how well a floor-ceiling assembly blocks the sound of footsteps, dropped objects, and other physical contact from reaching the unit below. The scale runs from the low 20s, where you can hear almost everything, up past 70, where normal walking is barely perceptible downstairs. Most building codes require a minimum IIC of 50 for new multifamily construction, and many condo associations demand higher numbers when owners want to replace carpet with hard flooring.¹ Knowing what the number represents, where it comes from, and where it quietly fails is the difference between a quiet floor and an expensive mistake.

What the Numbers Mean in Practice

The IIC scale is logarithmic, so each point of improvement represents a meaningful change in audible noise rather than a fixed, equal step. A jump from 40 to 50 does not cut the noise in half, but the subjective difference is dramatic. Here is roughly what occupants in the unit below can expect:

Below 40. Impact noise is easily heard and disruptive. Footsteps are clearly audible and dropped items can sound alarming. Common in older buildings with thin slabs and no underlayment.
40 to 49. Footsteps are noticeable, though somewhat muffled. This range fails current building codes for new construction and generates most neighbor-to-neighbor noise disputes.
50 to 55. Acceptable for most residential use and the minimum code-compliant range. Footsteps are present but not dominant. Heavier impacts, like children jumping, remain audible.
56 to 64. Good performance. Normal walking is faint and only heavy impacts draw attention. Many HOAs target this range in their governing documents.
65 to 70 and above. Excellent isolation. Normal foot traffic is essentially inaudible. Achieving these numbers usually requires some combination of carpet or thick underlayment, a substantial slab, and a decoupled ceiling.

Individual sensitivity to noise varies, so these descriptions are approximations. The practical takeaway: a floor in the low 40s will generate complaints, one at 50 will satisfy an inspector, and anything above 60 starts to feel genuinely quiet.

How IIC Is Measured

Acoustic engineers measure impact sound across 16 one-third octave frequency bands from 100 Hz to 3,150 Hz.² Those 16 points capture everything from the low thud of a heel strike to the higher click of a hard shoe on tile. The data is compared to a standard reference contour and shifted until the total of all points where the measured sound exceeds the contour is 32 decibels or less. The position of the fitted contour at 500 Hz becomes the IIC rating.³

The sound source is a tapping machine: five steel-faced hammers, each 500 grams, that drop 40 millimeters onto the floor and cycle at ten impacts per second.⁴ This is harsher than normal footfall by design. The machine simulates a worst-case scenario, and the rating reflects performance under stress.

Compressing 16 frequency readings into a single number is a deliberate simplification. The tradeoff is that two assemblies with the same IIC can perform quite differently at specific frequencies. One might excel at heel strikes but struggle with the rumble of a washing machine. For most residential purposes, though, the single number is a reliable shorthand.

Lab Testing vs. Field Testing

Laboratory testing under ASTM E492 happens in purpose-built chambers where the floor-ceiling assembly is the only path for sound between rooms. Side walls, plumbing, and structural connections are isolated.⁵ Lab results represent the ceiling of what an assembly can achieve. They are useful for comparing materials but do not predict real-world performance.

Field testing under ASTM E1007 happens in finished buildings, where sound travels through walls, pipes, ducts, and framing as well as the floor itself.⁶ The resulting Apparent Impact Insulation Class (AIIC) reflects the combined performance of the floor and all those flanking paths. Field scores almost always come in lower than lab scores for the same assembly, sometimes by five points or more. That gap is why codes set a lower threshold for field testing.

IIC vs. STC: Two Different Problems

IIC and Sound Transmission Class (STC) are companion ratings that measure different kinds of noise, and this distinction matters because many buyers assume one covers the other. If you can feel vibration by pressing your hand against the wall or floor, that is impact noise and IIC applies. If you hear sound but feel no vibration, that is airborne noise and STC applies.

Impact noise starts when something strikes a building surface: footsteps, furniture dragged across the floor, a dropped toy. The energy enters the structure directly and travels through solid materials, which is why it is so difficult to block. Airborne noise, like voices and television audio, moves through air and strikes the wall or floor from one side. The solutions diverge. A thick, dense wall can be excellent at stopping voices (high STC) and do little against footsteps (low IIC) because impact energy bypasses the air entirely. Building codes set separate minimums for both ratings, and an assembly meeting one but not the other will still fail inspection.

What Score You Actually Need

The International Building Code addresses impact noise under Section 1206.3. Every floor-ceiling assembly separating dwelling units, sleeping units, or units from public or service areas must achieve an IIC of at least 50 when tested in the lab under ASTM E492. When field testing is used instead, the code requires a Normalized Impact Sound Rating (NISR) of at least 45 under ASTM E1007.¹

NISR and IIC are not interchangeable even though both are single-number ratings. NISR is derived from field data that includes flanking contributions; IIC is a lab-only number. You cannot subtract five points from a lab IIC and call it a passing field score. The field test has to be performed and reported as NISR. Failing to meet the threshold can mean a withheld certificate of occupancy or a demand to tear out and rebuild the floor assembly.

HOA and Condo Requirements

Building codes set a floor; associations frequently raise it. Many write specific IIC thresholds into their CC&Rs, typically requiring an IIC of 55 or 60 for any hard-surface flooring installation.⁷ Enforcement usually runs through a pre-renovation approval process. Before swapping carpet for hardwood, the owner has to submit documentation, often a full acoustic report from a licensed consultant, proving the proposed system will meet the standard.

Skipping that step or installing flooring that falls short can trigger fines, mandatory removal of the new flooring, and in contested cases legal action. The combined cost of sound testing, investigation, and attorney fees can easily exceed $5,000.⁷ The worst outcome is not the fine but being ordered to rip out a brand-new floor. Pull your CC&Rs and check the acoustic requirements before you order materials.

Reading Product Labels: Why Delta IIC Matters

When you shop for underlayment or flooring, the IIC rating on the box deserves skepticism. A standard IIC rating reflects the whole assembly: structural slab, underlayment, finish floor, and sometimes the ceiling below. A manufacturer can test their underlayment on a thick, high-performing concrete slab, report an impressive IIC of 65, and that number tells you little about what will happen over your thinner slab. The real-world result could be ten or more points lower.

Delta IIC (ΔIIC) isolates the contribution of the flooring product itself. The test, standardized as ASTM E2179, first measures the IIC of a bare reference concrete slab, then measures it again with the flooring product installed.⁸ A product with a ΔIIC of 22 adds roughly 22 points of impact isolation to whatever assembly you install it over.

Comparing two underlayments? Ignore the headline IIC numbers and look for ΔIIC. Two products could both advertise “IIC 60” but have been tested on different reference slabs. Ask your supplier for the ASTM E2179 test report. If all they can produce is an E492 whole-assembly test, you have no way to isolate what the underlayment itself is doing.

Typical IIC Performance by Flooring Type

The finish floor material has an outsized influence on the final IIC. Carpet is the champion of impact isolation; hard surfaces are the usual source of complaints. Rough ranges for common flooring types installed on a standard concrete slab:

Carpet with pad. IIC 65 to 75. The pad does most of the work. A thick, high-quality pad can push well into the 70s. This is why many HOAs require carpet or demand proof of equivalent IIC for any hard-surface replacement.
Hardwood or laminate with acoustic underlayment. IIC 50 to 60. Without underlayment, these materials can score in the 30s.
Tile or stone with acoustic mat. IIC 50 to 60. Rigid flooring transmits impact energy efficiently. A decoupling membrane or acoustic mat is essential to reach code.
Luxury vinyl (LVT or LVP) with underlayment. IIC 50 to 58. Vinyl is lighter and slightly more flexible than tile, which helps a bit, but underlayment is still needed.
Hard surface on bare concrete, no underlayment. IIC 25 to 35. The scenario behind most neighbor disputes and HOA enforcement.

These ranges assume a standard six-inch concrete slab. Wood-framed floors with joist systems behave differently and are generally harder to bring up to high IIC numbers, because the framing itself is a flanking path. Wood construction typically requires a more aggressive combination of underlayment, resilient ceiling channels, and insulation in the joist cavity to hit similar scores.

Building a Floor That Actually Performs

No single layer produces a high IIC rating. Effective impact isolation requires multiple components, each addressing a different stage of how vibration moves through a building. A well-designed assembly works from the top down:

Finish floor. The surface you walk on. Carpet absorbs impact energy at the source. Hard surfaces do almost nothing on their own, which is why the layers below matter so much more when hard flooring is involved.
Acoustic underlayment. A resilient layer between the finish floor and the subfloor, made from rubber, cork, or closed-cell foam. It acts as a shock absorber, converting impact energy into heat before it reaches the structure. This is the single most impactful upgrade for hard-surface floors.
Structural subfloor. Typically a concrete slab or wood joists with plywood sheathing. Mass helps: a thicker, denser slab transmits less impact energy. Wood-framed floors are lighter and inherently more prone to transmitting vibration.
Ceiling isolation. Below the subfloor, resilient channels or sound-isolation clips decouple the drywall ceiling from the framing. Without decoupling, vibration travels through the framing into the drywall, which radiates sound into the room below.
Perimeter isolation strips. A buffer placed between the flooring edge and the wall. These break the lateral path by which impact energy travels through the flooring into wall framing and down into the unit below. They matter most with rigid materials like tile and laminate.⁹

Swapping one component changes the whole equation. A premium underlayment with no perimeter isolation, or resilient channels with an empty joist cavity, will underperform the product data sheets. The weakest link sets the ceiling. Acoustic consultants evaluate all the layers together for exactly this reason.

Fixing Impact Noise in an Existing Building

Improving impact isolation after the fact is harder and costlier than getting it right during construction, but several approaches work depending on budget and access.

If you are replacing flooring anyway, upgrading the underlayment is the most cost-effective improvement. A quality rubber or cork underlayment beneath hard-surface flooring can add 15 to 25 IIC points compared to hard flooring laid directly on the slab. A floating floor, which sits on underlayment without being fastened to the subfloor, prevents vibration from transferring mechanically into the structure and works well with laminate, engineered hardwood, and some vinyl products.

If you can access the unit below, treating the ceiling is powerful. Adding a resilient channel or clip system and hanging new drywall creates a decoupled layer that absorbs vibration before it radiates as sound. This works particularly well in wood-framed buildings. Filling the joist cavity with mineral wool or fiberglass batt insulation dampens vibration within the cavity. On its own, insulation will not solve a serious impact problem, but it meaningfully improves a ceiling treatment.

Flanking paths deserve attention too. Seal gaps around pipes, electrical boxes, and HVAC penetrations, and add perimeter isolation strips at the floor-wall junction. Individually these details seem minor; collectively they can account for several IIC points. The common mistake is treating one component as a silver bullet. Impact noise finds paths through a building, and only a systematic approach — source, structure, receiver, and flanking paths — delivers reliable results.

A Blind Spot in the Standard Number

The standard IIC rating emphasizes mid-to-high frequencies and underweights the low-frequency thumps that bother people most. Two newer metrics address this. Low-Frequency Impact Insulation Class (LIIC), defined by ASTM E3207, covers the 50, 63, and 80 Hz bands where heavy footfalls dominate. High-Frequency Impact Insulation Class (HIIC), defined by ASTM E3222, covers 400 to 3,150 Hz where lighter tapping sounds live.¹⁰

Neither has been adopted into the IBC or most HOA documents, but they are gaining traction among acoustical consultants who recognize that an IIC of 55 can hide poor performance at the low frequencies where barefoot walking on a lightweight wood floor generates the most complaints. If you are hiring an acoustic consultant for a renovation, ask whether LIIC and HIIC data is available for the products you are considering. The extra detail can prevent an unpleasant surprise after installation.

1
ICC Digital Codes. IBC 2021 Chapter 12 Interior Environment
2
ASTM International. ASTM E492 – Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine
3
ASTM International. ASTM E989 – Determination of Impact Insulation Class (IIC)
4
CESVA Instruments. MI006 – Tapping Machine
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Intertek. ASTM E492 – Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine
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ASTM International. ASTM E1007 – Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission
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Educational Community for Homeowners. HOA Duty to Address the Upstairs Units Noisy Floors
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Intertek. ASTM E2179 – Standard Test Method for Laboratory Measurement of the Effectiveness of Floor Coverings in Reducing Impact Sound Transmission Through Concrete Floors
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Ecore Sound Advice. Perimeter Isolation Strips: What Are They and How Do You Install Them
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European Acoustics Association. Design of Floor-Ceiling Assemblies Using Dual Ratings of Impact Noise Insulation